Abstract

The late Pleistocene deglacial history of the Laurentide ice sheet in Vermont, specifically the lowering elevations of the ice sheet surface levels and the receding positions of its margins through time, has never been established in its entirety. By this is meant the specific, explicit delineation of successive ice margin positions, levels, and times as deglaciation proceeded, not inferred ice margin positions as taken from evidence provided by proglacial water bodies, as was suggested long ago by Chapman and other subsequent researchers. This reflects the absence of end moraines in Vermont, as has long been recognized, which it turns out is itself significant, reflecting Vermont’s unique “Styles,” or environmental conditions along its margins, and it’s “Glacial Dynamics,” representing the interaction between the ice sheet and these conditions.

By contrast, considerable information about ice margins, levels, and deglacial history of the ice sheet recession in neighboring Quebec, New York, and New Hampshire has been published, particularly in recent years, reflecting the presence of end moraines in these areas and other evidence bearing on Laurentide ice sheet deglacial history. Thus, Vermont represents a gap in the historical record. This gap is crucial for the understanding of regional deglacial history of the Laurentide ice sheet in as much as the Champlain Basin in New York and Vermont, the Memphremagog Basin in eastern Vermont, and the Connecticut Basin in Vermont and New Hampshire likely served as major pathways for the Laurentide ice sheet. This gap in the historical record is important as a matter of the academic understanding of deglacial history and needs to be filled, including with correlations with neighboring regions. In addition, the evidence presented here, especially in the Addendum to this report, suggests that the late recessional history of the Champlain lobe in Vermont represents destabilization, perhaps a “collapse(?),” which bears on present-day issues of global warming with relevance to the stability of modern ice sheets, making the closing of this gap more important for these concerns.

This study was originally inspired by research reports in southern Quebec, near Vermont, which have identified moraines that can be correlated on the basis of their isostatically adjusted elevations, orientations, and alignment. This suggests a physiographic elevation “Bath Tub Model” might be used to correlate different types of ice margin features which are quite numerous in Vermont, such as stagnant ice deposits, on this basis as a general guide. In this study it was assumed, using the “Bath Tub Model,” that certain frontal as opposed to lateral type stagnant ice margin deposits and other ice margin features which occur at similar isostatically adjusted elevations formed at approximately the same times. The usage of such a Model is made more reasonable by the fact that much of the ice sheet recession in Vermont took place at a late time when the ice sheet was relatively thin, with relatively low ice surface gradients, and in a “reverse gradient” setting, much like a northward sloping, slowly draining bathtub, with step-down ice margin recession in most places influenced or controlled by both physiography and standing water along the ice margins. Multiple arguments in support of the usage of elevation as a guide for reliably developing Vermont deglacial history are presented. Both the collective weight of the evidence and as well independent evidence, including certain “Signature” type ice margin features which can be readily identified and correlated across Vermont semi-independently of elevation, support the validity of the Bath Tub Model, and thus the deglacial history so determined.

Previously unpublished information from my field mapping in the 1960s and 1970s, current online information provided by the Vermont Center for Geographic Information (VCGI), including LiDAR imagery, limited more recent field work, and pertinent previously published reports in the recent literature were used to explore Vermont deglacial history as presented in this report. This study identifies conventional types of ice margin features, such as stagnant ice deposits, and as well types which reflect Vermont’s varying Styles and Glacial Dynamics, including calving, streaming, and other types of ice margin features.

As noted in the above Preface, this report was “completed” in January 2026. However, discussions with colleagues about the late glacial history of the Champlain lobe led to further study which helped to identify new and different ice margin features, leading to a revision of deglacial history, as reported here in an Addendum to the original report. Most of the findings in the pre-Addendum report have not changed, with the Addendum findings providing new information about ice streaming, calving, and perhaps a collapse(?) in the late chapters of this deglacial history.

Ice Margin Features

This study identified a variety of different types of ice margin features which were mapped on VCGI “Project Sheets.” Some of these were conventional types but many were less familiar types identified by usage of LiDAR and required study to establish their nature.

Stagnant Ice Margin Deposits: Typical kame and kettle deposits and eskers are common and numerous. LiDAR imagery greatly helped to identify and map such deposits, in many cases with evidence within individual deposits showing progressive recession of lobe margins. In general, stagnant ice deposits formed along Warm, inactive ice margins which allowed meltwater penetration into, along, beneath, and from such margins.
Kame Deltas: Whereas kame deltas, also called “ice contact deltas,” have long been recognized, in my past mapping in the 1960s and 1970s in Vermont, these were not generally recognized or used in Vermont as ice margin features. However, LiDAR imagery helps to identify and map many ice contact deltas, both shoaling and Gilbert-type deltas, which are very common and important as they indicate the close proximity of the ice margin to proglacial water body strandlines at specific elevations of the receding ice margins. In general, ice margin recession was in a reverse gradient setting and tended to be associated with the progressive development of local proglacial lakes, in a recurrent step-down pattern for the development of lower proglacial water body levels as lower outlets were uncovered by ice margin recession, as commonly marked by kame deltas and associated ponded water and drainage features. This step-down pattern shows the progressive coalescence and enlargement of proglacial water bodies into successively lower and increasingly larger water bodies. This step-down coalescence is a very significant Glacial Dynamic element in that changes in the levels of larger,regional water bodies had the capability of substantially altering the stability of the ice margin.
“Bedrock Grooves:” Bedrock Grooves formed by fluvial meltwater erosion along Cold, active ice margins capable of restraining or impounding meltwater, with drainage along margins typically forming relatively large, deep, incised, channels, which in many places reflect bedrock structural elements which were enhanced by ice marginal drainage erosion. Such Grooves commonly are associated with fluvial sedimentary deposits and stagnant ice deposits as part of hybrid margins, and tend to be graded into kame deltas, all of which together represent a type of “Drainage Lines.”
“Drainage Lines:” Fluvial meltwater drainage along receding ice margins tends to be marked by erosional and depositional evidence, which is readily detectible on LiDAR imagery, often as linears, commonly with soil textural differences on published surficial geology maps, Vermont Highway Department maps, and USDA Soil Conservation Service soil maps, all provided by VCGI. These Drainage Lines, which mark the margins of the receding ice sheet at different places, elevations, and times represent temporal “connective tissue” which aids correlation, extending a) downgradient from or along ice margins within individual basins, b) across interfluves between neighboring side-by-side basins, and c) across drainage divides via spillways between opposing neighboring basins.

Drainage Lines in some places can be traced and correlated for long distances, serving as important links, thereby helping to correlate regional ice margins across the State in support of the Bath Tub Model. For example, an approximate 30 mile long (48 km) Drainage Line from a specific (T4) ice margin near the Quebec border in the Memphremagog Basin can be traced southward in a downgradient direction, linking to the strandline for Lake Winooski (a major proglacial water body in the Lamoille and Winooski Basins), which in turn can then be traced southward for another 30 miles (48 km) in close association with a T4 ice margin and its associated strandline; and further, this strandline can then can be linked via another Drainage Line into the Champlain Basin to a Lake Vermont strandline, which likewise can be traced further southward to T4 ice margin positions in southern Vermont. Thus, such Drainage Lines provide support for the viability and usage of the Bath Tub Model as links for correlating margins across the State and are important elements of Vermont deglacial history.

“Scabby Terrain:” This is a newly identified type of deposit with a peculiar LiDAR signal mapped in many places on VCGI, along the margins of ice masses which became “Disconnected,” a term and concept identified in the literature for modern ice sheets and glaciers where ice masses become detached from the parent ice sheet and as a consequence stagnate en masse. Both individual “patches” and long “tracts” of Scabby Terrain are mapped, the latter being especially important and interpreted as indicating the margins of large portions of the Connecticut Basin lobe in Vermont which became Disconnected. Such Disconnections are identified at three times and places related to the lowering of the ice sheet across cols on topographic divides, cutting off the active ice supply from the parent ice sheet in the Champlain and Memphremagog basins into the Lower, Middle, and Upper Connecticut Basins. These “Disconnections” reflect the higher terrain along the perimeter of the Connecticut Basin where and when at different times and places, ice supply from the parent ice sheet became critically limited. This history pertains to a longstanding debate about the deglacial history of the Connecticut basin and the issue of receding active margins versus en masse stagnation.

A similar Disconnection is identified in deep drainage basins on the east side of the Green Mountain front range along the western perimeter of Lake Winooski whereby localized masses of possibly temporarily active ice became separated and isolated from the main ice sheet. Such Disconnections and en masse stagnations are marked by Scabby Terrain and represent both important information about Glacial Dynamics and as well evidence for deglacial history.

“Ice Marginal Channels:” These are erosional features identifiable on LiDAR imagery which are very numerous and quite common in Vermont, especially in certain areas, along certain ice margins, and at certain specific elevations and therefore times. Such channels have been widely reported in previous literature both in Vermont and elsewhere, with different terminology and theories for their formation. In general, Ice Marginal Channels in Vermont have been carefully examined and are here interpreted as having formed: 1) sub-glacially, 2) laterally along active, “Cold Ice” margins, 3) vertically beneath immediately overlying “Warm” basal ice, 4) by sub-glacial meltwater drainage, 5) under elevated hydrostatic head, confined by inverse polythermal Cold over Warm ice, 6) in a manner analogous to spillways for drainage of surface water proglacial lake impoundments across drainage divides, but in this case for drainage of impounded subglacial meltwater across terrain protuberances.

Once again, these features provide evidence for deglacial history but as well about Glacial Dynamics. The general absence of such features in the Lower and Middle Connecticut Basins is consistent with en masse stagnation in those basins. And likewise, the absence of Ice Marginal Channels associated with the late glacial T7 lobe in the Champlain Basin is an indicator of fully Warm ice, which was a critical part of the late glacial recessional history associated with ice streaming and perhaps the “collapse(?)” of the Champlain lobe.

Ice Margin Channels are numerous in the Champlain Basin and are especially prominent in the Memphremagog Basin, where they show the recession of the ice in an early “Nunatak Phase,” with progressive development of lobes in the following “Lobate Phase.” Certain Late T3/T4 Ice marginal Channels in the Upper Connecticut Basin in Vermont, represent active ice margins which are correlated with the early portion of the White Mountain Moraine System (WMMS) in New Hampshire, which has been reported as representing a significant readvance of the ice sheet. These particular Ice Marginal Channels are believed to have formed in a similar way to the preceding explanation, but with the base of the ice sheet warmed by the T3/T4 ice readvance over terrain that had been warmed during the preceding recession.

Importantly, T3/T4 readvance features occur in conjunction with lower, slightly younger (“hybrid”) type stagnant ice deposits in the WMMS, which together represent a “Signature” pattern which can be identified and correlated across Vermont, semi-independently from the usage of ice margin elevations in the Bath Tub Model. This correlation includes readvance evidence reported by others in the Winooski Basin and by water well logs and soil borings records in the Vermont Valley in the Rutland and Bennington vicinities. Thus, these particular features serve as an important marker of a readvance of the ice sheet that occurred at an early time across Vermont, near the end of the Nunatak Phase and the beginning of the Lobate Phase.

Further, as just suggested, Ice Marginal Channels together with stagnant ice deposits define “Hybrid” margins, with the stagnant ice margin components being longer lasting than their active ice counterparts, owing to the insulation of their sediment cover, resulting in a “dance” of receding active ice margins to newer, lower and later active ice margin positions while their stagnant margin counterparts persisted. This “dance” is referred to as a Style termed “Everything, Everywhere, All at Once and Continuing.”

“Ice Tongue Grooves:” These are erosional grooves at the mouths of the Missisquoi, Lamoille, Winooski, and Otter Creek Basins which are believed to have formed in conjunction with the sudden and substantial lowering of proglacial Lake Vermont from the Coveville to the Fort Ann levels for the Lamoille, Winooski, and Otter Creek basin features, and for the Fort Ann to Champlain Sea lowering for the Missisquoi features. These water level changes resulted in destabilization of the ice margins, with the Grooves marking increased surface ice gradients associated with the transformation of the eastern margin of the Champlain lobe from a lateral type of margin toward a frontal type of margin

Shattuck Mountain Pothole Tract: This tract, which has been reported previously by others and interpreted in different ways, is believed to have formed from drainage of either supraglacial meltwater drainage via moulins to the ice sheet base, or by ice marginal drainage, or both, in either case associated with extraordinary meltwater drainage volumes and velocities related to a physiographic barrier that served to collect and direct meltwater from a large catchment area of the Champlain lobe. This Tract, which represents drainage closely associated with the deglacial recession of the Champlain lobe, is part of the evidence of Glacial Dynamics associated with the eventual warming of the entire Champlain lobe and penetration of meltwater to its base, which is an important part of its accelerated recession.
Calving Ice Margin Features: Calving is identified at receding lobe tips for five discrete, narrow ice streams(?) in physiographic re-entrants in the “Middlebury Bench,” a slightly raised eastern portion of the Champlain Basin floor, beginning in late T6 and early T7 time, associated with the sudden and substantial Coveville to Fort Ann water level lowering, as a destabilizing event. Features indicative of calving include 1) “Headless Deltas,” 2) “Ribbed Lacustrine Deposits,” and 3) “Thickened Bouldery Ponded Water Deposits.” Whereas these features are interpreted as indicating destabilization of the Champlain lobe and calving, for which substantial supporting evidence exists, these features do not specifically mark grounding lines, which generally are unmarked. Nevertheless, the evidence indicates that the calving margins of these lobes receded in a downgradient, generally westward direction in T7 and Fort Ann time.

In later T7 and Fort Ann time, these calving margins receded from the Middlebury Bench to the eastern margin of the “Trough,” a lower, flatter portion of the basin floor. This resulted in the loss of buttressing support for the Champlain lobe, which triggered a first phase of ice streaming and associated calving. A prominent Transverse Morainic Ridge in the Champlain Basin is identified, marking a pause in the receding grounding line of the calving and streaming Champlain lobe at a late T7 and Fort Ann time, when it was confined to the Champlain Basin “Trough,” immediately west of the Middlebury Bench.

Streaks: These are ice margin features identified in the Addendum study, detectible as linears on LiDAR, found at the same correlative isostatically adjusted late T6 and early T7 levels and times in the Champlain Basin, again indicating a Glacial Dynamic associated with the sudden and substantial lowering of Lake Vermont from the Coveville to the Fort Ann levels. “Streaks” are believed to represent both compressional and shear Glacial Dynamics along the Champlain lobe margin when it was destabilized by the lowering of Lake Vermont from the Coveville to Fort Ann level, when calving of re-entrant ice masses in the Middlebury Bench as described above also began, and as well when the aforementioned Ice Tongue Grooves formed in conjunction with the transformation of the eastern margin of the Champlain lobe from a lateral toward a frontal margin. Streaks are numerous features and provide a readily identifiable LiDAR signature which can easily be correlated across the Champlain Basin independent of elevation in the Bath Tub Model.
Ice Margin Steps: These features are stepped terrain, usually in glacial till ground moraine, reported previously by others in Vermont and elsewhere, with different theories for their formation, such as imprinting by ice blocks on till substrate or as push moraines. Such Steps in Vermont are commonly associated with Ice Marginal Channels at the transition from the Nunatak Phase to the Lobate Phase. The Addendum study suggests that certain Steps in some places and times formed by imbricate thrusting of glacial till in ground moraine at destabilized ice margins, in association with “Streaks” as part of the Champlain lobe destabilization.
Scarps, Mega-Scale Lineations, and Transverse Morainic Ridges: Identified in the Addendum study, “Scarps” are linear features which are numerous and readily identifiable on VCGI LiDAR imagery, likewise representing Signature ice margin features. These clearly delineate ice margins of the Champlain lobe in later T7 and Fort Ann time when recession of the Champlain lobe from the Middlebury Bench led to its being confined to the Trough, which removed buttressing support and triggered streaming of the Champlain lobe.

Scarps are distinctive geomorphic features marking the eroded frontal faces of topographic benches, similar to and likely representing Ribbed Lacustrine deposits, which formed from stagnant ice, with lacustrine sediment veneer cover representing the incursion of standing waters while stagnant ice remained. Rounded erratic pebbles and boulders commonly are found on the tops and frontal faces of Scarps and associated benches, in places with exposures of a distinctive type of till, likewise with abundant rounded pebbles and boulders. Scarp faces are very evident as steeper inclines of the terrain, with shallow bedrock outcroppings, in places forming bedrock cliffs, indictive of substantial erosion. The bases of Scarps are distinctive, readily identifiable marks on LiDAR imagery, which facilitates mapping.

Scarps are believed to represent lateral shear margins at the interface between fast moving active ice streams and adjacent slow moving or stagnant ice, at consistent (isostatically adjusted ) elevations in late T7 and Fort Ann times, with Scarp bases remarkably and consistently close to but slightly above the elevations of the nearby Champlain Sea marine limit strandline. Scarps mark the first phase of ice streaming when the Champlain lobe was confined to the Trough. Again, this ice streaming likely was triggered by the loss of buttressing support previously provided by the Middlebury Bench, but was aided and abetted by the warming of the ice, the presence of basal meltwater, and the predominance of fine grained sediment and shales as the substrate for the streaming Champlin lobe.

In addition, Mega-Scale Lineations are also identified, further documenting streaming of the Champlain lobe associated with Scarp formation. These are long, linear, multiple, parallel grooves in fine-grained sediment on the basin floor, formed beneath the ice stream.

Transverse Morainic Ridges are identified on the Champlain Basin floor, marking the grounding lines of calving ice shelves associated with streaming.

Again, Scarps and the associated Mega-Scale Lineations are interpreted as representing an early phase of ice streaming of the Champlain lobe in the Trough in late T7 and Fort Ann time, triggered by the loss of the buttressing support for the Champlain lobe when the lobe cleared the Middlebury Bench and thus became confined to the Trough with diminished obstructions for ice flow and the base of the ice sheet resting primarily on shales with meltwater at the ice sheet base (as indicted by rounded gravel sheared into till), which together offered lower resistance to ice flow.

Importantly, Scarps are limited to the southern and central Champlain Basin and are generally absent in the northern Champlain Basin, north of an Onset Zone in the Shelburne/Burlington area at the head of this ice stream. As discussed below, this absence is believed to be significant, interpreted as indicating a second accelerated phase of streaming or perhaps Champlain lobe collapse(?) when the Onset Zone was cleared by further ice recession, in conjunction with the sudden and substantial lowering of Lake Fort Ann to the Champlain Sea level.

Onset Zone: The northern head of Champlain lobe first phase of ice streaming, as marked by Scarps and MegaScale Lineations in the southern and central Champlain Basin, is at an “Onset Zone” located just south of the Burlington area. This Onset Zone is likened to an ice dam for the Champlain lobe associated with the necking of the basin Trough physiography and an overdeepened Trough floor. Unusual LiDAR detected curvilinear grooves in a Scarp face in this Onset Zone mark erosion in the terrain associated with the progressive, likely rapid clearing of the Onset Zone in a manner likened to a dam break, leading to accelerated subsequent streaming and possible “collapse(?),” likely as well triggered by the reportedly sudden, rapid, and substantial fall of Lake Vermont at the Fort Ann level to the Champlain Sea level, associated with the incursion of the Sea into the Basin. The Onset Zone thus marks the position on the Champlain lobe frontal tip at the beginning of Champlain Sea time.
Collapse(?) lobe Ice “Margin:” As just noted, Scarps which mark the margin of the streaming Champlain lobe in the southern and central portions of the Champlain Basin in late T7 and Fort Ann time are generally absent in the northern Champlain Basin, north of the Onset Zone. This absence is taken as indicating the second phase of accelerated streaming, possibly the “collapse (?)” of the Champlain lobe, entailing the unusual and difficult logic of proving a finding by the negative absence of evidence. Other than peculiar grooves in the Onset Zone, no ice margin features mark the ice margin in conjunction with the incursion of the Champlain Sea, again raising the possibility that the Champlain lobe collapsed, whereby it lacked a coherent ice margin and therefore no ice margin features could form. However, both freshwater Lake Vermont and marine Champlain Sea bouldery bottom-set silt-clay deposits suggest calving of the Champlain lobe proceeded northward to and beyond the Quebec border, again in T7 time, first in Fort Ann and then Champlain Sea time. Evidence for subsequent ice presence is found in the Missisquoi Basin, representing a readvance in T8 time, after the opening of the Basin in early Champlain Sea time.
Miscellaneous and Uncertain: In this category are a wide variety of markings, commonly identified on LiDAR imagery, which are believed to be related to ice margins, which are of interest, but their origin and meaning are as yet unknown. This is particularly the case for the Memphremagog Basin, and to some extent for the Champlain Basin, especially in late glacial times. For example, on the floor of the Memphremagog Basin are many features which are recognized on LiDAR and which appear to be related to both ice margins and associated drainage. Whereas these features represent details which do not need to be understood for the regional perspective on deglacial history given here, it is likely that much can be gained from more detailed study of such features.

Ice Margin Types

Different types of ice margins are recognized, having to do with the different natures of ice margins and associated Glacial Dynamics. Again, LiDAR imagery is especially helpful for identifying and delineating such margin types.

Active ice margins: Such margin types are marked by “Ice Marginal Channels,” “Bedrock Grooves,” “Ice Tongue Grooves,” and perhaps parts of the “Shattuck Mountain Pothole Tract,” all of which are believed to have formed beneath or along the margins of basically hard, solid, cold, and active ice, where meltwater was firmly restricted by and along the ice margin.
Stagnant ice margins: These margin types are marked by typical hummocky, kame and kettle topography which is very evident on LiDAR. The larger of such deposits, such as in the Rutland, Williston, and Jeffersonville vicinities tend to have LiDAR linear markings suggestive of progressive recession during the formations of these deposits.
Hybrid margins: Individual T times tend to be marked by both active and stagnant ice margins. Owing to the insulating property of sediment cover stagnant ice margins tend to persist longer than their active ice counterparts, while their active ice partner recedes to the next lower ice margin position, with drainage features graded from the older stagnant ice margin to the next younger active ice margin where meltwater tends to be diverted or impounded. This relationship adds temporal and spatial complexity to deglacial history study. All T times are marked by hybrid margins.
Step-down margins: Most ice margins in Vermont formed in reverse gradient conditions and thus are associated with progressively lower terracing, commonly marked by kame deltas and associated ponded water sediments, in association with a variety of different types of other ice margin features such as Bedrock Grooves and Drainage Lines. Most of the late T3 to T7 margins are step-down types, commonly associated with “Disaggregated” ice margins and more or less open water corridors.
Nourished Ice Margins: Ice margin features generally are discontinuous, having formed only where conditions were favorable depositional environments.
Destabilized, Disaggregated, and Calving Ice margins: Such margins in some cases are marked by specific types of features, as for example Streaks, Scarps, and Ice Tongue Grooves which are specifically interpreted as indicating destabilization. However, other ice margins are inferred as being destabilized, based on the context taken from deglacial history interpretation. For example, calving margins are inferred, based on the interpretation of Ribbed Lacustrine, Headless Delta, and Thickened Bouldery Lacustrine or Marine deposits.
Diachronous Margins: The evidence strongly supports the interpretation that ice margins formed progressively through time. For example, ice margin features are mapped and interpreted as representing early T7 time, later T7 time, and still later T7 time. All ice margins identified as T times are believed to have been diachronous.
Oscillatory versus Readvance margins: This distinction is or can be a difficult, thorny, and contentious issue. Stratigraphic and structural evidence, such as multilayered or deformed sediment such as till and lacustrine or marine deposits, commonly has been interpreted as indicative of a “readvance” of the ice sheet. The evidence here indicates that great care in identifying advances is needed. For example, the readvance evidence identified in the Missisquoi Basin by Cannon (1964) and Wagner (1972) included till deposits interbedded with or overlying deltaic deposits and many other exposures consistent with a readvance. This type of evidence contrasts with till interbedded with other materials. The evidence here indicates that this is common, in conjunction with calving and streaming. However, local exposures of interbedded and in some cases deformed till, which often is interpreted as indicative of a glacial readvance, is found in many places to represent calving or other destabilized ice margin conditions.
Warm versus Cold margins: As indicated above, Ice Marginal Channels have been studied in detail, leading to the interpretation that these formed along generally Cold ice margins with the basal ice of the outer fringe of the ice margin warmed. In general, certain ice margin features, such as Bedrock Grooves, are interpreted as having formed along Cold, hard, and impervious ice (meaning lacking open fractures or crevasses) whereby meltwater is restricted by and along ice margins. In contrast, Warm ice is inferred to have been associated with stagnant ice margins, or along ice lobes where Ice Marginal Channels are not present. Scarps provide direct evidence of meltwater presence at the base of Warm ice.

Vermont Deglacial History

As just stated above, this study was inspired by research reports in Quebec which suggest that ice margins in Vermont might be differentiated and correlated on the basis of their elevation, orientation, and alignment in a “Bath Tub Model.” Numerous ice margin features were identified and mapped, as described above, leading to the recognition that ice margin features are more numerous and substantial at certain isostatically adjusted elevations and can be widely correlated within and between physiographic basins, leading to a deglacial history in eight time steps from T1 through T8, beginning in a Nunatak Phase and continuing in a subsequent Lobate Phase.

The following map gives a sense of T1-T8 times for the entire State. T1 – T3 times were associated with progressive recession in the “Nunatak Phase,” as marked by Ice Marginal Channels. Most but not all nunataks in this phase are depicted by the russet color on this map. Whereas these features are very evident and quite numerous on LiDAR imagery, clearly showing the progressive development of nunataks as the ice sheet levels lowered, they were not correlated and delineated as discrete ice margins. As stated above, Ice Marginal Channels in the Nunatak Phase are interpreted as representing subglacial erosion by drainage of hydrostatically confined and impounded subglacial meltwater across terrain protuberances associated with polythermal ice margin fringes along Cold, active ice margins capable of restricting meltwater. Taken together, Ice Marginal Channels show the progressive lowering of the ice sheet surface in this Phase, gradually leading to the development of ice lobes.

Late T3 to T8 ice margin features represent progressively younger times in the Lobate Phase, marking the lowering of ice sheet levels and recession of ice margins. Ice margin features in this Phase are numerous over a range of elevations suggesting the progressive and continuous lowering and recession of the ice sheet in a manner which again can be likened to “multiple rings in a slowly draining bathtub.” However, ice margin features at T times tend to be larger and more numerous than features at intervening elevations, and are found in diverse physiographic settings across the State. T times are believed to represent stillstands only in the sense that time was required for the formation of ice margin features representing these times, and do not necessarily represent or require climatic variations. Instead, as discussed herein, T times are believed to represent pauses in ice sheet recession associated with hypsometric or other Glacial Dynamic control.

Owing to the fact that ice sheet recession in Vermont was almost entirely in a reverse gradient setting, ice margin features in the Lobate Phase tend to be closely associated with and as well marked by ice margin deposits in a step-down recessional type pattern. Again, the pattern of step-down recession of Hybrid margins represents a complex overlapping spatial and temporal recessional “dance” between active and stagnant ice margins. Owing to physiographic differences in the terrain within and between basins, this deglacial history tends to be extraordinarily complex both to decipher and describe.

Late T3 to T8 times are marked by diverse types of ice margin features in the “Lobate Phase,” with distinct lobes in the Champlain Basin, the Memphremagog Basin, and the Connecticut Basin, with a multitude of sub-, sub-sub-, etc., lobes associated with physiography as recession progressed. The Champlain lobe was more substantial and longer lasting owing to its lower elevation floor and more direct, less obstructed opening to the parent ice sheet to the north. The Memphremagog Basin became deglaciated earlier than the Champlain Basin owing to the elevation of its floor being significantly higher than the Champlain Basin floor. The Connecticut lobe was nourished primarily by ice from the Champlain and Memphremagog Basins, with ice flow across cols on basin ridges and saddle divides, with de minimus flow from the northern Connecticut Basin headwaters owing to the relatively high and restricted physiography of the headlands of this basin.

Ice sheet lowering below these cols and divides resulted in “Disconnections” of ice masses in the Lower, Middle, and Upper Connecticut Basins, leading to three en masse stagnation events, as marked by Scabby Terrain, in T1, T2, and T4 times, as identified on the above map. Also, as noted above, Ice Marginal Channels, which formed along active ice margins, are absent in the Lower and Middle Connecticut Basins, which is a significant evidentiary clue consistent with the lack of active ice in these basins. The first Disconnection event in T1 time was in the Lower Connecticut Basin, and the second in T2 time in the Middle Connecticut Basin. The third Disconnection occurred in the Upper Connecticut Basin after the late T3/ early T4 time White Mountain Morainic System (WMMS) readvance.

Late T3 and early T4 time margins mark the beginning of the Lobate Phase in the Memphremagog and Champlain (Lamoille) Basins, extending across basin divides and cols into the Upper Connecticut Basin where these features mark a well-defined lobe extending southward to the Bradford vicinity, with an outwash plain graded into a delta associated with Lake Hitchcock. Ice marginal features associated with this lobe are traced into New Hampshire where these margins are correlated with the White Mountain Morainic System (WMMS), which has been previously reported by Thompson et al as a significant readvance associated with the Older Dryas cooling event and dated at approximately 13,800-14,000 years BP. Based on the reported orientation of isobases, the elevation of this margin compares with the Frontier moraine in Quebec.

Subsequent ice sheet margin recession and surface lowering in T4 time, following the readvance, led to Disconnection of this ice mass in the Upper Connecticut Basin by loss of its ice supply from the Memphremagog and Lamoille Basins, leading to the third en masse stagnation event, again as marked by Scabby Terrain in the Upper Connecticut Basin.

The findings here of Disconnections and en masse stagnation in the Connecticut Basin relate to a long-standing debate in the literature as to the manner of ice recession in this Basin, whether by massive stagnation of large portions of the ice sheet, or by progressive recession of discrete, active ice margins. The findings here support both theories in the debate, albeit in a unique way.

The Late T3 and early T4 margins mapped on VCGI, marked by closely spaced Hybrid active Ice Marginal Channels and associated stagnant ice margin deposits provide a distinctive visual “Signature” pattern, which can be readily traced widely across Vermont, semi-independently of elevation. In the Memphremagog Basin, this Late T3/early T4 lobe margin extends northeastward on the eastern side of the Memphremagog lobe, to the Quebec border where T4 Ice Marginal Channels are correlated with the Dixville moraine in Quebec on the basis of elevation, orientation, alignment, and comparable physiographic position at the foot of the same upland, around the corner from each other.

The western margin of this Late T3/T4 Memphremagog lobe extends into the upper Missisquoi Basin on the eastern side of the Green Mountain front range, thence wraps northward around the eastern and northern flanks of the front range in the Jay Peak area, continuing westward into the northern portion of the Champlain basin, marking the eastern margin of the Champlain lobe. This late T3/T4 Champlain lobe margin extends southward from the Missisquoi Basin along the western flank of the Green Mountain front range and the foothills, with tributary lobes in the major Champlain subbasins, including the Missisquoi, Lamoille, Winooski, and Otter Creek basins, and as well with smaller sub-lobes in the many smaller tributary basins. The map-view pattern of this margin delineates a long, irregular, highly convoluted and convex Late T3/T4 lobe margin in the Champlain Basin associated with physiography, extending southward to a bifurcated southern lobe tip, with one tip extending into the Vermont Valley (Otter Creek Basin) in the Bennington and Rutland areas, and the other on the floor of the main Basin on the western side of the Taconics, extending southward into New York. Previous mapping by DeSimone and LaFleur in southwestern Vermont and east-central New York identified ice margins which almost exactly correspond with late T3/T4 ice margins as mapped here (prior to their mapping being seen).

As noted, the late T3 and T4 margin is correlated with the WMMS as a readvance. In the Winooski basin this margin correlates with readvance evidence previously reported by Larsen, Wright, and others, supporting a correlation with the WMMS readvance, albeit along a very convoluted margin. Soil boring and well drilling records likewise support and are consistent with a readvance of this late T3 and T4 margin in the Vermont Valley near Bennington and Rutland, but this evidence requires further study. DeSimone and LaFleur specifically state that they found no evidence of a readvance of their ice margins, which again correlate with the margins identified here as T3 and T4 time.

Returning to the deglacial history story in the Memphremagog Basin, the major drainage sub-basins of the Memphremagog Basin are physiographically long, narrow, and finger-like, extending into headwater areas of the Basin, marked by over-deepened, enclosed bedrock basins near basin divides, similar to the Finger Lakes region of New York. As with the Finger Lakes with “Valley Heads” moraines at basin divides, the late T3/T4 ice margins in the Memphremagog Basin mark finger-like ice lobes extending to and slightly across basin divides, again similar to the Valley Heads pattern. It is likely that the Memphremagog over-deepened basins represent long term erosion, possibly by multiple glaciations, as suggested in the literature for the Finger Lakes.

Ice margins in the Memphremagog Basin mark a step-down type recession of Hybrid margins from late T3/T4 to T6. The T6 margin represents the last ice presence of the ice sheet in the Memphremagog basin, marked by a Disaggregated ice margin with large, detached ice blocks in conjunction with proglacial Lake Memphremagog. Numerous deep kettle holes which were occupied by large, long-lasting ice blocks, along this T6 margin are mapped, interspersed with kame deltas, marking a “Disaggregated” type ice margin. The T6 margin is correlated with the Sutton moraine in Quebec.

This same step-down pattern was also identified in the Champlain Basin in T3 through T7 time. As step-down recession of the Champlain lobe continued, smaller proglacial water bodies progressively coalesced to form larger, lower water bodies associated with the opening of lower outlets. This coalescence and growth in size of proglacial water bodies and their lowering, especially when the lowering was sudden and substantial for larger water bodies, was an important Glacial Dynamic which destabilized the Champlain lobe. In particular, the Coveville to Fort Ann and then Fort Ann to Champlain Sea level lowerings both involved large water bodies along the ice margin, with sudden, substantial water level changes which caused destabilization of the Champlain lobe.

The evidence for the deglacial history of the Champlain Basin was originally interpreted here, prior to the Addendum study, as indicating that the Champlain lobe persisted as a long convex lobe, extending far to the south in the Champlain Basin, including a readvance in T8 time. This interpretation of a long, late active ice lobe was based on consideration of substantial evidence, importantly including the correlation of readvances previously reported by Cannon (1964) and Wagner (1972) in the Missisquoi Basin, by Wright in the Charlotte area, and by Connally in the West Bridport area. Connally’s Bridport readvance has subsequently been questioned, but photographs in his report clearly show structural deformation of sediment indicating significant of movement of the ice margin.

Thanks to discussions with colleagues, this long convex, late lobe of active ice was re-examined in the Addendum study, finding substantial, new LiDAR evidence, which was confirmed by focused field study, showing that the Charlotte and West Bridport readvance features instead represent deformation along the T7 lateral shear margin of the streaming Champlain lobe. Further, the Addendum finds that the Champlain lobe stood as a long convex lobe in T7 time but became destabilized by multiple Glacial Dynamics, with calving, streaming, and perhaps collapse(?) of the Champlain lobe, followed by a readvance restricted to the northern Champlain basin in T8 time. The original findings for much of the earlier, pre-T7 history remained largely unchanged by this Addendum. Both the original and the Addendum findings are presented in order to preserve and present the considerations, deliberations, and arguments on this matter.

Stagnant ice deposits and associated kame delta features show that in T6 time in the Champlain Basin the progressive step-down recession and coalescence of proglacial water bodies led to regional Coveville Lake Vermont. Coveville was restricted to a narrow, open water corridor along a “Disaggregated” type ice margin, controlled by its outlet in New York. The head of this corridor progressively extended northward from New York into Vermont in T6 time, along the western flank of the Taconic Mountains, around the nose of the Taconics, into the mouth of the Vermont Valley just north of Rutland, and further northward along the western margin of the foothills, close to the Basin floor, eventually reaching the vicinity of the mouth of the Winooski Basin. In late T6 and early T7 time, the Coveville level fell to the Fort Ann level. Again, this lowering was sudden and substantial, causing a major Glacial Dynamic which resulted in destabilization of the Champlain lobe. Also adding to this destabilization at about this time was the outbreak of proglacial Lake Iroquois and the draining of proglacial Lake Mansfield into the Champlain Basin, both of which likely added substantial volumes of warmed water which contributed to the destabilization. Like Coveville, Fort Ann waters occupied a long narrow, more or less open water corridor along a Disaggregated ice margin, the head of which rapidly progressed northward along the eastern margin of the Champlain lobe, extending to and beyond the Quebec border in T7 time.

Such long corridors along ice margins were significant in that substantial and sudden changes of the water levels for these corridors, as for example Coveville to Fort Ann in late T6/early T7 time, and Fort Ann to Champlain Sea levels in later T7 time, resulted in significant Glacial Dynamic changes for the Champlain lobe, including its destabilization.

Evidence of destabilization associated with the Coveville to Fort Ann lowering in late T6 and early T7 time is provided by Streaks which were identified on LiDAR in the Addendum study. As noted above, Streaks are numerous and widespread markings on LiDAR imagery along the Champlain lobe eastern margin in Vermont at the late T6 and early T7 levels and times, showing that that this destabilization was a “jolt” to the entire Champlain lobe. This destabilization also resulted in the development of calving of ice masses occupying the deeper floors of re-entrant basins in the “Middlebury Bench.” This Bench was an important and significant raised portion of the Champlain Basin floor which at that time provided buttressing support for the Champlain lobe. Owing to scale limitations, these calving ice masses in re-entrant basins can not be shown on the above map.

The evidence reported here regarding the deglacial history associated with this time and associated destabilization in the vicinity of the Winooski Basin mouth is complex, and to be understood requires an understanding and familiarity with the local geography and physiography. The following side-bar gives a sense of this complexity:

Stagnant ice deposits and multiple kame deltas are mapped, demarcating the late T6 and early T7 ice margin and Coveville strandline, as for example at Benson Landing, Castleton, Proctor (near Rutland at the mouth of the Vermont Valley), Pittsford, Brandon, East Middlebury, Bristol, and South Hinesburg. As reported previously by others, including Wagner (1972), the Bristol delta at the Coveville level was formed by outwash from an ice margin here identified as T6, followed by ice margin recession leading to the formation of the Coveville delta at South Hinesburg formed by drainage from Lake Mansfield 1, a successor to Lake Winooski. Lake Mansfield required an ice dam across the mouth of the Winooski Basin. Again, the evidence indicates the progressive, diachronous nature of ice margin recession, in this case in T6 and Coveville time, with the progressive northward recessional opening of the Disaggregated type ice margin and Coveville corridor. This northward penetration of Coveville waters along the eastern margin of the Champlain lobe resulted in the shape of the Champlain lobe becoming a progressively longer, more convex ice lobe.

Subsequent recession of the late T6 and early T7 ice margin resulted in drainage through the ice dam, leading to the lowering of Lake Mansfield from its level 1 to level 2, with a new control for level 2 by a spillway south of Oak Hill. Evidence indicates that weakening and opening of the Winooski Basin ice dam for Lake Mansfield 2 took place by a basal ice tunnel discharge and by subglacial seepage, with both the dispersed and channeled drainage leading to a small water body controlled by the Oak Hill spillway at the Lake Mansfield 2 level. Drainage from this spillway grades into deltaic deposits (along Sucker Brook) at both the Coveville and Fort Ann levels.

Further, Streaks are identified along the northern margin of Oak Hill, extending into the mouth of the Winooski Basin, both at the Coveville and Fort Ann levels. These Streaks represent destabilization as a Glacial Dynamic associated with the Coveville to Fort Ann lowering, and as well the opening of the Winooski Basin to Lake Vermont at this time.

The opening of the Winooski basin in late T6 and early T7 time, associated with the lowering of Coveville to Fort Ann waters is marked by Ribbed Lacustrine and Headless delta deposits in the Winooski Basin, both near Richmond and in the Waterbury area, marking a long, narrow destahilized Winooski lobe. Whether or not Coveville waters actually penetrated the Winooski Basin is uncertain, the difference between Lake Mansfield 2 and Coveville being very small.

As an aside, notwithstanding the identification of many features as just noted as representing Coveville Lake Vermont, the evidence for a regional Coveville stage is here regarded as questionable. A close examination of Chapman’s Coveville strandline features shows that these do not mark a single strandline. It is possible that so-called Coveville features instead are local proglacial water bodies which have been mistakenly correlated as a regional water body by Chapman and subsequent researchers, including myself in my 1972 report. The validity of the Coveville strandline is here assumed.

To step back, readvance evidence reported previously by Larsen, Wright, and others in an interior lowland area east of the Green Mountain front range in the Winooski and Lamoille Basins is here correlated with the late T3 and T4 margins, and with the WMMS readvance, as noted above. The late T3 and T4 step-down margins show the associated presence of local proglacial water bodies leading to the development of Lake Winooski, as a large waterbody in this upland interior lowland in the Winooski and Lamoille Basins, again east of the Green Mountain front range. Lake Winooski drained southward across a divide with a spillway near Williamstown, with drainage into the Connecticut Basin. The evidence indicates rapid penetration of Lake Winooski waters around the perimeter of the Late T3 and T4 ice mass which then occupied the footprint of the Lake Winooski Basin along a Disaggregated ice margin. On the eastern margin of Lake Winooski Basin ice mass the evidence indicates the rapid step-down of a hybrid margin (in the Style of “Everything, Everywhere, All at Once, and Continuing” ) leading to the progressive development of Lake Winooski, but again in a narrow corridor. On the western margin of the Basin, the penetration of Lake Winooski, again along a narrow corridor, led to a Disconnection of ice masses remaining in deep drainage basins on the eastern flank of the Green Mountain front range. Isolated ice masses in these basins may have initially been active but subsequently stagnated en masse as marked by Scabby Terrain tracts.

In time, melting of the ice mass in the central portion of the Lake Winooski footprint led to its becoming an open water body dammed by ice lobes in the lower Winooski and Lamoille Basins. Further step-down recession of these ice dams led to the opening of drainage outlets for two stages of Lake Mansfield, again as reported by others and described above, with drainage into the into the Champlain Basin, initially into Coveville Lake Vermont, as marked by a major delta at South Hinesburg in T6 tine, while the Winooski ice lobe served as a dam for Lake Mansfield 1. Further ice recession in late T6 and early T7 time led to the drainage for Lake Mansfield 2, with evidence indicating the aforementioned subglacial drainage at the base of the Winooski lobe, by both dispersed leakage beneath the ice and as well by ice tunnel drainage, leading to a spillway near Williston, immediately south of Oak Hill, with early spillway drainage marked by a Coveville delta, followed by a Fort Ann delta. Again, the Coveville to Fort Ann lowering was a destabilizing event associated with the opening of the Winooski Basin, as marked by Streaks on the northern flank of Oak Hill and Ribbed Lacustrine deposits near Richmond and Waterbury, associated with the termination of Lake Mansfield 2.

As stated above, the destabilization caused by the Coveville to Fort Ann lowering in late T6 and early T7 time also resulted in calving of narrow ice masses occupying the re-entrant basins in the Middlebury Bench (specifically the LaPlatte, Lee, Little Otter Creek, New Haven, and Otter Creek basins), while the intervening uplands of the Middlebury Bench remained ice covered, providing buttressing support for the Champlain lobe. This calving is marked by “Ribbed Lacustrine, Headless delta, and Thickened Bouldery Lacustrine deposits. In T7 time, these calving margins receded downgradient, generally in a westerly direction, as the ice cover on the intervening uplands of the Middlebury Bench thinned.

Returning to the deglacial history of the Champlain lobe, as noted this lobe was the largest and longest lasting of Vermont lobes, owing to its basin floor being the lowest in Vermont, with a direct, open connection to the parent Laurentide ice sheet to the north. VCGI mapping shows the traces of the ice margins marking the progressive step-down recession of the Champlain lobe margins from the late T3/T4 levels and times as just described, into T7 time. By T6 time the Champlain lobe had become quite thin and largely confined to the Basin floor with the ice margin along the foothills. For example, ice margins for the Missisquoi Basin sub-lobe in T6 time show an intricate pattern of sub-sub-lobes closely associated with the local physiography, showing a complicated ice flow pattern related to ice supply from the Laurentide ice sheet to the north via multiple pathways. This T6 margin is correlated with the Sutton moraine in Quebec.

By T7 time, meltwater associated with the Champlain lobe had become substantial, as marked by diverse features, including along its margin and at the ice sheet base, which by then was a fully Warm type, as shown by the absence of Ice Marginal Channels (which, as noted, are associated with Cold ice). The thinning and warming of the Champlain lobe led to its increased fragility.

In addition, related to and adding to this fragility was the physiography and geologic nature of bedrock substrate on the Basin floor. The floor of the basin is not flat, but as noted above consists of a raised portion referred to as the “Middlebury Bench” which is underlain by dolostone and quartzite which form the Bench and represent hard, durable, less erodible materials. To the west of the Bench is the “Trough,” a lower flatter portion of the floor separated from the Bench by a pronounced riser in the terrain. The Trough is underlain predominantly by soft, readily erodible shales. Further, whereas the Trough tends to be a simple chute shape: 1) in plan view, the northern extent of the Middlebury Bench in the Shelburne area, just south of Burlington, serves as a narrowing constriction of the Trough floor, which widens to the north, and 2) the floor of the Trough includes a narrow, over-deepened bathymetric trench on the floor of Lake Champlain at elevations below present-day sea level. These physiographic differences had profound impact on Glacial Dynamics and the deglacial history of the Champlain lobe related to an Onset Zone for ice streaming that took place when the margin of the Champlain lobe receded from the Middlebury Bench, resulting in the loss of buttressing support, triggering streaming.

The position of the ice margin for the Champlain lobe in late T6 time stood along the margin of the foothills, along the eastern margin of the Middlebury Bench, with Coveville Lake Vermont occupying a narrow corridor along the lobe margin. The lowering of Lake Vermont from the Coveville to Fort Ann levels in late T6 and early T7 time was caused by an “externality,” and was both sudden and substantial, resulting in destabilization of the Champlain lobe as a major Glacial Dynamic, again as marked by LiDAR features referred to as Streaks, at many locations along the eastern margin of the Champlain lobe at late T6/early T time. These features are readily identifiable on LiDAR as Signature-type ice margin markers which are mapped extensively in the Champlain Basin, showing that, as noted above, the entire Champlain lobe in Vermont was destabilized by this event.

Again, the evidence indicates that Coveville Lake Vermont occupied a narrow, “Disaggregated” ice margin corridor, the head of which extended northward to near Burlington at the time of lowering to the Fort Ann level, in late T6 and early T7 time. This Fort Ann corridor then subsequently continued to extend rapidly northward to and beyond the Quebec border, still in T7 time. This recession is documented by substantial evidence, as for example:

Eskers in the Essex area, just north of Burlington, with drainage graded to deltaic deposits at the Fort Ann level.
A Drainage Line in The Creek associated with proglacial drainage along the T7 margin in the Lamoille Basin, leading to Fort Ann deltaic deposits at Underhill, just northeast of Essex.
Step-down ice margin and associated meltwater features near Bakersfield, just north of the Lamoille leading to a kame deltaic deposit with ice margin markings at the T7 level and Fort Ann level.
A substantial Ribbed Lacustrine deposit in the headwaters of the Black Basin, a long, narrow tributary sub-Basin of the Missisquoi Basin, marking the destabilization of the ice margin associated with the Coveville to Fort Ann transition.
Headless deltaic deposits further north in the Black Basin again at the Fort Ann level.
Numerous kame deltas in the headwaters of tributary basins on the south side of the Missisquoi Basin at the Fort Ann level.

The evidence further indicates that the destabilization of the Champlain lobe caused by the Coveville to Fort Ann lowering was associated with a transformation of its eastern margin, again along the Disaggregated corridor, from a lateral toward a frontal margin, and that the Champlain lobe became fully warmed in T7 time, with the presence of substantial meltwater on the ice surface, along its margins, and at its base. Such warming and meltwater presence contributed to Champlain lobe instability.

The map below shows the deglacial history, specifically regarding the Glacial Dynamic events in the Champlain Basin:

Again, the destabilization caused by the Coveville to Fort Ann lowering triggered calving of ice masses in narrow re-entrant basins in the Middlebury Bench, as marked by features referred to as “Ribbed Lacustrine,” “Headless Delta,” and “Thickened Bouldery Lacustrine” deposits. Progressive downgradient recession in T7 time of these calving margins resulted in the Champlain lobe clearing the Middlebury Bench, resulting in the loss of buttressing support, as a Glacial Dynamic, which triggered a first phase of streaming as marked by distinctive LiDAR identifiable features termed “Scarps.” Streaming was also favored by the relatively erodible shale substrate of the Trough floor and by the relatively smooth chute-like shape of the Trough.

Scarps, which are numerous, generally along the eastern margin of the Trough, are restricted to the southern and central Champlain Basin, south of the narrow restriction of the Basin associated with the Middlebury Bench in the Shelburne area. This restriction is interpreted as an “Onset Zone” for this T7 time ice streaming. Both LiDAR imagery and as well field evidence indicates that Scarps represent Lateral Shear Moraines of the streaming Champlain lobe. Mega-Scale Lineations and Transverse Morainic Ridges are also identified and mapped, as identified on the above map, further documenting this early phase of ice streaming and calving of the southern and central portions of the Champlain lobe, south of the Onset Zone.

A second phase of streaming and calving in later T7 time is indicated by the absence of Scarps in the northern Champlain Basin, suggesting that by this time the Champlain lobe streaming accelerated and lost its integrity as a coherent active ice mass, and as such may represent its collapse(?). Erosional LiDAR marks on the Onset Zone indicate that this second phase of accelerated streaming and possible collapse(?) was triggered by the lowering of Lake Vermont from the Fort Ann level to the Champlain Sea level, again representing a Glacial Dynamic. This lowering was as well both sudden and substantial, indicating that the narrow eastern Disaggregated Fort Ann corridor had extended northward into Quebec at this T7 time to allow for the draining and lowering of Fort Ann Lake Vermont, and the associated incursion of the Champlain Sea. Again, this northward progression of the Fort Ann corridor is documented by numerous stagnant ice deposits in close association with Fort Ann kame deltas. In addition, the opening of the Onset Zone is likened to a dam break which also may have been a Glacial Dynamic contributing to this second phase of accelerated streaming and possible collapse(?).

The incursion of the Champlain Sea into Vermont has been dated as previously reported by Cronin et al as about 13,000 years BP. Thus, almost the entire deglacial history of Vermont took place rapidly over a short interval of time, on the order of 800 -1000 years , at a late glacial time from the WMMS readvance , dated as about 13, 800- 14,000 years BP, in late T3 and T4 time through T7 time with the Champlain Sea incursion. The recession of the possible collapse(?) of the Champlain lobe took place in likely much less than 800 -1000 years, at a rate which is comparable to calving and collapse recession rates reported in the literature elsewhere.

This second, accelerated phase of streaming resulted in the rapid recession of the Champlain lobe from Vermont, accompanied by calving as marked by both bouldery lacustrine and marine deposits on the Basin floor extending northward to the Quebec border. Again, this calving evidence for both freshwater and marine bottom sediments supports the hypothesis of a possible collapse(?) of the Champlain lobe.

Further, it is suggested that this late T7 vestige of the Champlain lobe may have been associated with a similar recession of the Ontario and St. Lawrence lobes, which together represented a tri-lobe remnant of a portion of the Laurentide ice sheet which previously had extended southward into a large region of eastern North America. The map below gives a conceptual sense of the envisioned spatial relationships:

This “model” is intended to suggest the shape of the T7 Champlain lobe prior to its collapse (?) in late T7 and Fort Ann time, when the lobe extended southward in the basin, in concert with the St. Lawrence lobe and the Ontario lobe, as marked by the orange dashed line. The positioning of the ice lobe margin on this map is conceptual only, and not intended to mark exact positions. The larger pale blue arrows are intended to represent suggested ice flow directions associated with these lobes. Additional arrows are shown on the northern Appalachians to suggest the presence of an isolated ice mass as has been reported by Quebec researchers. The smaller, darker arrows correspond with the overdeepened basins in the Finger Lakes, the headwaters of the Memphremagog basin, and the floor of the Champlain Basin. And the russet colored area is added to depict the Disconnected ice mass in the Upper Connecticut Basin. It is not suggested that these elements were correlated in time, but are shown as a reminder of the associated Glacial Dynamics.

Finally, a readvance is identified in northern Vermont, restricted to the Missisquoi Basin in T8 and Champlain Sea time. This readvance is documented by Cannon (1964), Wagner (1972), and VCGI mapping in this present study. This readvance may correlate with a recently reported Champlain Sea readvance in Quebec by Parent (2023), said to have briefly re-established Fort Ann Lake Vermont in the Champlain Basin.

Ironically, whereas Vermont has long been thought of as a gap in the record, with “normal” moraines being absent, it now turns out, largely thanks to LiDAR evidence in this Addendum study, that this absence resulted from important Glacial Dynamics associated with streaming, calving, and possible collapse(?). Thus, for me these findings represent a kind of “Rosetta Key-Stone” for the center of the tri-lobed Laurentide ice sheet “arch” shared with New York and Quebec. The findings of this report bear on both deglacial history as a matter of academic interest but as well also on present-day issues of concern regarding global warming and the stability of modern ice sheets related to Glacial Dynamics.

Interestingly, both the deglacial history and Glacial Dynamics for Vermont as just described fit with and support the larger regional picture that has been developed by recent research by others, specifically that:

The entire Vermont deglacial history as reported here occurred rapidly at a late and brief time.
This recessional history was associated with multiple Glacial Dynamic elements, including a) the progressive warming of the ice sheet, b) the production of meltwater in a reverse gradient setting and its eventual penetration to the base of the ice sheet, c) the progressive coalescence of standing water bodies, in narrow corridors penetrating the ice margin, whereby sudden and substantial changes in water levels led to destabilization of the ice sheet, d) the contribution of erodible substrate at the base of the ice sheet, e) Disconnections of substantial masses of ice, including both locally still active ice masses and as well en mass stagnation , and f) perhaps most fundamentally, the role of physiography in this Dynamic.
The recession of the Champlain lobe at a late glacial time fits with the recession of the Ontario lobe and St Lawrence lobe as a tri-lobed projection of the Laurentide ice sheet in Quebec, New York, and Vermont.
Late glacial streaming, was two phased, including an accelerated second phase, possibly representing a collapse(?). which likewise has been reported by others in the Ontario and St Lawrence basins.
The over-deepened terrain associated with the Onset Zone for Champlain lobe ice streaming fits with similar evidence reported in the Ontario basin and the Finger Lakes area of New York, including the suggestion that this over-deepening represents long term geomorphic evolution of the physiography, including multiple glaciations.

The findings reported here for the deglacial history of Vermont, along with the following Glacial Dynamics, for me represent what I think of, at the risk of sounding melodramatic, as a “voyage of discovery,” with many new “Epiphanies” along the way, leading me to a new paradigmic understanding of this history. Remarkably, this “voyage,” on its own and independently, led to a perspective which is astoundingly similar to the regional picture of these events and times as has been emerging in recent reports in the literature.

Report Organization

As just stated, this report is organized into two parts. The great bulk of this report, as the first part, was completed in January 2026. This first part presents background information about this study and the evidence leading to the identification of a long, convex Champlain ice lobe in late deglacial times (identified as T7 and T8 times), continuing into Champlain Sea time. As indicated above, conversations with David Franzi and Etienne Brouard led to re-examination of this issue, including detailed study of LiDAR imagery which identified new, never before recognized, and for me quite remarkable features, as described above, attributed to calving and streaming of the Champlain lobe, triggered by multiple, different Glacial Dynamics with different associated Styles. This LiDAR study was then followed by a focused field examination, which confirmed the LiDAR findings (albeit with some of the LiDAR features not evident in the field). These new findings are presented in an Addendum which primarily focused on T7 and T8 times in the Champlain Basin. Much of the pre-addendum findings for T1 -T6 times have not been changed by the Addendum. Whereas the Addendum study led to important new insights, most of the pre-Addendum report remains valid, and is retained. The Addendum serves to flesh out and document the final chapter of this story.