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 (VCGI), 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.
As is well known, LiDAR imagery is a game changer for surficial geology, but as well the diverse information provided by VCGI proved to be enormously helpful. VCGI information was used to examinee remote imagery across the State of Vermont. But as well previously unpublished information from my mapping in the 1960s and 1970s, along with recent focused field study, were used to examine the deglacial history of Vermont.
This study encompasses 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.”
As a consequence, this report is not only about the “When” of deglacial history, but also the “Why” and “How” this deglacial history unfolded. It turns out that in order to decipher the When of Deglacial History it became necessary to better understand the Hows and the Whys associated with Styles and Glacial Dynamics. In the end, this report became as much a story about Glacial Dynamics and Styles as it is about deglacial history.
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, though this report which is the first to establish this deglacial history for the State of Vermont, and relationships with neighboring regions, in my opinion has merit – but instead is intended to provoke further study.
The determination of deglacial history was “completed” in January 2026. These findings indicted that the Champlain lobe stood as a long convex lobe with its tip in the southern portion of the basin at late glacial T7 and T8 times, associated with Lake Fort Ann and the Champlain Sea. Thanks to discussions with colleagues, this finding was revisited by an Addendum which was concluded in September 2026. This Addendum identified new evidence related to Glacial Dynamics showing that these early findings were incorrect for late glacial T7 and T8 times. These pre-Addendum findings are retained in order to preserve the arguments which are intriguing. The deglacial history prior to T7 and T8 times was not changed. The September 2026 Addendum presents the new, revised deglacial history for T7 and T88 times.
Deglacial history of Vermont is divided into eight times, designated T1-T8, as depicted on the following map:
A close examination of the above map shows the progressive recession of the ice margins in Vermont through time from T1 to T8 times in the Memphremagog and Champlain Basins. The deglacial history identified on this map and discussed in the report in this report represents the progressive lowering of the Laurentide ice sheet in Vermont with a step-down recession of its margins in a manner referred to as a “Bath Tub” model, with ice margins likened to multiple rings in a slowly draining bath tub. This step-down progression can be seen on the above map by the pattern of T times. The elevations of ice margin features, adjusted for isostatic rebound, were used to help with correlation. In addition, certain ice margin features are distinctive and easily recognizable, as “Signatures,” further supporting such correlation. However, , the determination of deglacial history entails taking into consideration substantial information, in the end involving judgement as to what makes geological sense.
T time ice margins are thought to represent stillstands primarily in that time was required for their formation, with such stillstands likely representing Glacial Dynamics controlled by proglacial water bodies along the receding margin and perhaps the hypsometric configuration of basin physiography. Whereas the data used for mapping are silent with regard to climate, it is believed that the recession was generally associated with a progressively warming climate, without requiring or implying climate changes. However, as discussed below, correlation of late T3 and T8 time ice margins with previously reported ice margin readvances in neighboiring regions indicate climatic cooling.
Not shown on the above map are early margins of the ice sheet in the “Nunatak Phase” when the higher elevations of the terrain progressively emerged. However, the above map gives a sense of these early nunataks by the ochre-colored areas. Ice margins in the Nunatak Phase are marked by Ice Marginal Channels which were identified and mapped but not correlated. These are features formed at the base of the ice sheet along the T1 to late T3 ice margins when the ice was predominantly Cold and Active ice, marking the progressive, recessional lowering of the ice sheet margins, again in the Nunatak Phase.
Careful study of the evidence associated with Ice Marginal Channels indicates that they formed by meltwater fluvial erosion across subglacial terrain protuberance dams in a manner similar to ponded surface water spillways, for hydrostatically confined meltwater in Warm basal ice beneath an overlying Cold ice confining layer. Such features mark the early warming of the ice sheet when it was still predominantly Cold, with the formation of limited meltwater, as a Glacial Dynamic.
The “Lobate Phase” represents a time when the ice became confined into discrete lobes, sub-lobes, sub-sub-lobes, etc, initially in the Champlain, Memphremagog, and Connecticut Basins, which are marked on the above map as delineated by nunatak uplands. The late T3 ice sheet cover and margin, marking the beginning of the Lobate Phase, is identified on the above map.
The Champlain lobe was the most robust, owing to its relatively lower elevation floor and direct and open connection to the parent Laurentide ice sheet to the north in Quebec. The Memphremagog lobe in northeastern Vermont was less directly connected, with a higher floor, and thus became ice free earlier than the Champlain lobe in T6 time. Owing to the higher elevation and constricted terrain of its headwaters, the Connecticut lobe was nourished by ice flow predominantly from the Memphremagog Basin to the northeast and the Champlain Basin to the west, by ice flow across high elevation cols along drainage basin ridgelines between these basins. Ice margin features clearly document ice tongues extending across these cols as feeder ice for the ice sheet in the Connecticut Basin. This ice supply to the Connecticut lobe became terminated at three early times when the level of the ice sheet lowered at these cols, resulting in en masse stagnation of first the ice sheet in the Lower Connecticut Basin in T1 time, followed by the ice sheet in the Middle Connecticut Basin in T2 time, and lastly the Upper Connecticut Basin in T4 time, after a readvance in late T3 time based on a correlation of this ice margin with the earlier, higher portion of the White Mountain Morainic System in northwestern New Hampshire as reported by Thompson et al. The margins of these stagnant ice mass “Disconnections” are marked by “Scabby Terrain,” a newly recognized, distinctive type of ice margin feature on LiDAR imagery. Scabby Terrain forms large tracts along these three Disconnected ice masses, the larger of which are identified by the lavender colored areas on the above maps. Again, this history represents a significant, albeit different type of Glacial Dynamic than described above.
As just described above, Ice Marginal Channels formed at an early time and continuing in the Nunatak Phase, with meltwater penetrating beneath the outer fringe of the ice sheet to form the Channels in a unique way. In general, the evidence indicates that ice warming and meltwater volumes increased in time, exceeding the capacity of subglacial drainage, leading to the development of proglacial surface water bodies along ice margins in the Lobate Phase. These surface water bodies progressively lowered, enlarged and coalesced as new outlets were uncovered, along a step-down type ice margin recession. In Vermont this recession took place predominantly in a reverse gradient setting. The evidence indicates that meltwater served as a very effective thermodynamic penetrant for warming and melting ice, both along a) lateral margins and b) beneath the ice sheet. The warming and melting of ice along both lateral margins and the base of the ice sheet served as an important Glacial Dynamic.
Regarding the Glacial Dynamic along the lateral margin of the Champlain lobe, the evidence indicates that standing water formed in and along “Disaggregated” type margins of the lobe, as described in the literature, with water penetration in a highly crevassed ice margin labyrinth of narrow, more or less open water corridors along the eastern margin of the Champlain lobe. The first such corridor was in Coveville time along the late T6 margin. The evidence indicates that the head of the Coveville corridor was diachronous, reflecting the penetrant ability of standing water, extending progressively northward, reaching the Burlington vicinity when Coveville lowered to the Fort Ann level.
As a consequence of this northward corridor penetration the shape of the Champlain lobe became increasingly long and convex. The lowerings of water levels in these corridors, first from Coveville to Fort Ann in late T6 and early T7 time and subsequently Fort Ann to the Champlain Sea in later T7 time, were both sudden and substantial, serving to a) destabilize the Champlain lobe, b) transform the eastern margin of the Champlain lobe from a lateral type margin toward a frontal type margin, and c) trigger calving in the re-entrant basins within the Middlebury Bench.
“Streaks” are ice margin features identified in the Addendum studyat numerous locations throughout the Champlain basin along the late T6 and early T7 margins, northward beyond the Burlington area to near the Q1uebec border. These Streaks are interpreted as representing a Glacial Dynamic “jolt” to the entire Champlain lobe associated with and caused by the Coveville to Fort Ann lowering in late T6 and early T7 time. To account for this jolt being marked along the entire Champlain lobe margin, including north of the Burlington area, it is suggested that by late T6 and early T7 time:
- The Champlain lobe had become substantially Warm, as evidenced by the absence of Ice Marginal Channels in association with the late T6 and early T7 time Champlain lobe.
- Substantial meltwater existed at the base of the Champlain lobe over a large area in the form of both an extensive network of subglacial tunnels and as well a basal seepage zone at the ice/bedrock contact. Again, this is supported by evidence for such features, testifying to the ability of standing water to penetrate both along the lateral margins and as well the ice sheet base.
- A Cold intermediate layer capable of serving as a hydraulic confining stratum still remained, such that the sudden and substantial reduction of hydraulic head in a basal Warm layer associated with the Coveville to Fort Ann lowering had the ability to destabilize the Champlain lobe as a pressure reduction which rapidly penetrated the entire lobe in conjunction with slower actual drainage.
Whereas the above map gives a sense of these Dynamics, the following map is intended to provide more detailed information for the Champlain Basin:
The blue colored line on the above map represents the Fort Ann strandline, which was closely associated with the late T6 and early T7 margin at the time of the Coveville to Fort Ann lowering. Again, Streaks are mapped throughout the basin, including as far north as the Missisquoi Basin (even though Coveville itself was restricted to the Basin south of the Burlington area), at a range of elevations corresponding with the Coveville and Fort Ann levels and the late T6 and early T7 margins. This lowering is also marked by “Ribbed Lacustrine,” “Headless Delta,” and ”Thickened Bouldery Lacustrine deposits associated with the triggering of calving in the re-entrant basins in the Middlebury Bench at late T6 and early T7 time associated with the Coveville to Fort Ann lowering, as identified on the above map.
This calving was associated with the progressive recession of the calving margins in these re-entrant basins in subsequent T7 time, leading to the clearing of the Middlebury Bench with the Champlain lobe margin along the Trough, as marked by the white line on the above map. The recession of the Champlain Lobe margin to this Trough location resulted in loss of buttressing support, which triggered a first phase of ice streaming and calving in mid-T7 time still at the Fort Ann level, as marked by 1) “Scarps,” which represent the lateral shear margin of the ice streaming Champlain lobe, 2) Mega-Scale lineations associated with streaming, 3) bouldery deposits associated with calving, 4) a Transverse Morainic Ridge marking a pause in the calving recession, and 5) a physiographic constriction pivot point Onset Zone.
Further, in late T7 time, the lowering of Fort Ann to the Champlain Sea, which again took place suddenly and was substantial , triggered a second phase of accelerated ice streaming leading to the possible collapse (?) of the Champlain lobe, accompanied by calving recession, as documented by evidence, northward to and beyond the Quebec border. The Fort Ann to Champlain Sea lowering is marked by erosional gouges on the Onset Zone, which thus give a fix on the position of the Champlain lobe tip at the time of the Champlain Sea incursion. It is suggested that the lowering of Fort Ann and opening of the basin for the incursion of the Champlain Sea took place via a narrow corridor that progressively extended northward into Quebec.
The preceding is intended to emphasize the role of Styles and Glacial Dynamics as important elements in deglacial history. The T3/T4 time, level, and ice margin is correlated with the White Mountain Morainic System in New Hampshire, associated with a readvance which reportedly is dated as about 13,800 -14,000 years BP (Thompson et al). The earlier, higher portion of the WMMs is correlated with the Frontier moraine and the lower, later T4 margin with the Dixville moraine, both in Quebec. The late T3 and T4 readvance is correlated with reports of a readvance in the Winooski Basin by Larsen, Wright, and others. Also, soil and well boring 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 T6 margin is correlated with the Sutton moraine in Quebec.
A late T8 readvance in the Missisquoi Basin, as reported by Cannon (1964) and occurred in Champlain Sea time, shortly after the incursion of the Sea into Vermont which reportedly is dated as about 13,000 years BP. In the original January 2026 report this readvance was correlated with reported readvances further south in the LaPlatte Basin and Bridport area. The Addendum study found and suggested instead that the readvance evidence more likely was associated with the lateral shear margin of the streaming Champlain lobe, thereby disproving this correlation. The readvance evidence in T8 and Champlain Sea time is now interpreted as being restricted to the Missisquoi Basin.
The dates cited above for the T3/T4 and T8 ice margins indicate that the entire deglacial history of Vermont as found in this study is surprisingly young, and took place very rapidly over a brief span of time. The final times of this recession may represent the “collapse(?)” of the Champlain lobe. It is suggested that this “collapse(?)” may have led to the last vestige of the Champlain lobe in Vermont, conjoined with the remnant Ontario and St Lawrence lobes and marked by a minor readvance restricted to the Missisquoi Basin in northern Vermont in T8 time.
The identification of a possible “collapse(?)” of the Champlain lobe is significant, and pertains to present day concerns for global warming. This term and concept of collapse is not used here lightly. Establishing and proving a “collapse,” as to what exactly constitutes a “collapse,” how a “collapse” is defined, and what serves as evidence for and proof of a “collapse,” are challenging and difficult, whether for the geological record for past glaciations, or for modern day ice sheets and glaciers. However, the recession of the T7 margin of the Champlain lobe as found and reported here is regarded as being as close to proof of a collapse as can be established by geological studies. In my opinion, these findings likely represent a collapse. This finding has significant implications for present day sheets and modern ice global warming.
As discussed in the Addendum report, the readvance of the Champlain lobe in T8 time in the Missisquoi Basin, subsequent to the initial Champlain Sea incursion may correlate with findings reported by Parent (2023). This observation leads to a regional conceptual model for late glacial mid T7 to T8 times, as depicted on the following map:
In this model, the Champlain lobe stood as a long convex lobe in mid T7 time, with rapid recession, perhaps as a collapse, triggered by the Fort Ann to Champlain Sea lowering, leading to the exit of the ice sheet from Vermont, followed by a readvance in T8 time back into Vermont, restricted to the Missisquoi Basin. Obviously the entire matter of Vermont deglacial history deserves further study, especially in the context of Glacial Dynamics which bear on modern day global warming and present-day ice sheet instability.