Abstract

The late Pleistocene deglacial history of the Laurentide ice sheet in Vermont, specifically meaning 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 margins in Vermont as deglaciation proceeded, not inferred ice margin positions as taken from evidence provided by proglacial water bodies as have  been suggested by Chapman and other previous researchers. This reflects the absence of end moraines in Vermont which can be readily identified, mapped,  differentiated,  traced, and correlated across the State.

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. 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 pathways for a significant portion of the ice sheet.

This report was inspired by research reports in Quebec which have identified moraines that can be correlated on the basis of their elevation, orientation, and alignment. This suggests a physiographic “Bath Tub Model” might be used to correlate other types of ice margin features which are quite numerous in Vermont,  such as stagnant ice deposits, on the basis of their elevation differences, as a general guide.  

In this study it was assumed, using the “Bath Tub Model,” that frontal margin deposits (as opposed to deposits formed at lateral margins) which formed at the same time along thinning ice margins, oriented perpendicular to ice sheet flow lines, tend to occur at similar elevations. 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, and in a “reverse gradient” setting, much like a “Bath Tub,” with the ice margin in most places influenced or controlled by both physiography and standing water  associated with proglacial water bodies, all of which is discussed in detail in the text. Multiple arguments in defense  and support of the usage of elevation, again as a guide, in such a Model for reliably developing Vermont deglacial history are presented. Both the collective weight of the evidence and as well independent evidence support the validity of the “Bath Tub Model,” and thus the deglacial history so determined.

Quebec reports also suggest that environmental conditions along the ice sheet margins varied and that these conditions represent different “Styles,” and that these conditions were associated with  “Glacial Dynamics” whereby the physical nature and condition of the ice sheet was influenced by ice sheet boundary conditions. This report thus explores Vermont’s deglacial history by the usage of a “Bath Tub Model,” including identification of ice margins, “Styles” and “Glacial Dynamics.”

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 recent field work, and  pertinent previously published reports in the 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 varying Styles and Glacial Dynamics, including calving and other types of ice margins. A section of this report identifies and describes the ice margin features identified and used in this study. However, as the study progressed, the growing understanding of deglacial history led to the recognition of other types of features which formed along ice margins. Some of these features could only be understood in the context of glacial history. In as much as these are best presented in this  context, such features are identified, mapped, and added to the ice margin feature catalog in later sections of the report as appropriate.

The following is a summary of ice margin features:

• Stagnant Ice Margin Deposits: These  deposits,  as conventionally understood, include a wide array of features  formed in conjunction with stagnant ice, such as kame and kettle deposits and eskers. As discussed below, kame deltas likewise are ice margin features even though they tend to be regarded as  standing water features.

In VCGI mapping the State Surficial Geology Map was frequently consulted and used for the identification of stagnant ice deposits.  However, LiDAR imagery helped to identify and map numerous additional such deposits, with better definition of the topographic expression and  boundaries of such deposits.

Whereas eskers were recognized in the 1960s and 1970s, generally speaking they were then few in number. LiDAR imagery has led to the identification of many eskers, in some places being commonly found in a multitude of small neighboring physiographic basins occupied by local ice sheet sub-lobes. Such eskers identified on LiDAR imagery  are extremely numerous, perhaps in the hundreds, in many places identified as  very minute, subtle traces.  One prominent and important “esker,” previously identified and reported in the Missisquoi Basin, has been shown to possibly instead represent an end moraine, illustrating the need for great care in such study.

Stagnant ice deposits generally are associated with “hybrid  margins” which are a combination of stagnant and active ice margins. Owing to the insulating property of sediment cover on stagnant ice, stagnant ice margins commonly were long lasting, remaining in place  while associated active ice margins receded t0 a younger, lower level. Drainage evidence  commonly shows   meltwaters from buried ice extending downgradient to the next younger active ice margin.  As is well known, active ice is capable of blocking and impounding meltwaters whereas stagnant ice allows for the drainage of meltwaters along crevasses.  As discussed, both active  and stagnant ice margins are identified. “Hybrid” ice margins are  identified and mapped as pairs of active and stagnant ice margins formed at the same time, with the upper part of the pair represented by the  active ice margin features and the lower part its associated stagnant ice margin partner. This overlapping temporal and spatial relationship is referred to as  Style described  as “Everything, Everywhere, All at Once and Continuing.”All T time ice margins as mapped here are believed to be hybrid margins.

Hybrid margins in the Memphremagog Basin mark  a style of ice margin recession referred to as “Everything, Everywhere, All at Once , and Continuing” to underscore the overlapping temporal and spatial “dance” of active and stagnant ice margins during recession. In the Champlain Basin, hybrid margins are important  elements associated with deglacial history, as for example  the development of open water “corridors” and calving ice margins, and as such their recognition and understanding is important and necessary for the delineation of deglacial history which was locally quite complex.

Whereas  recognition and mapping of both stagnant and active ice margins is an important element for understanding deglacial history, many of the ice margin features  identified here take on special importance owing to the role of meltwater associated with their formation. This study suggests that meltwater  and its drainage, both within and along ice margins is an important element which by itself deserves study. To be clear, this suggestion is intended to indicate that it likely would be possible, via future research,  to  identify  and delineate “equipotentails” for both confined and unconfined meltwater  hydraulic heads, within, beneath, and along the ice margins, at least as conceptual models, similar to conventional models associated with hydrogeologic surface and groundwater studies in hydogeology. To some extent, this present work begins to lay the groundwork for such studies.

An important distinction is made between stagnant ice deposits formed along lateral versus frontal ice margins, with ice sheet flow lines oriented more or less parallel to the former and perpendicular to the latter. This distinction is important for the usage of the Bath Tub Model, because frontal  but not lateral margins are believed to provide markers for identifying ice margins of approximately the same age, and likewise conversely for differentiation of features of different ages.

• “Bedrock Grooves:” Bedrock Grooves are formed by fluvial meltwater erosion along active ice margins, typically as deep, incised, channel-like grooves, 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, and tend to form in conjunction with other deposits such as kame deltas  which together represent “Drainage Lines (see below).” Bedrock Grooves are especially common along the western margin of the Green Mountains, along the receding eastern  margin of the Champlain lobe, especially north of the Lamoille Basin, but as well at scattered locations to the south, including drainage from the upland interior associated with proglacial Lakes Winooski and Mansfield as identified and mapped by others.
• Kame Deltas: Whereas kame deltas, also called ice contact deltas,  have long been recognized, in past mapping in the 1960s and 1970s  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. Whereas kame deltas generally are regarded as marking the levels of standing, proglacial water bodies, these features are so intimately and closely related to ice margins as to be regarded as ice margin features.

In general, ice margin recession 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. This step-down pattern shows the progressive coalescence of proglacial water bodies into lower and increasingly larger water bodies. This step-down coalescence is a very significant element pertaining to Vermont deglacial history   because larger regional water bodies had the capability of substantially controlling and altering the stability of the ice margin, which was particularly important for  the Champlain lobe, as part of a significant Glacial Dynamic. As discussed herein such coalescence is likewise believed to be significant for modern day global warming concerns. Kame deltas are a major part of this step-down recessional history. Drainage features associated with both active and stagnant ice margins commonly show meltwater  drainage in a downgradient direction toward and into standing proglacial water bodies as marked by kame deltas.

In addition,  LiDAR imagery for large kame deltas on the floors of major  basins, specifically the Missisquoi, Lamoille, and Otter Creek Basins,  shows markings which represent the progressive recession associated with crevasse fracture and drainage patterns  on  ice lobe tips at different times.

• “Disaggregated Ice Margin Water Corridors:”  As indicated in the preceding,    step-down recessional ice margins of the receding ice sheet tended to be penetrated by standing water, forming  more or less open water corridors  marked by a close association with  stagnant ice deposits, commonly including kame deltas. This reflects the fact that water in liquid form is a thermodynamically  effective agent for melting ice  As reported in the literature, such corridors  are associated with “Disaggregated” ice margins, highly fragmented ice margins penetrated by  standing water. Whereas in this report such margins are not specifically and formally identified as ice margin features per se, they are so common and important as to merit identification as such features. Such Corridors are marked in many places, most commonly by stagnant ice deposits and kame deltas, in close association with fine-grained silt-clay fresh water or marine ponded water bottom sediments. For example:

• • A corridor is identified in the Memphremagog Basin in association with proglacial Lake Memphremagog in conjunction with the T6 ice margin. The literature regarding Disaggregated ice margins underscores the highly fragmented nature of such margins, with meltwater, usually but not necessarily associated with standing water, penetrating a “chicklet-like maze  of ice blocks. In the Memphremagog Basin such blocks tended to include very large blocks of ice, now identifiable as kettle holes with unusually deep present-day ponds.

    Also in the Memphremagog Basin, many of the larger present day lakes, such as  Lake Willoughby, likely formed in overdeepened basins similar to the Finger Lakes in New York (which in itself deserves further study) . These overdeepened basins as well were occupied by long lasting ice blocks as part of the Disaggregated margins.

  • A corridor is also mapped along a T4 margin in conjunction with proglacial Lake Winooski, a major proglacial water body in the interior uplands as identified and mapped in previous literature. Along the eastern margin of Lake Winooski  this corridor shows the development of this water body by rapid penetration of standing water along  its strandline around the eastern perimeter of  an ice mass within the footprint of  the  Lake Winooski Basin, with associated stagnant ice margin deposits such as eskers graded downward to kame deltas at the Lake Winooski strandline, basically a step-down hybrid ice margin record.   Along Lake Winooski’s western  margin this corridor was associated with “Disconnections” of active ice masses in deep valleys on the eastern flank of the Green Mountain front range, as marked by Scabby Terrain in those basins.The “Disconnection” term and concept is taken from the literature.
  • Similarly, a corridor is identified and mapped along the west-, south-, and east-facing margins of the Champlain Lobe. This corridor was associated with the rapid recession of this lobe, by penetration of first Coveville, then Fort Ann Lake Vermont waters, and finally  leading to the incursion of the Champlain Sea waters into the Basin. The ice margin associated with these corridors was a calving type margin. As a consequence of the rapid growth of this corridor along the eastern and western ice margins, with residual ice remaining on and buttressed by a raised portion of the Basin floor identified here as the “Middlebury Bench,” the Champlain lobe became  long and convex in plan-view shape. As a consequence, this convexity resulted in and was associated with ice margin recession which had both westward and northward recession components, along highly irregular ice margins. Thus, the ice margin and its recession was not a simple pattern of northward recession of  an ice margin as conventionally regarded. This spatial configuration was especially  important  in regard to the finding that an ice margin readvance took place in Champlain Sea time (at T8 time); such a margin makes it easier to account for possibly correlative readvance evidence as reported by others at southern locations in the Champlain Basin without requiring long distance readvance.
• “Drainage Lines:” Fluvial meltwater along receding  ice margins tends to be marked  by erosional and depositional evidence which is readily detectible on LiDAR imagery, but as well such Lines are associated with soil textural differences  on published surficial geology maps, Vermont Highway Department maps,  and on USDA Soil Conservation Service soil maps, all provided by VCGI. These Lines  mark the  margins of the receding ice sheet at different places and times. Drainage Lines extend a) downgradient from or along ice margins within individual basins,  b) across interfluves between neighboring basins, and c) across drainage divides via spillways between neighboring basins.

Such 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  the Lake Winooski in the Lamoille and Winooski Basins, which in turn can then be traced southward  for another another 30 miles (48 km)  in close association with a T4 ice margin, 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 can 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 deposit with a peculiar LiDAR signal identified and mapped in many places  on VCGI. These deposits  formed along the margins of ice masses which became “Disconnected” (again, a term and concept identified in the literature for modern ice sheets and glaciers) from the parent ice sheet and as a consequence stagnated 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 (not necessarily to the south in this Basin beyond Vermont), which became “Disconnected.” Such Disconnections are identified at three times and places related to the lowering of the ice sheet across divides, cutting off the active ice supply from the parent ice sheet via 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.
• “Ice Marginal Channels:” These are erosional features which have been widely reported in recent literature, with different terminology and theories for their formation. Such features are numerous and quite common in Vermont, especially in certain areas, along certain ice margins, and at certain specific times. In general, Ice Marginal Channels are interpreted as having formed subglacially along “Cold”ice but beneath Warm ice, specifically along  active ice margins, and tend to mark ice margins in an early “Nunatak Phase” of recession. In many places these Channels are very numerous, clearly marking the progressive lowering of the ice sheet in the Nunatak Phase, leading to the development of ice lobes in the subsequent Lobate Phase.

Certain Ice Marginal Channels  at a particular T level and time  (T3/T4) occur at the base of the Nunatak Phase where together with stagnant ice deposits they define a “Hybrid” margin at the beginning of the Lobate Phase.  These  particular T3/T4 Hybrid margin features, which are mapped across Vermont, represent a “Signature” that facilitates mapping, semi-independently from the usage of ice margin elevations in the Bath Tub Model.

Further, the T3/T4  Ice marginal Channels in the Upper Connecticut Basin in Vermont, representing active ice,  are  correlated with a 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 along active ice margins, beneath Cold Ice, the base of which was warmed by the T3/T4  ice readvance over terrain that had been warmed during the preceding recession. These Channels are thus interpreted as associated with inverse polythermal ice. Further, these Channels are believed to have  formed by erosion by sub-glacial melt water under confined elevated  hydrostatic  head,  across terrain protuberances,  in a manner analogous  to spillways for drainage of  subglacial lake-like impoundments.  This  T3/T4 readvance is part of the aforementioned “Signature,” which can be identified and correlated across Vermont, including   correlation with readvance evidence reported by others in the Winooski Basin and the Vermont Valley in the Rutland and Bennington vicinities. Thus, the T3/T4 margin is regarded as an important marker of a readvance of the ice sheet that occurred at an early time across Vermont. 

The Ice Marginal Channels in the Upper Connecticut Basin are associated with Scabby Terrain and stagnant ice deposits indicating the en masse stagnation of the ice mass in this basin following the WMMS readvance. Together, these particular Ice Marginal Channels and associated deposits mark the margin of a lobe in this Basin at T3/T4 time which stagnated en mass in conjunction with the recession associated with the WMMS readvance. This lobe shaped Upper Connecticut ice mass  is mapped as extending southward to a stagnant ice margin position near Bradford, Vermont with an  associated outwash plain which is graded to deltaic deposit in Lake Hitchcock.

The deglacial history in the Upper Connecticut Basin as just described is related to a long standing debate in the literature as to the manner of ice recession, whether by massive stagnation of large portions of the ice sheet in that basin as opposed to progressive recession of discrete active ice margins. The evidence presented here indicates that the recession initially involved active ice margin recession associated with and following the WMMS readvance, but that the ensuing recession then led to en masse stagnation of the Connecticut lobe in the Upper Connecticut Basin, again with a terminous near Bradford, Vermont, with drainage evidence, essentially an outwash plain, graded to the Lake Hitchcock strandline.

Significantly, Ice Marginal Channels generally are absent:

1. 1. In the Lower and Middle Connecticut Basins, where  as noted the ice sheet in these basins  became Disconnected and stagnated en masse at  two early times (T1 and T2). The absence of Ice Marginal Channels is taken as evidence consistent with the en masse stagnation of ice in these basins.
   2. In the northern Champlain Basin where and when the waning Champlain lobe  in  T7 and especially T8 times is believed to have been a fully Warm Ice  type, such ice being unfavorable for Ice Marginal Channel formation.

• “Ice Tongue Grooves:” These features are erosional grooves believed to have formed at the mouths of the major basins, including the Missisquoi, Lamoille, Winooski, and Otter Creek Basins in conjunction with the  sudden and substantial lowering of proglacial water bodies in these basins, resulting in disequilibrium marked by increased surface ice gradients associated with the transformation of the eastern margin of the Champlain lobe  from a lateral type margin  to a frontal type margin. Further study of these features is needed.
• Shattuck Mountain Pothole Tract: This tract, which has been reported previously, is believed to have formed  from drainage of either supraglacial meltwater drainage via moulins or by ice marginal drainage,or both, as suggested by others, in either case associated with  extraordinary meltwater drainage volumes/velocities associated with a physiographic barrier that served to collect and direct meltwater from a large catchment area of the Champlain lobe. Again, such features represent drainage history closely associated with deglacial ice margin history, which likewise deserves much more study.
• Calving Ice Margin Features: Calving is identified for discrete ice streams in five physiographic re-entrants in the vicinity of the “Middlebury Bench,”  a slightly raised eastern portion of the Champlain Basin floor, and in the  “Trough” and “Deep Lake” portions to the west, mostly in New York State. Calving ice margin  features include 1) “Headless Deltas,” 2) Ribbed Lacustrine Deposits,” and 3) “Thickened Bouldery Ponded Water Deposits” associated a with  unusual, conspicuous, and mappable  flat valley floor terrain. Whereas these features are interpreted as indicating calving, for which substantial supporting evidence exists, these features likely do not specifically mark grounding lines. Whereas calving generally is regarded as associated with ice streams, an alternative interpreation is raised suggesting  that these ice streams may have been associated with intermittent oscillations, as suggested in recent literature for collapsing ice margins , especially for reverse gradient settings as in this case, associated with the rapidly receding, perhaps collapsing  margins beyond a tipping point. This alternative interpretation is important as it may indicate that the previously reported readvance evidence in several locations in the southern Chmplain Basin may correlate with bonafide readvance evidence in the Missisquoi Basin, but without requiring active ice extending southward for a long distance in the convex shaped Champlain lobe. The distinction between oscillations versus readvances is important but difficult to make. This issue  pertains to the nature of dying ice sheets associated with global warming which is of considerable present day importance.
• “Ice Margin Lines:” These are long linear shallow groove-like elements, the origin of which is uncertain but in some places may include “Mega-Scale Linears” associated with calving,  or other ice marginal drainage marks.
• Ice Margin Steps:  These are features resembling stepped terrain, usually in glacial till ground moraine, interpreted a having formed along ice margins by a mechanism which may represent imprinting by ice blocks on till substrait or by some other mechanism which is as yet uncertain. Such Steps are less common but closely related to Ice Marginal Channels, and believed to be part of the larger, regional ice margin record at the transition from the Nunatak Phase to the  Lobate Phase.
• 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, and likely  are very important 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, nevertheless  it is likely that much can be gained  from more detailed study. Some of these features near Newport appear to correspond, for example with features identified by Wright in his detailed mapping of the Memphremagog Basin, but these need to be better understood as to what they mean and how they fit in the larger perspective of deglacial history. As suggested  in a conversation with Jeff Munroe, until recently at Middlebury College, the power of LiDAR  is incredible and no doubt much more information leading to a better understanding of the Laurentide ice sheet and its history, and of ice sheets including modern analogs,  remains to be studied and understood.

The mapping reported here identifies and delineates eight ice margin positions, levels, and times from the oldest and highest at T1, through the youngest and  lowest at T8 (with the hint of a possible T9). It is suggested that the prevailing, conventional thinking of ice margins as like simple, well-defined lines on a map, and that recession of these ice margins took place progressively and predominantly from south to north, is over simplified. Ice margins are shown to have been highly convoluted and  irregular in plan view, again reflecting the influence of physiography. Further, as noted , a distinction is made between  active ice margins and stagnant ice margins, both together marking individual T times, referred to as “hybrid” ice margins, which had overlapping   spatial  and temporal relationships  during recession,  and were  very irregular in shape, consistent with physiography, and  receded  both laterally and longitudinally along irregular but generally  lobe-shaped ice margins.  As just stated, all T time ice margins are believed to have been hybrid types, with the identification of stagnant and active  margin components being critical to a full understanding of deglacial history.

This report finds that the eight ice margin levels, positions, and times represent “stillstands” in that time was required for ice margin feature development at various levels, during the progressive recession of the ice margin from an early “Nunatak Phase” through a “Lobate Phase,” leading to the final exit of the ice sheet from Vermont. Whereas global warming generally was responsible for the recession, the cause of such stillstands with regard to climatic variations is undetermined. As suggested, stillstands  may be attributable to or influenced by climatic variations but other glacial dynamic factors such as hypsometric variations in the “Bath Tub” ice volumes during recession seem more likely. Deglacial history was generally progressive and relatively steady from T1 to T8 time, and is repeatedly likened to a  step-down, recessional progression of  “closely spaced multiple rings in a slowly draining bath tub.”

Evidence for ice margins in the early “Nunatak Phase” were identified but generally not correlated and mapped  across Vermont except in the Memphremagog Basin where they are remarkably numerous, indicating a T4 margin which correlates with the Dixville moraine in Quebec and the WMMS in New Hampshire.  The evidence indicates that the ice sheet in the subsequent  “Lobate Phase” developed distinct lobes in the Champlain Basin, the Memphremagog Basin, and to some extent the Connecticut Basin, with a multitude of sub-, sub-sub-, etc., lobes 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 by ice primarily  from the Champlain Basin and the Memphremagog Basin.

In T1 and T2 times ice sheet recession and surface lowering resulted in thinning of ice at divides  between first the Champlain Basin and the Lower Connecticut Basin in T1 time, and then with the Middle Connecticut Basin in T2 time, resulting in en masse stagnation as marked by Scabby Terrain and the absence of Ice  Marginal Channels or other evidence of active ice. The surficial geology of the Lower and Middle Connecticut Basins in Vermont consists mostly of widespread, thin till ground moraine and scattered small stagnant ice deposits, consistent with  the lack of active ice and associated Lobate Phase margins in these Basins.

T3-T4  times, levels, and margins correlate with the White Mountain Moraine System (WMMS)  in New Hampshire, in what is referred to here as the Upper Connecticut Basin. The WMMS has been reported  as marking a significant readvance, associated with the Older Dryas cooling event and dated at approximately 13,800- 14,000 years BP. The ice sheet in T3 time associated with the readvance  extended across divides from the Memphremagog Basin, with insignificant flow from  the north in the Connecticut Basin itself owing to the constricted physiography and high elevation of the terrain in the headwaters of the Connecticut Basin. The active ice margin  associated with this readvance as mapped by Thompson et al  in conjunction with the WMMS is believed to have been associated with an ice lobe extending southward in the Upper Connecticut Basin, again approximately to the Bradford, Vermont  vicinity where, as noted above, the evidence indicates a margin with drainage into Lake Hitchcock.

Recession in T4 time with lowering and thinning of the ice sheet the supply of active ice  from the Memphremagog Basin to the Upper Connecticut Basin ended, as marked by the  later  and lower moraines at the WMMS and by Scabby Terrain related to  en masse stagnation of the ice mass in the Upper Connecticut Basin, and  as marked by widespread  and substantial stagnant ice deposits  extending across  divides between the  Memphremagog and Upper Connecticut Basins.  

The T3-T4 margin is mapped across Vermont, including its distinctive “Signature,” in the Memphremagog and Champlain Basins, representing the beginning of the Lobate Phase. In the Memphremagog Basin the  T3/T4 margin is well and substantially marked by  Ice Marginal Channels and stagnant ice deposits, as a hybrid margin at T4 time with the Ice Marginal Channels indicative of active ice correlated with the Dixville moraine in Quebec.   Continued recession in the Memphremagog Basin  is marked by a step-down sequence  of hybrid margins marking T5 and T6 times, with the T6 margin representing the last ice presence in this basin.

In the Champlain Basin,  the T3/T4 margins are likewise well marked with the T3 margin marked by numerous Ice Marginal Channels  showing the  recession  of a well developed lobe in the Vermont Valley, with a substantial T3 stagnant ice deposit in the Bennington vicinity. The T3 margin is mapped on VCGI as extending northward on the eastern flank of the Champlain Basin to the Lincoln vicinity, north of which the ice sheet was in the Nunatak Phase.  VCGI evidence shows progressive recession of stagnant ice margins  from the T3 margin near Bennington to the T4 margin near Rutland, with  an almost continuous recession of stagnant ice deposits on the floor of the Vermont Valley. The T3  and T4 stagnant ice deposits at Bennington and Rutland are correlated with T3/T4 features at the WMMS. Evidence reported in the literature supports a possible readvance of the ice sheet at these locations, although further research on this issue is needed. The T3 and T4 margins  in southwestern Vermont were mapped independently, but later found to closely correspond with ice margin positions mapped by DeSimone and LaFleur, mostly in New York. However, those authors specifically state that they found no evidence of an ice readvance.

T3 and T4 margins mapped on VCGI in the Champlain Basin are relatively well defined as marked by stagnant ice margin deposits and Ice Marginal Channels as part of the aforementioned “Signature” pattern. The trace of these margins extend across divides between the Champlain Basin and the Memphremagog Basin, with evidence suggesting ice as well extending across low col divides between the Winooski and Upper Connecticut Basins.  The T3 and T4 margins in the headwaters of the Winooski Basin show recession leading to the  development of Lake Winooski in T4 time, again as mapped by others . Evidence reported by Wright, Larsen and others for a readvance , mostly within the footprint of Lake Winooski, is here taken as T4 time, but as well at slightly higher and earlier T3 levels,  is correlated with the WMMS readvance but the  trace of the T3/T4 ice margin from the Winooski Basin into the Memphremagog Basin and thence into the Upper Connecticut Basins was   quite convoluted and irregular owing to physiographic influence on the ice margins.

The Champlain lobe, was the largest and longest lasting of the ice sheet lobes, in Vermont.  VCGI mapping shows the  shows the traces of the ice margins marking the progressive recession of the Champlain lobe from the T3/T4 levels as just described, through T8 time. This recession is marked by diverse types of ice margin features showing the progressive step-down ice margin recession, likened to “multiple rings on a slowly draining bath tub,”  with the close association of the ice margin and many  local proglacial water bodies, as “Corridors,” which became progressively larger as these coalesced.

Stagnant ice deposits and associated kame delta features at the early T6 time  show that Coveville Lake Vermont extended as a narrow open water corridor, likely as a “Disaggregated Ice Margin, extending northward from its outlet in New York, into Vermont  along the western flank of the Taconic Mountains, around the nose of the Taconics, and continuing northward to the vicinity of  the mouth of the Winooski Basin.  Stagnant ice deposits and multiple kame deltas are mapped demarcating this margin, the latter 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, and the South Hinesburg delta at the Coveville level  was formed by drainage from Lake Winooski, which required an ice dam across the mouth of the Winooski Basin. The T6 margin in the Missisquoi Basin extends into Quebec so as to correlate with the Sutton moraine. However, the information here suggests that the ice sheet in T6 time in the Missisquoi Basin was quite thin, with an intricate pattern of sub-lobes closely associated with the local physiography.

An initial, first phase of calving is inferred at early T6 and Coveville time, based on the physiography of the basin floor, with low elevations of the terrain, mostly on the New York side of the Basin which resulted in substantial water depth capable of supporting calving.  It is believed that configuration of the Champlain lobe at this time was substantially convex, demarcated by  the early T6 open water Coveville corridor along the south facing and east facing margins , with significant assymetry imparted by the more substantial  spanse of the open Coveville water Corridor  on the western, New York side.

In late T6 and early T7 time, Coveville Lake Vermont lowered to the Fort Ann level. As suggested by Franzi (personal communication, 2024) this may have been causally associated with the breakout of Lake Iroquois in the Ontario Basin into the Champlain Basin. This was a substantial and sudden change in  water levels which is shown to be associated with the beginning of a second phase of calving. This time was also closely associated with  the opening  of the Winooski Basin , by subglacial leakage, not catastrophic ice dam failure,  leading to the draining of Lake Mansfield (the successor to Lake Winooski) in the Winooski Basin, briefly opening the Winooski Basin to Coveville waters, shortly followed by Fort Ann entry into this Basin. 

The  second phase of calving is identified as  beginning at the heads of re-entrant basins within the “Middlebury Bench,” such as the LaPlatte River, Lewis Creek, New Haven River, Little Otter Creek, and Otter Creek, as marked by calving ice features as referred to above.   The evidence indicates the progressive recession of the grounding line in each of these re-entrant basins, from early Fort Ann to late Fort Ann time. The evidence also indicates that this water level lowering destabilized the eastern margin of the Champlain lobe whereby it transformed from a lateral to a frontal margin, as suggested by Ice Tongue Grooves. The evidence also indicates that the Disaggregated ice margin and associated  narrow, more or less open, “Disaggregated”  Fort Ann  Corridor, rapidly extended northward in T7 time and Fort Ann time, to and beyond the Quebec border, as marked for example by Ice Tongue Grooves at the mouth of the Lamoille Basin,  by Fort Ann strandlines in portions of the Missisquoi Basin, and as well by the reported evidence of Fort Ann (termed Candona) features in Quebec.

Further, recession led to the opening of the Corridor for incursion of the Champlain Sea into the Champlain Basin. The evidence for the Champlain Sea in Vermont is shown by numerous deltas  and other strandline features at this level, as reported by Chapman and many subsequent researchers. The evidence indicates that the calving margins in the re-entarnt basins in the Mddllebury Bench, receded progressively in Fort Ann time, and that the lowering from the Fort Ann level which was substantial, and reportedly occurred rapidly, triggered a third calving event. Further, the evidence suggests that the configuration of the Champlain lobe continued to be substantially convex, and likely was even more asymmetrical, with a larger spanse of open water on the western, New York side of the Basin. Further, evidence reported by Cannon (1964) and Wagner (1972)in the Missisquoi Basin likewise suggests that after the initial incursion of the Champlain Sea, the Champlain lobe readvanced in T8 time. In as much as the date of the Champlain Sea incursion   has been  estimated previously by others to be about 13,000 years BP, the T8 readvance was slightly younger. Intriguingly, this may fit with evidence reported of a freshening of the Champlain Sea. The T8 readvance apparently blocked the basin, resulting in a higher water level, but the nature and extent of this raised level are unknown.  Also, the T8  readvance may have overrode some of the older, previous Fort Ann features in the Missisquoi Basin, accounting for the absence of such features in parts of the Basin. Remarkably fresh looking, bouldery ground moraine terrain in parts of this Basin may likewise be associated with the T8 readvance.

In addition, the VCGI evidence suggests that the T8 ice margin was in the close proximity to the Champlain Sea strandline. Further,  readvance evidence reported by Wright et al in the  Charlotte area and Connally ( ) in the Bridport area, likely as well may correlate with this T8 readvance, both in close proximity to the Champlain Sea strandline. This suggests that in plan view  the Champlain lobe may have had a long, convex configuration in T8 and Champlain Sea time.

Franzi (2025,  personal communication) has not found evidence supporting such a long convex lobe, and believes that the Champlain lobe ice margins in Coveville, Fort Ann, and Champlain Sea times were substantially flattened, again in plan view , approximately as drawn for example by Chapman. Further, Franzi has examined model information of ice sheet profiles which he believes suggest it is unlikely that the Champlain lobe  was physically able to sustain such a long convex lobe of active ice for such a readvance.nce.

My rejoinder is that his evidence for the  ice margin in Coveville, Fort Ann, and Champlain Sea times  being far to the north is consistent with the opening of the large open water spanse on the western, New York side of the basin, and that the theoretical inability of the ice sheet to sustain such a long convex lobe needs to be viewed in the context of the Champlain lobe at T8 time  in the basin being the remnant of the much larger ice mass left remaining from an earlier time.

Franzi’s observations are helpful as they raise the question regarding the  nature of the recession of the Champlain lobe. The evidence indicates that the recession was rapid, associated with calving, with suggested instability, essentially the collapse of the Champlain lobe. Substantial recent literature exists suggesting that collapsing ice lobes, especially in reverse gradient settings,  exhibit erratic, “spasms” of short lived surge-like oscillations.  Thus, whereas the evidence in the Missisquoi Basin for a readvance is substantial, it is possible that the readvance evidence in the Charlotte and Bridport areas associated with the long convex lobe in T8 time was  part of a “spasm” associated with the Champlain lobe collapse. By this explanation, the long convex lobe in T8 time, south of the Missisquoi Basin may have been stagnant ice, not active ice, marked by “spsams” when the ice was reactivated in local, narrow ice streams in the Middlebury Bench.  Clearly, this is an intriguing matter deserving  further study. 

This present report identifies numerous aspects where further study may prove fruitful, which in fact is the intended purpose of this  study.

Whereas this study addresses the deglacial history of Vermont, findings of this study as well pertain to three correlative topics:

1. Archaelological evidence pertaining to the migration of Paleoindians into and around Vermont: Whereas this is a topic largely beyond the scope of this study, as noted, this is a long standing interest. As is well known, Paleoindian migration tended to shadow the receding Laurentide ice margin. This matter is briefly examined, primarily as a suggestion of the potential benefit of cross-discipline research.
2. Global warming: As is made clear, I am not an expert on global warming, but I share the concern for the potential effects of modern day global warming on modern ice sheets. It is believed that the information gained from this study of the recession of the Laurentide ice sheet in Vermont may provide insights about potential impacts of global warming on present day ice sheets, especially Greenland. This is touched on briefly.
3. Pre-T1 Vermont glacial history: The finding here that the entire deglacial record here represents a history of the ice sheet presence and recession during a very late portion of the last glaciation is, for me, as stated, quite surprising and remarkable. This obviously leads to the larger question of  evidence for the preceding times when Vermont, for a much longer time, was covered by ice. A chance encounter with Paul Bierman whose research  includes studies of the long term Pleistocene record of glaciation has prompted me to add some thoughts in a separate section of this report having to do with this issue what happened in Vermont in pre T1 time. . At issue, and still needing to be considered, are questions as to when and how erosional features such as striations, roche moutonnée,  overdeepening, etc, and other erosional features occurred, and  as well depositional features, such as glacial till itself formed,  including the possibility that larger elements of the regional physiography may have been shaped by long term multiple glaciations, which is the thrust of Bierman’s continuing studies. Accordingly, thoughts are offered in regard to possible avenues for his further studies in Vermont which may be fruitful.