The deglacial history of Vermont, based on VCGI mapping, as just presented in the preceding, is represented by eight times, ice sheet levels, and ice margin positions, designated as T1 through T8. Whereas the preceding presents deglacial history by displaying the positions of ice margins across the State for selected T times on maps, through space as it were, the following provides a narrative summary of this history through time.
This is a complex story. The following only gives a summary overview, with details provided in various sections of this report, such as descriptions and explanations of calving ice margins, Ice Tongue Grooves, and so on. Also, the presentation of Locales in the Appendix gives substantial historical information.
As a prefatory note, to put this story into perspective, whereas VCGI mapping does not give absolute dates, the deglacial history reported here for Vermont suggests correlations with neighboring regions and events as reported in the literature. This indicates that:
- .the Vermont deglacial history reported here took place at a late time in the larger, regional recessional history of the Laurentide ice sheet,
- occurred rapidly, over a relatively short time, and
- was book-ended by two readvances.
These observations stem from the following:
- The T3 and T4 margins, marking the beginning of Vermont deglacial history in the Lobate Phase, are correlated with the White Mountain Morainic System (WMMS) in New Hampshire, which as reported by Thompson et al represents a significant readvance in Older Dryas time, approximately 14.0 -13.8 KA B.P.. This correlation, which fits together well with the deglacial history of the Connecticut Basin, the Memphremagog Basin, and the Champlain Basin, has major implications, because T3 and T4 are the oldest times, levels , and margins in the VCGI mapping of the Lobate Phase of Vermont deglacial history.
- Likewise, T8 time in Vermont represents a readvance, based on evidence in the Missisquoi Basin, reported previously, first by Cannon and subsequently corroborated by me in my 1972 report. Cannon recognized, though he could not bring himself to formerly accept the idea, that this readvance occurred in Champlain Sea time. My 1972 findings unequivocally document a readvance after in the Basin had already opened with the Champlain Sea time, at its oldest, highest, maximum level, while still at the maximum, marked by recession leading to the exit of the ice sheet from Vermont as the Champlain Sae levels were lowering. Given a revised age date for the intial development of the Champlain Sea of about 13 KA B.P , 1 This date was taken from a report based on a study in part based on sediment cores from the Champlain Basin,, by: Cronin T., et al, 2008, Impacts of post-glacial drainage events and revised chronology of the Champlain Sea Episode 13-9 KA; Paleogeography, ,Paleoclimatology, Paleoecology ; V. 262 Issues 1-2, PP. 46-60 it is suggested that the T8 readvance may represent the Younger Dryas cooling event which is dated as 12-11.7 KA B.P.
These findings, which came from my late report writing “Epiphany,” as discussed further below, are surprising and in my opinion are quite significant, both in terms of degalcial history and as well global warming. These findings indicate that the Vermont deglacial history, in its entirety, took place at a late glacial history time and was quite rapid. This rapidity likely reflects the influence of physiography and standing water on the recession of the Laurentide ice sheet in Vermont, which is not only an intriguing academic matter pertaining to deglacial history but as well is relevant to modern day global warming concerns. In general, from an academic point of view I try to be as dispassionate as possible, but for me personally, this history likely documents a history represents a collapse beyond the ice sheet’s in Vermont’s tipping point.
Much of the Vermont deglacial history as mapped here on VCGI has to do with the ice sheet response to a generally warming climate in which ice lobes developed in accordance with physiography, marking the recession of the margins of these lobes in conjunction with meltwater, both in fluvial and standing water body form. Meltwater from the receding Laurentide ice sheet increased in volume dramatically with time, beginning from an early time when meltwater was limited to the fringes of nunataks. Added to this is the recession of the melting ice sheet in a reverse gradient setting, meaning that the ice sheet margin was continually “bathed in water,” to use Chapman’s phrase. This too is significant for present day concerns in that much of Greenland and large parts of Antarctica are in reverse gradient settings. As the Laurentide ice sheet in Vermont receded, not only did the volume of meltwater increase, leading to the development of proglacial lakes as part of a step-down sequence, proglacial lakes became larger and increasingly regional via progressive coalescence of smaller, local, to larger, more regional water bodies. Again, this has to do with physiography, ie., the configuration of the “Bath Tub.” Whereas the lowering of water levels for such water bodies was generally progressive and gradual, at times this took place suddenly, with major hydraulic head differences related to the opening of new drainage pathways and lower outlets. Water, by its nature, is very effective at finding the lowest possible “weakness” points for new drainage pathways, leading to ever newer, larger, lower and more regional proglacial standing water bodies. The Vermont deglacial history shows that at multiple times new proglacial water bodies developed at significantly lower levels, leading to sudden changes. These were destabilizing events, which likely accelerated recession (again, perhaps beyond a tipping point).
In terms of deglacial history, the earliest ice margins, levels, and times mapped in this VCGI study are identified in the “Nunatak Phase,” marked by Ice Marginal Channels, which were not correlated, but show the early emergence of mountain peaks as the ice sheet thinned and its surface lowered. VCGI mapping identified T1 and T2 times which mark the thinning of ice across regional physiographic divides, resulting in “Disconnections,” whereby ice supply to downgradient portions of the ice sheet in first the Lower and then the Middle Connecticut Basins ceased, causing en masse stagnation of much of the ice sheet in those Basins.
T 1 and T2 times are marked by Scabby Terrain tracts. These tracts represent the margins of Disconnected ice in early times, not the ice margins of the receding ice sheet. This is an important conceptual distinction. These features show the importance of physiography, specifically upland divides of irregular terrain beneath the ice sheet, which served to isolate large masses of ice from the parent accumulation area of the ice sheet. This is an important “Glacial Dynamic,” representing a significant aspect of ice sheet recession in a time of global warming, which is of interest with regard to present day concerns about global warming.
T3 and T4 times are extensively marked across the State by many ice margin features, specifically closely associated correlative high level Ice Marginal Channels and stagnant ice deposits, marking the beginning of the “Lobate Phase” in the Memphremagog Basin, the “Upper Connecticut Basin,” and the Champlain Basin. The T3 and T4 ice margins, levels, and times in a sense represent the start of the Vermont deglacial history clock, marking the beginning of the progressive recession of mappable ice margins of the receding ice sheet in the Lobate Phase, as mapped and identified on VCGI across the State. As just noted, T3 and T4 times, levels, and margins are correlated with the White Mountain Moraine System, at a late time in the Laurentide history in conjunction with a readvance.
T3 margins at the beginning of the Lobate Phase are marked by stagnant ice deposits at elevations just below Ice Marginal Channels at the bottom of the Nunatak Phase in southern Vermont (in the Bennington area), while the ice sheet in northern Vermont was still in the Nunatak Phase. As discussed in different sections of this report, including my late “Epiphany,” the T3 and T4 margins together represent a pattern involving both Ice Marginal Channels and stagnant ice deposits which is so well marked and identifiable as to constitute a “Signature.” This T3 and T4 margin “Signature” marks the beginning of the receding ice margin deglacial record in Vermont, with correlative stagnant ice deposits in close association with Ice Marginal Channels, together marking a “hybrid” type ice margin consisting of separate but closely related active ice margin and a stagnant ice margin features. All T time ice margins likely were hybrid types.
Hybrid ice margins are important components of the three major elements of the triumvirate concepts identified in the Introduction of this report of Deglacial History, “Styles” and “Glacial Dynamics. ” The active ice margin component of hybrid margins, as marked by Ice Marginal Channels, served to restrict meltwater, which as suggested in the text above is believed to represent polythermal conditions, with warmed basal ice along the ice margin. In contrast, stagnant ice margin features, also part of the hybrid duo, formed in fully warmed ice margins, fully penetrated by meltwater. Recession of discrete, individual hybrid active and stagnant ice margins took place in tandem with the active ice margins receding to new, lower T positions, levels, and times, while stagnant ice remained in the earlier, higher stagnant ice margin “partner,” as evidenced by Drainage Lines from the longer-lasting stagnant ice margin to the receding, lower active ice margin. This cycle then took place repeatedly as a kind of dance between active and stagnant ice margins during recession. Such hybrid ice margins represent a Style in this report referred to as “Everything, Everywhere, All at Once, and Continuing.” Again, this is of interest from the points of view of both deglacial history and global warming.
As already noted, in the Upper Connecticut Basin, the T3 and T4 Vermont margins are correlated with the White Mountain Morainic System (WMMS) in New Hampshire, as reported by Thompson et al. This is significant, again as noted above, in that the WMMS ice margins were reportedly associated with a readvance of the ice sheet in a time of climatic cooling in Older Dryas time. The extent of the recession prior to the readvance is uncertain. This readvance is significant, again as noted above, in that T3 and T4 times a) represent the beginning of the deglacial history clock in Vermont, b) taking place in conjunction with a readvance, and c) occurred at a relatively late time in the larger, longer deglacial history of the Laurentide ice sheet. Again, as noted above, the entire Vermont deglacial history record thus represents only a small, late portion of this larger history, associated with rapid recession.
T3 and especially T4 ice margins mark the top of a step-down sequence of ice margins and closely associated fluvial and ponded water drainage features, continuing through T6, T7, and T8 times, showing the progressive enlargening and coalescence of local ponded water bodies, becoming more regional in extent. Coalescence of local water bodies into larger regional water bodies is significant for global warming concerns because larger, regional water bodies are subject to changes caused by “externalities, such as the opening of lower water body level controls, which can be sudden and substantial, as was the case in Vermont, resulting in destabilization of a significant, albeit regionally local, portion of the ice sheet.
The step-down sequence is identified in both the main Champlain and Memphremagog Basins, and as well in the major tributary sub-basins, especially the Missisquoi, Lamoille, Winooski, and Otter Creek basins. This step-down sequence across the State was associated with T3/T4 time ice margins as a starting time, but reached different end points in terms of T times and water regional water bodies, at different locations.
The following is a brief summary of the step-down sequence for the main Champlain lobe, interwoven with major aspects of the step-down sequence in major tributary basins. The step-down pattern of ice recession in the Memphremagog Basin is substantially and well documented. In fact, the concepts of hybrid margins and the “Everything, Everywhere, All at Once, and Continuing” Style of recession of these margins came from VCGI mapping in this Basin. However, in the interest of brevity this history is not reviewed here.
In the main Champlain Basin, the beginning of the step-down sequence is marked by the T3 margin in the Bennington area which can be traced northward in the Vermont Valley to the Rutland vicinity at the mouth of the Vermont Valley, continuing northward along the Champlain Basin foothills to the Lincoln Vermont vicinity. North of the Lincoln area the T3 margin is in the Nunatak Phase in which ice margins are marked chiefly by Ice Marginal Channels, which are not correlated so as to delineate the trace of the T3 margin. Further, more careful and detailed study might show the delineation of the T3 time ice margin in northern Vermont. For example, in the headwaters of the Lamoille and Winooski Basins T3 and T4 ice margin features mark the ice margins leading to the development of Lake Winooski in the conjoined lowland of these two basins.
The T4 margin is identified in the Rutland vicinity at the mouth of the Vermont Valley. Stagnant ice deposits on the floor of the Vermont Valley show an essentially continuous step-down recession of the ice margin from the T3 position near Bennington to the T4 position near Rutland.
Evidence in the vicinities of both Bennington and Rutland, as discussed above, supports the possibility of a readvance, consistent with the correlation of T3 and T4 margins with the WMMS. Whereas these locations are about 50 mile apart, such that a readvance of this magnitude seems extreme and implausible, the low gradient of the Vermont Valley basin floor favored a distended ice margin in contrast to the WMMS where the ice sheet abutted against a substantial upland with steep terrain such that the margins were compressed. Further, as noted by Thompson et al the specific details of the readvance remain unclear, requiring caution. The T3 and T4 margins are also correlated with ice margins in New York as mapped by DeSimone and LaFleur, where the authors specifically report that they found no evidence of a readvance. The reported absence of readvance evidence in New York at correlative margins suggests that the regional ice sheet recession was not uniform or monolithic.
It needs to be said, as a general observation side bar:
The Vermont deglacial history record, specifically as reported in the literature, as for example by Wagner (1972) in the Missisquoi area, by Calkin in the Middlebury area, by Wright et al in the Charlotte area, and by Connally in the Bridport area, identifies evidence which indicates the oscillatory back and forth movement of the ice margin during recession. Calkin in particular specifically recognizes raises the distinction between “oscillations” versus “readvances.” For the Middlebury area, for reasons given, he favors oscillations. By contrast, Connally and Wright et al favor readvances. Connally’s Bridport readvance has been subsequently cast in doubt, but it would seem that his evidence clearly marks some sort of oscillation, if not a “true readvance,” the distinction being another matter. None of us put the evidence to the test of the possibility that such ice margin fluctuations relate to calving, though Calkin to his credit recognizes this possibility. Whereas the distinction between oscillations and readvances can not be resolved here, the Thompson et al study of the White Mountain Moraine System (WMMS) is unique for its level of detail and careful study. Their findings clearly, substantially, thoroughly, and firmly document that the WMMS entailed a fluctuation of the ice margin corresponding with climatic variations, which favors a “readvance” interpretation. However, whereas this reflects a very detailed study, nevertheless, even with such detail, Thompson et at wisely urge caution.
In Vermont, the studies thus far, whether by Wright et al or Wagner (1972), or others have not reached such a detailed level. It is one thing to recognize conceptually that recession took place by a step-down sequence of ice margins associated with the development of progressively larger and lower proglacial water bodies, leading to regional water bodies. But it is quite another to actually map the individual water bodies. The work by Wright et al for Lakes Winooski and Mansfield comes closest to such detail, but lacks the specific delineation of associated ice margins. Further such study is needed, in the context of ice margin delineation and recession. The VCGI mapping given here at the mouth of the Winooski, as discussed in detail above, illustrates the complexity of the challenge. It seems that certain locations in Vermont are especially important and promising, where such more detailed study might prove fruitful, such as the mouths of the major sub-basins in the Champlain Basin, specifically the Missisquoi, Lamoille, Winooski, and Otter Creek basins. Likewise, the mouths of the five sub-basins in the Memphremagog area similarly deserve detailed study. Further, the linkage between these basins by drainage features is an essential part of a more fully developed “Bath Tub” model based history which ultimately will provide a more complete documentation, including the matter of oscillations versus readvances.
The T4 margin is traced northward from the Rutland area along the foothills to the Quebec border, including major indentations in the major sub-basins, demarcating the eastern margin of the Champlain lobe. Again, the T4 margin represents the top or uppermost level of a step-down sequence of ice margin features, marked by substantial and increasing evidence of meltwater, including Bedrock Grooves, Drainage Lines, and Kame Deltas. In the Rutland area the bottom of the step-down sequence is at the at the early T6 level, time, and margin, giving way to Coveville Lake Vermont. T5 and T6 times, levels, and margins represent an intermediate part of the step-down sequence, as just noted, with Coveville Lake Vermont representing a proglacial water body controlled by regional physiography.
An early phase of calving along the south-facing Champlain lobe ice margin in early T6 time, in Coveville Lake Vermont, is tentatively identified in conjunction with the lower portion of the Champlain Basin floor, related to deeper Coveville waters. Coveville Lake Vermont also extended northward along the eastern margin of the Champlain lobe in a narrow, “Disaggregated,” more or less open, water corridor. The northernmost extent of Coveville Lake Vermont in this corridor was in the Winooski Basin, in association with late T6 and early T7 ice margin features. Thus, the Champlain lobe was substantially convex in plan view shape, with bounding water bodies. This convexity was to become more pronounced in time, which is part of the paradigm discussion above. The ChatGPT generated image on the frontispiece of this report was specifically developed with this concept in mind.
In late T6 and early T7 times, Lake Vermont lowered from the Coveville to the Fort Ann level, owing to an “externality” (the opening of a lower outlet for Lake Vermont), representing a very substantial and sudden drop in the regional hydraulic head base level. This had a major Glacial Dynamic effect, specifically: a) the progressive northward extension of Lake Vermont waters in the eastern “Disaggregated,” narrow open water corridor, along the Champlain lobe eastern margin, rapidly and progressively extended northward to and beyond the Quebec border, again giving the Champlain lobe a stronger, convex shape, b) the destabilization of the eastern margin by which it proceeded to convert from a lateral to a frontal margin, and c) the development of a second, more substantial phase of ice margin calving associated with multiple ice streams in the “Middlebury Bench.”
The T7 Fort Ann lowering as well is correlated with the breakout of proglacial Lake Iroquois waters in the Ontario Basin into the Champlain Basin at Covey Hill and the draining of Lake Mansfield, the successor to Lake Winooski as described above, which likely added to the destabilization.
Whereas the preceding description summarizes the stepdown sequence in the main Champlain Basin in T3 to T6, leading to T7 time, a similar step down sequence is identified in the major tributary basins. The Missisquoi, Winooski, Lamoille, and as well the Otter Creek sub-Basins extend from the main Champlain Basin through the Green Mountain and Taconic Mountain fronts eastward and southward into upland interior areas. Physiography was a critical control for the development of sub-lobes, or ice tongues, in these basins. Not only are the mouths of the Missisquoi, Lamoille, Winooski, and Otter Creek Basins relatively low openings into the Green Mountain and Taconic uplands, but, as well, higher terrain on the Champlain Basin main floor, such as the Middlebury Bench, served to deflect ice flow toward these openings. Extensive T4 ice margin features delineate ice sub-lobes in these tributary basins.
In the Winooski and Lamoille Basins these ice tongues extended into the upland interior where they merged to form a conjoined ice mass in a large physiographic embayment in this interior upland area behind the Green Mountain front range (the Morrisvoille-Stowe-Waterbury-Waitsfield lowland). The T4 margin is closely associated with strandline features related to a major proglacial water body in this interior area, as “Lake Winooski” and its successor Lake Mansfield, again as identified and discussed above. These lakes were large, substantial water bodies representing the coalescence of local proglacial water bodies in a step-down sequence, contributing to a Glacial Dynamic whereby ice masses in deeper valleys on the eastern flank of the Green Mountain front range and western perimeter of Lake Winooski became “Disconnected” by the penetration of standing waters along the Lake Winooski strandline. Scabby Terrain marking Disconnected ice masses is mapped on VCGI in multiple narrow, deep valleys extending eastward on the west side of the Green Mountain front range, with drainage graded downgradient, eastward to the Lake Winooski strandline. These Disconnected ice masses became separated from the main parent ice sheet, leading to en masse stagnation.
On the east side of Lake Winooski, ice margin features likewise indicate that the recession of the ice margin occurred quickly, leaving large blocks of stagnant ice masses behind while the ice contained in these stagnant ice masses continued to drain into Lake Winooski via eskers to form “Headless Deltas” at the Lake Winooski level. Whereas such features are part of the evidence for calving in the Champlain Basin where water depths were substantial, in this interior area the water depths apparently were insufficient for the development of a calving margin. Nevertheless, this history added to the destabilization of the Champlain lobe. The close association of T4 ice margin deposits and Lake Winooski strandline features, chiefly kame deltas, indicates that Lake Winooski waters extended along the perimeter of the ice mass, likely as a long, narrow open water corridor which again as in the main Champlain Basin is identified as a “Disaggregated” ice margin (referred to in the literature as resembling “Chicklet-like ice blocks permeated by a maze of narrow open water channels). In fact, the T4 margin is marked in places by numerous kettle like ice blocks, which represent these “Chicklet-like” masses. This evidence is interpreted as indicating the rapid penetration of the T4 ice margin by Lake Winooski waters, progressively encircling the interior ice mass.
Again, this history is here taken as evidence of the penetrant action of standing meltwaters along ice margins, which is associated with ice margin instability, and is seen as a significant part of the story having implications regarding present day global warming concerns related to the Glacial Dynamics between the ice sheet and meltwater, closely related to physiography. Per reports by Wright et al, with time and progressive westward ice margin recession and the opening of lower outlets Lake Winooski lowered to Lake Mansfield, which itself represented two levels.
Larsen, and Wright et al also identified and reported evidence of a readvance of the ice sheet in the Winooski Basin, most of which is located within the footprint of Lake Winooski which is here mapped as representing T4 time, but with some readvance features at slightly higher levels which can be correlated with the T3 margin as mapped here. Thus, it appears that this readvance is correlative with the WMMS, with the aforementioned recessional features thus representing subsequent events. The trace of the T3 and T4 margins shows that the connection of the T3/T4 margin between the Winooski Basin and the WMMS was quite irregular and convoluted, via an ice margin trace whereby the Champlain lobe was connected to the memphremagog lobe, which was then connected to the lobe in the Upper Connecticut Basin prior to its en masse stagnation.
As mapped by Stewart and MacClintock, and more substantially defined and delineated by Wright et al, Lake Winooski and its successor Lake Mansfield, represent a large proglacial water body in the upland interior area of the State, within the Champlain Basin. The ice sheet at T3/T4 time is correlated with Lake Winooski and slightly earlier times and levels, which fits with readvance evidence reported by Larsen, Wright, and others within and slightly above the footprint of Lake Winooski. As discussed below, Lake Winooski and then Lake Mansfield persisted and enlarged through T5 and T6 times to T7 time, with recession of the Champlain lobe eastern margin.
By T7 time the eastern portion of the Memphremagog Basin had become mostly ice free. In the main Champlain Basin, substantial recession of the Champlain lobe in T7 time ice margin occurred, with both calving of the ice streams in the Middlebury Bench and northward extension of the narrow open water Fort Ann corridor. Whereas the step-down sequence of ice margin recession south of and including the Winooski Basin was from the T4 to T6 levels, stepping down to Coveville Lake Vermont features, north of the Winooski the step-down sequence was from T4 to T7 stepping down to Fort Ann level features. T7 and Fort Ann time was a period of rapid northward recession of the open water corridor along the eastern margin of the ice sheet.
As discussed at length elsewhere in this report, T8 time was marked by a readvance of the ice sheet in Champlain Sea time, following a recession which led to the lowering of Fort Ann waters to the Champlain Sea. The readvance is marked by:
- Glacial till and ponded water sediment on Champlain Sea deltas in the Missisquoi Basin, and as well by reatures reported by Cannon and Wagner, such as absence of ponded water sediment in the Lake Carmi lowarea and fresh looking boulder strewn till ground moraine.
- Readvance evidence reported by Wright in the Charlotte area and by Connally in the Bridport area.
The T8 lobe was long and convex in plan view shape, extending from the Quebec border southward to the Bridport area, with the readvance evidence associated with the lateral and frontal margins of this lobe. However, the extent of the recession prior to the readvance is not known.
Thus, in sum, taken together, most of the deglacial history of Vermont is bracketed by readvances, the first at T3/T4 time is dated as 13,800-14,000 yrars BP. The second in T8 time likely slightly postdated the incursion of the Champlain Sea in Vermont which is dated as about 13,000 years BP. Again, this finding indicates that the recessional history of Vermont occurred in a very late portion of the larger recessional history of the Laurentide ice sheet. The recession was fast, and was accelerated by destabilization related to physiography and meltwater, bringing together elements of deglacial history based on the “Bath Tub model,” with associated Styles, and Glacial Dynamics elements. It is likely that once started the ice sheet recession reached a “tipping point,” an irreversible condition, spelling the end of the ice sheet in Vermont.