Much of the recessional deglacial history of the Champlain lobe is largely related to:
- a readvance in T3/T4 time, marked by a “Signature” pattern of Ice Marginal Channels and stagnant ice deposits,
- followed by a progressive step-down sequence of ice margin and proglacial water body features marking the lowering, expanding, and coalescing local proglacial water bodies,
- which gave way to regional water bodies, first in early T6 time to regional Coveville Lake Vermont,
- followed by Fort Ann Lake Vermont in late T6 and early T7 time,
- and then by the Champlain Sea with a second readvance in T8 time.
The lowering from first Coveville to Fort Ann levels and then Fort Ann to Champlain Sea levels was both substantial and sudden, resulting in destabilization of both the south-facing and east-facing margins of the Champlain lobe. In T8 time a readvance of the Champlain lobe ice margin occurred in close association with early Champlain Sea features at the marine limit.
Owing to substantial proglacial water body depths, the south-facing and east facing margins of the Champlain lobe developed a calving ice shelf, as marked by Ribbed Lacustrine, Headless Delta, and Thickened, Bouldery Silt-Clay deposits, and possibly grounding line Wedges, associated with ice streams, first in the “Deep Lake” portion of the Basin, and subsequently in the re-entrant basins within the “Middlebury Bench, continuing with rapid recession of the south-facing margin northward to the Quebec border, leading to the final exit of the ice sheet from Vermont. These ice margins are described in detail in the preceding , and is not repeated here.
It is believed that whereas the Middlebury Bench initially served as a buttress support for the Champlain lobe, the thinning of the ice sheet over the Bench, as marked by ice margin features, resulted in crevasse penetration to the base of the ice sheet in higher terrain between re-entrant basins in the Bench , which likewise contributed to the destabilization and rapid recession of the Champlain lobe frontal margin. These observations are a testament to the role and importance of physiography of the “Bath Tub.”
In addition, a narrow, more or less open water ”Disaggregated” ice margin corridor concurrently developed along the eastern margin of the Champlain lobe as part of the step-down sequence pattern of ice margin recession, marked by step-down ice margin and proglacial water body features and by Ice Tongue Grooves at the mouths of the Missisquoi, Lamoille, and Winooski Basins, and perhaps Otter Creek Basins. As discussed elsewhere in this report, this eastern corridor likewise developed and opened progressively northward along the eastern margin, from T6 to T8 times, which is a testament to the capability of standing water to rapidly penetrate the ice margin, leading to its destabilization and transformation from a lateral to a frontal margin. It is likely that in late T7 time this corridor extended northward along the ice margin into Quebec, along the south facing margin of the Champlain lobe in the St Lawrence lowland, joining a similar open water corridor penetrating westward along the ice margin, whereby the Champlain Basin was opened to the Champlain Sea.
It is suggested that the conventional thinking of the recession of the Champlain lobe frontal margin as taking place progressively northward is correct, but represents an incomplete and misleading perspective of the receding ice margin recessional history. Instead this recession is believed to have been marked by the recession of both the south-facing and east-facing margins of a persistent convex ice lobe, with a flattened south-facing lobe frontal tip, reflecting the nature of the ice margin. As discussed, the identification of ice margins as being destabilized is not only part of the deglacial history, but as well relates to modern day global warming concerns. The nature of this ice margin is an important part of the deglacial ice margin history story.