As stated in the Introduction, the goal of this study had a modest beginning, but thanks to the services of the Vermont Center for Geographic Information, which makes LiDAR imagery available across the State, along with diverse other information about soils, topography, and so on, this study expanded throughout the State of Vermont to encompass three elements, specifically:
- the late Pleistocene deglacial history of the ice sheet recession, as marked by progressively lower levels of the ice sheet surfaces and the associated recession of its margins,
- the nature of the ice margin environments, or “Styles,” and
- the dynamic interplay between the ice sheet and its surroundings, or “Glacial Dynamics.”
This report is presented here, not to declare victory – the history of the ice sheet in Vermont is not conclusively established and understood as a finality – but instead to provoke further study.
At the heart of this study was the identification of ice margin features, including both long established, well known types, and as well newer features. These include:
~ End moraines and stagnant ice deposits.
~ Ice Marginal Channels – features believed to have formed at the base of and close to the margin of the active ice sheet. As noted, these features are believed to have significance along with associated ice margin features as “signature markers which are helpful and important for the study of deglacial history.
~ Ice Margin Steps – terrace-like features similar to Ice Marginal Channels formed along the ice sheet margin, generally in glacial till.
~ Drainage Lines – meltwater features formed along the margins of the ice sheet.
~ Kame Deltas – deltaic deposits formed in close proximity to the ice sheet margin.
~ Scabby Terrain – a peculiar and distinctive LiDAR observed texture associated with deposits formed by “Disconnected” ice sheet masses, leading to en masse stagnation.
~ Bedrock Grooves – shallow to deep ravines cut into bedrock by meltwater along frontal or lateral ice margins.
~ Ice Tongue Grooves – similar deep ravines eroded into bedrock along the lobe tips of ice tongues, related to ice sheet destabilization by proglacial water bodies.
~ Calving ice margin features including, Thickened Bouldery Lacustrine deposits, Ribbed Lacustrine Deposits, which are hummocky stagnant ice deposits composed either entirely of fine grained, ponded water sediment or with veneers of fine grained sediment overlying coarser clastic materials, and Headless shoaling deltas, which are similar to “normal” deltas but originating from drainage associated with remnant stagnant ice margins.
~ Ice Margin Lines, which are linear to curvilinear features on VCGI maps, with uncertain origin and meaning for deglacial history but in southwestern Vermont possibly representing grounding line markings for calving ice margins.
These, and in addition other features of less certain or unknown origin and historical meaning, are identified, again all deserving further study.
This study reached several intriguing and unexpected findings. One is that ice margins were not simple, like lines on a map as conventionally regarded, mapped, and conventionally drawn. Instead, ice margins are identified as wide complex zones, not narrow knife edged margins as suggested by ice margin lines, including features formed at both active and stagnant ice margins, having overlapping temporal and spatial relationships, and confronted by increasing volumes of meltwater in close association with standing proglacial waters. Ice margins are characterized as active, stagnant, hybrid (which include both stagnant and active ice), step-down, destabilized, calving, Disaggregated, Diachronic, Calving, Oscillatory versus Readvance types, and Warm vs Cold.
As discussed, the interpretation of deglacial history for Vermont as presented here required the exploration of new ways of thinking, or paradigms. The nature of ice margins and their recession in Vermont is explored in terms of deglacial history, using elevation as a guide, referred to as a “Bath Tub Model.” The evidence indicates that the deglacial history of Vermont was late in the regional recession of the Laurentide ice sheet when it had substantially thinned relative to the physiography, such that elevation can reliably serve as a helpful guide for delineating ice margin recessional history, as suggested and supported by previously published reports in Vermont and neighboring regions of Quebec, New York, and New Hampshire.
In addition, the environment of the ice margin, or “Styles,” and the dynamic interaction between the ice sheet and these environmental conditions, or “Glacial Dynamics,” are examined. Styles and Dynamics represent an unexpected finding. Whereas the Bath Tub model used for deducing deglacial history initially represented an assumption, basically a hypothesis, its application and testing led to not just the determination of deglacial history, but as well the role of meltwater, especially standing or ponded water, as a Dynamic which interacted with the ice sheet, as a major factor – a controlling agent affecting the ice margin positions, nature, and configuration. This Dynamic is reflected by what has come to be seen as a standard step-down sequence, involving stagnant ice features in close association with kame deltas and Drainage Lines, giving way to increasingly larger and more regional water bodies, remarkably similar to lowering levels in a “bath tub,” albeit with a highly irregular margin configuration. The links between ice margin features and meltwater features both within and between basins not only provide supporting evidence to the validity of the “Bath Tub Model,” but as well show the role of meltwater in largely influencing and in some places and times actually controlling ice margins.
Standing and flowing water along ice margins had pronounced effects on the ice sheet and its margins, as a major Glacial Dynamic, in multiple different ways: 1) ice margins became increasingly associated with and likely at least partially controlled by standing or ponded water; 2) proglacial waters penetrated and were intimately associated with ice margins, in certain locations and circumstances resulting in Disconnections; 3) lateral margins of ice lobes became fringed by long narrow standing water corridors; 4) these corridors in some cases served to destabilize the ice sheet, as marked by a unique type of ice margin feature; (“Ice Tongue Grooves”) 5) as part of the destabilization, calving ice margins developed, again as marked by multiple types of unique features; 6) once initiated, such destabilization appears to have reached a “tipping point” whereby the demise of the ice sheet in Vermont was accelerated and its end fated and sealed.
In general, the lowering of the ice sheet began with a “Nunatak Phase,” followed by a “Lobate Phase,” in which the ice lobes in the Champlain, Memphremagog, and Connecticut Basins developed, their levels lowered, and their margins receded.
Ice margin recession in Vermont generally was progressive, with a step-down pattern of the receding ice margins, likened to “multiple rings in a slowly draining bathtub.” However, some “rings” were more substantial, helping to define eight levels and times, marking ice margin recession, levels, and ice margin positions in a proposed chronology. However, as just suggested, these “rings” and the associated recession were temporally and spatially complex, with overlapping spatial and temporal relationships described as “Everything, Everywhere, All at Once, and Continuing.” The eight successive levels and times are correlated across the State, representing “stillstands” which may or may not have been climatically related, but other possible causes are identified and briefly explored.
At multiple times in the early history, emerging high elevation physiographic divides served to “Disconnect” different masses of ice in the Connecticut Basin from the parent ice sheet, resulting in en masse stagnation. This finding adds to a long standing debate about wholesale stagnation of ice in this basin.
The evidence indicates that the receding Champlain lobe developed a calving ice margin, with several phases of calving, including 1) a possible initial phase associated with a low portion of the Basin floor, a second phase triggered by the lowering of Lake Vermont from the Coveville to Fort Ann level, and a third phase associated with the lowering from the Fort Ann level to the Champlain Sea. Evidence is found indicating that the receding, calving ice sheet was more convex in shape than conventionally depicted, with both northward and westward recession of the Champlain lobe margin, with ice margin presence close to proglacial strandlines, including both Lake Vermont and Champlain Sea, possibly even continuing into the latter’s, lowering stages.
Correlations of Vermont deglacial ice margins with neighboring regions of Quebec, New York, and New Hampshire, are tentatively suggested, again requiring further study. Evidence of ice margin “oscillations,” is identified and two “readvances”, as suggested by previous research. Whether or not these had climatic significance can not be determined based on the VCGI information in this study. Nor can the scale of such “readvances” be established meaning a) the distances of recession and readvance of the ice margin, and b) the regional versus local geographic extent. The T3/T4 time, level, and ice margin is correlated with the White Mountain Morainic System in New Hampshire, which reportedly is dated as about 13,800 -14,000 years BP. This readvance may correlate with reports of a readvance in the Winooski Basin by Larsen, Wright, and others. Also, evidence is found here which suggests the possibility of such a readvance in the Vermont Valley in the Rutland and Bennington areas. However, reports from the western Taconics in New York indicate that no evidence for a readvance for the equivalent T3 and T4 time has been found.
The later readvance was associated with T8 ice margin feature in the Missisquoi Basin. This readvance occurred in Champlain Sea time shortly after the incursion of the Sea into Vermont which reportedly is dated as about 13,000 years BP. This readvance is correlated with reports in the literature by others further south in the LaPlatte Basin and Bridport area. But neither the climatic significance, if any, nor the scale of this as a bonafide “readvance,” versus a more minor and less significant oscillation and its local or regional geographic event, is established. The possibility is raised that this readvance may be related to the “collapse” of the Champlain lobe.
The dates above for the T3/T4 and T8 ice margins indicates that the entire deglacial history of Vermont as found in this study is surprisingly young and brief. As discussed, physiography around and beneath the Laurentide ice sheet during its recession, specifically in a reverse gradient setting, in conjunction with the development of coalescing water bodies led to destabilization of the ice sheet in Vermont and its accelerated recession, probably beyond a tipping point The possibility is raised that this readvance may be related to the “collapse” of the Champlain lobe. It is suggested that this history bears importantly on modern day ice sheets with similar conditions, especially the Greenland ice sheet. And it is suggested that early humans may have been witness to the ice sheet dramatics.