As mentioned above, a close association of stagnant ice deposits, kame deltas, and ponded water sediments shows that the eastern lateral margin of the Champlain lobe in time became penetrated by standing waters in a narrow, more or less open water corridor, in a highly crevassed ice margin maize, as described in the literature for modern, present day glaciers and ice sheets as a “Disaggregated” ice margin, as is common for recession in reverse gradient settings. Further, the evidence indicates that standing waters associated first with Coveville Lake Vermont and subsequently Fort Ann Lake Vermont rapidly and progressively penetrated the ice margin and extended northward, along the eastern margin in such a corridor, effectively resulting in the Champlain lobe becoming a progressively longer, more convex shaped ice mass. In time, this continued to and beyond the Quebec border, eventually leading to the incursion of the Champlain Sea.
A “Disaggregated” type of margin is also suggested by the evidence in the Memphremagog Basin at T5-T6 time, marked by kame deltas at the proglacial Lake Memphremagog level interspersed with and closely associated with large ice stagnant ice blocks as marked today by many large kettle holes. It is also part of the story with both the eastern and western margins of Lake Winooski in the interior uplands of Vermont’s Green Mountains.
Further, Ice Tongue Grooves, at the mouths of major Champlain Basin tributary basins, including the Otter Creek, Winooski, Lamoille, and Missisquoi basins, while requiring further study, are interpreted as being associated with the northward penetration of this corridor and the destabilization of the Champlain lobe’s eastern margin, with its transformation from a lateral type margin toward a frontal margin. This destabilization, particularly in conjunction with the lowering of Lake Vermont from the Coveville to the Fort Ann level, is also indicated by the development of calving ice margin features, specifically Ribbed Lacustrine deposits, Headless deltas, and Thickened Bouldery Lacustrine deposits, located in the headwaters of re-entrant basins in the Middlebury Bench, in the proximity of the corridor.
It is believed that the penetration of corridor waters, all in T7 time, associated with destabilization and calving of the eastern ice margin of the Champlain lobe, transforming it from a lateral to a frontal margin, speaks to the power of meltwater, especially standing water along ice margins.
In terms of the nature of ice margins, which is the focus here, the evidence indicates that the close association of the Champlain lobe with both running and standing meltwater, again and importantly in a reverse gradient setting, played a major Glacial Dynamic between the ice sheet and water along its margins. The entire deglacial recessional history of the ice sheet in Vermont including the Champlain and Memphremagog lobes is about both ice margin recession and associated meltwater. This is both an ice sheet and a meltwater story. This Dynamic, between meltwater and the ice sheet, began at an early time in the deglacial history of Vermont but became increasingly important with time, especially so in later times. The evidence suggests that meltwater played a major role in this history, again not just as a Style but as a Glacial Dynamic, whereby the initial recession of the ice sheet in a generally warming climate regime was aided, abetted, accelerated, and to some extent controlled by meltwater, again beginning at an early time and increasing substantially and progressively thereafter.
The role of meltwater is evident in the various stages of the ice sheet recessional history and can be summarized in the following:
- Ice Marginal Channels mark the Nunatak Phase representing the presence of meltwater at an early T3 time.
- With time these channels became more numerous and substantial, and in conjunction with stagnant ice deposits, provide a “Signature,” marking the beginning of the Lobate Phase and a readvance of the ice margin that can be traced across the entire State in Late T3 and T4 time. As suggested, this readvance led to the warming of a fringe portion of the basal ice, which as described, involved unique conditions associated with the readvance favoring Ice marginal Channel formation in conjunction with warming of basal ice in a polythermal “sandwich,” with subglacial meltwater “ponding” under confined hydrostatic head conditions, with Ice Marginal Channels forming at subglacial “spillways” related to terrain irregularities.
- Step-down features mark the progressive recession of the ice margin in T4 to T6 times, with evidence of increasing volumes of meltwater marking the recession of the lateral margin of the Champlain lobe, including Bedrock Grooves, Drainage Lines, and kame deltas, and as well the impressive Shattuck Mountain potholes. Likewise, LiDAR imagery shows the penetration of meltwater into crevassed stagnant ice margins in the frontal tips of ice lobes, as for example on the floors of the Otter Creek and Lamoille Basins.
- As the recession progressed, local proglacial water bodies coalesced along the lateral margin into progressively larger, more regional water bodies, resulting in the development of a “Disaggregated” ice margin, as discussed above, penetrated by meltwater, again forming a more or less open water corridor along the lateral margin. In late T6 time this corridor developed into Coveville Lake Vermont, as marked by closely associated lacustrine deposits, kame deltas, and stagnant ice deposits at this level and time. The recession of the Champlain lobe lateral margin associated with the progressive northward penetration of this “more or less” open water corridor in T6 and Coveville time led to and is marked by an intricate dance-like pattern of both active and stagnant ice margin features, as a “hybrid” ice margin recessional history, providing a record of ice margin recession that was quite complex, as described in detail elsewhere in this report.
- The lowering of Coveville Lake Vermont to the Fort Ann level in early T7 time was a major destabilizing event. It is suggested, as stated above, that this destabilization was associated with the transformation of the eastern margin of the Champlain lobe from a lateral toward a frontal margin, as marked by Ice Tongue Grooves and calving margin features. Whereas, as noted above, the Ice Tongue Grooves require further study, the evidence for this recession is well marked by other features, such as for example marking the opening of the Winooski Basin and the drainage of Lake Mansfield, the successor to Lake Winooski. This destabilization progressed northward in T7 and Fort Ann time along the eastern margin of the Champlain lobe, leading from the opening of the Otter Creek Basin to the Winooski Basin, then the Lamoille Basin, and finally the Missisquoi Basin, ultimately leading to the incursion of the Champlain Sea.
- In early T7 time the destabilization associated with the lowering of Lake Vermont from the Coveville to the Fort Ann level led to ice streaming and associated calving of ice streams in the Middlebury Bench, as marked by Headless Deltas, Ribbed Lacustrine Deposits, and Thickened Bouldery Lacustrine features in these re-entrant basins.
- Subsequently, this recession led to the development of the Trough ice stream as marked by Scarps along the lateral shear margin, Transverse Morainic Ridges marking the grounding line, and Mega-Scale Lineations marking the lubricated base of the ice stream immediately upgradient of its grounding line. The lateral shear zone was a vigorous, highly erosive environment, as indicated by the presence of shallow bedrock along the steepened slopes of many Scarps. Also, the peculiar till with rounded pebbles and a foliated structure identified in the LaPlatte Basin, (described by Wright as “Lodgement Till” as distinct from his “Deformation Till”) has been identified in close association with Scarps elsewhere, indicating the shearing environment associated with Scarps, resulted in the mixing of basal meltwater gravel, with and into this distinctive till at Scarps. Further, the Ribbed Lacustrine deposits which in many places are associated with Scarps, indicate the presence of slow moving, more likely predominantly stagnant ice, along the lateral margins of shear zones. All of this is consistent with the nature of ice streams as reported in recent glaciology literature, providing further evidence about the nature of such ice margin environments.
It should be noted that as explained elsewhere, my understanding of the deglacial history of the Champlain lobe in this later time has evolved, as described per the following sidebar summary:
In my original “final” version, dated January 2026, as given below, I identified three phases of calving. The floor of the Champlain Basin can be divided into three portions, including 1) a slightly higher Middlebury Bench, bounded to the west by 2) a lower and flatter Trough, and 3) a “Deep Lake” (referring to present day Lake Champlain) portion along the Vermont/New York border, with substantially lower elevations. In my January 2026 interpretation I suggested a phase of calving first beginning with Coveville Lake Vermont in late T6 time in the “Deep Lake” sector of the basin, mostly in New York, perhaps helping to explain differences between the deglacial record on the New York versus Vermont sides of the Basin. This was followed by a second phase of calving when Coveville lowered to the Fort Ann level in Early T7 time. The Champlain lobe then stood as a long convex lobe in the Basin at this time, with Fort Ann Lake Vermont waters occupying a narrow, more or less open water corridor along the eastern lobe margin. As already noted, this corridor rapidly penetrated the margin in a northerly direction, resulting in a long, convex Champlain lobe.
Further, again in my January 2026 interpretation, I believed that the lowering from the Fort Ann to the Champlain Sea level in T8 time caused a third phase of calving, while the Champlain lobe remained as a long convex lobe with a lobe tip far to the south, with its margin along the foothills at the eastern margin of the Champlain Bain floor, and with a readvance marked by evidence reported by Cannon (1964) and Wagner (1972) in the Missisquoi Basin, correlated with readvance evidence reported by Wright in the Charlotte vicinity, and by Connally in the West Bridport vicinity. Thus, by this original interpretation, the Champlain lobe stood as a long convex ice mass into a later glacial T8 time. Again, this lobe was closely associated with a narrow, more or less open water corridor along its eastern margin. It was suggested that this corridor may have extended into Quebec, associated with the incursion of the Champlain Sea, prior to a T8 readvance. Owing to the long convex configuration of the Champlain lobe, with both Fort Ann and later Champlain Sea waters in the narrow corridor, a long distance for this this T8 readvance was not indicated or required.
As stated elsewhere, a discussion with Franzi about the presence of a long convex lobe at a late glacial time prompted a follow-up LiDAR and associated field study, out of which came a revised deglacial history in April 2026. Whereas the possibility of an early phase of calving remains, it is now believed that in T7 time the lowering of Coveville to Fort Ann was associated with both the progressive northward penetration of the “Disaggregated” ice margin corridor along the eastern margin of the Champlain Lobe, and the initiation of calving of ice streams in the Middlebury Bench. This resulted in the destabilization of the eastern ice margin with a steepened gradient, as marked by “Ice Tongue Grooves” at the mouths of the Otter Creek, Winooski ,and Lamoille Basins, and associated calving of ice streams in the re-entrant basins of the Middlebury Bench as marked by Headless Deltas, Ribbed Lacustrine Deposits, and Thickened Bouldery Lacustrine deposits, with evidence of the rapid recession of the calving ice tips of these ice streams in T7 time. Importantly, the Middlebury Bench between these ice streams served to buttress the Champlain lobe, but this buttressing support was lost when further recession led to the Champlain lobe being restricted to an ice stream in the Trough portion of the Champlain Basin floor west of the Middlebury Bench. The loss of this buttressing support then led to the rapid northward recession of the Trough ice stream, essentially a “collapse” of the Champlain lobe, to and beyond the Quebec border, still in T7 and Fort Ann time, as marked by Scarps, Transverse Morainic Ridges, and Mega-Scale Lineations. Finally, a readvance of the Champlain lobe restricted to the Missisquoi Basin is marked by the aforementioned evidence in T8 time. By this interpretation the Wright and Connally readvance evidence is interpreted as shearing along a lateral shear margin in T7 time. It is believed that in general the interpretation of “readvances” as found and reported in the literature in many places, including in Quebec, needs to be carefully re-examined with the possibility that this evidence instead was caused by shearing. This underscores the need for direct evidence of climatic cooling for readvances.