As just stated, much of my new way of thinking about glacial history has to do with LiDAR, which reveals an astounding array of evidence about ice margins. As described above, Ice Marginal Channels represent one of the most dramatic aspects of LiDAR imagery in Vermont. As I noted previously, these features are widely reported in the literature, with different terminology and theories about how they formed. Ice Marginal Channels are very common in Vermont and provide substantial information about glacial dynamics and glacial history. As repeatedly stated above, I spent a lot of time looking at and thinking about the evidence associated with these features.
I first observed Ice Marginal Channels in the Memphremagog Basin where they are very numerous as nested channels at progressively lower elevations, suggestive of the ice margins in the Nunatak Phase transitioning into the lobate phase. Ice Marginal Channels are notably common at slight terrain protuberances, providing a clue about the mechanics of their formation. As discussed previously, these Channels in Vermont are interpreted as ice margin features marking the progressive recessional lowering of the ice sheet, with the Channels formed subglacially by erosion across terrain protuberances in a manner similar to surface water spillways, by hydrostatically confined meltwater, along cold, active ice margins, in and beneath warmed basal ice, overlain by confining cold ice. Progressive recessional lowering of these margins thus led to nested series of such Channels. In some places these features together suggest a hydrodynamic flow systems pattern, with meltwater runoff on the ice surface toward lateral margins, thence in a downgradient direction along receding lateral ice margins, joining with other similar basin drainage to form multi-basin meltwater systems. This interpretation is an expression of Glacial Dynamics and the power of LiDAR.
Ice Marginal Channels in Vermont are thought to represent a Style and Glacial Dynamic related to and indicative of basal ice warming, associated subglacial meltwater drainage, along active and not stagnant ice, again with subglacial ponding behind terrain protuberances, all as part of a Glacial Dynamic. Warming of the ice sheet was progressive, and represents an important Glacial Dynamic element in that warmed ice sheets are more susceptible to destabilization, streaming and collapse. More specific details about Ice Marginal Channels in Vermont are provided in discussions regarding specific Locales. Intriguingly, the absence of Ice Marginal Channels at certain times and locations is likewise recognized as being an important Glacial Dynamic clue. For example, the absence of Ice Marginal Channels for large portions of ice masses in the Connecticut Basin, along with the presence of Scabby Terrain, is evidenceof Disconnections and associated en masse stagnation. Similarly, their absence in association with Streaks, Scarps, and other features in the Champlain Basin at a late T7 time is recognized as evidence indicative of streaming and calving of the Champlain lobe in T7 time, by which time the lobe had become fully warmed. These examples point to the importance of studying ice margin features in terms of their Glacial Dynamic origin as part of the progressively changing and evolving Glacial Dynamic systems as a means for examining and understanding ice sheet deglacial history.
As my study progressed, my understanding of Ice Marginal Channels in terms of both Glacial Dynamics and Dynamic systems and therefore deglacial history evolved. At an early time in this study I recognized that Ice Marginal Channels in the Memphremagog Basin, at the late T3 level and time are closely associated with stagnant ice deposits at the early T4 level and time. Evidence was found indicating that these represent hybrid margins, and that the faster recession of the active ice margin component, while stagnant margins persisted, led to a Style termed “Everything, Everywhere, All at Once, and Continuing.” Such hybrid margins are identified in all basins, and serve as important evidence of a relatively complex deglacial recessional history. I subsequently recognized that whereas Ice Marginal Channels in the Nunatak Phase in T3 time generally indicate the warming of the outer fringe of the ice margin by the penetration of slightly warmed meltwater, the late T3 Channels and their associated stagnant hybrid ice margins are associated and correlated with the White Mountain Moraine System, as part of the readvance of the ice margin as reported by Thompson et al. It thus was postulated that the readvance of the ice sheet over warmed terrain served to more substantially warm basal ice. 1 This Epiphany occurred when I revisited the evidence for the White Mountain Moraine System (WMMS) in New Hampshire, with the realization that the moraines and associated readvance evidence reported by Thompson et al, correlate with many features mapped on VCGI in many locations throughout Vermont, with many pieces fitting together to tell a substantial story. For example, Ice Marginal Channels and associated curvilinear markings on LiDAR at the mouth of the Ammonoosuc Basin mark an ice margin at the T3 and T4 levels and times correlative with WMMS features, for an ice lobe in the Upper Connecticut Basin extending southward to the Bradford, Vermont vicinity where drainage features appear to be graded to Lake Hitchcock. Further, VCGI mapping evidence indicates that the ice supply for this Connecticut basin lobe was from a) ice flow across cols along the physiographic divide between the Lamoille and Connecticut Basin at the T3 level and time, and likewise more substantially b) ice flow across col divides between the Memphremagog and Connecticut Basins at the T4 level and time. When the ice thicknesses across these divides diminished and no longer was able to sustain active ice flow, the ice sheet in the Upper Connecticut Basin stagnated en masse, as marked by Scabby Terrain, and was followed by the development of Stewart and MacClintock’s St Johnsbury “moraine” and Passumpsic esker, both of which are regarded by them as substantial and significant features.
Further, I came to realize that Late T3-early T4 Ice Margin Channels and associated stagnant ice deposits as hybrid margins are easily recognizable on LiDAR as a “Signature” pattern, which greatly facilitates ice margin mapping and lends support to the usage of elevations in the bath tub model. This late T3 and early T4 hybrid Signature can be traced widely across the State. In the Winooski Basin these features are found to be part of the development of Lake Winooski, showing the rapid penetration of Lake Winooski waters around the perimeter of an ice mass in its basin, with rapid recession of a hybrid ice margin as a Glacial Dynamic on the east side of the basin marked by associated eskers , Ribbed Lacustrine deposits, and Headless Deltas, and on the west side of the basin by Disconnections of ice masses in the deeper basins on the east flank of the Green Mountains, with such Disconnected ice masses possibly remaining briefly as active ice masses. Ice Marginal Channels, along with Bedrock Grooves, and stagnant ice deposits document the progressive recession of hybrid ice margins through time, showing that ice margins and associated ponded water strandlines were diachronic, which greatly complicates the study of deglacial history, but again is part of the Glacial Dynamic.
Still further, the late T3 and T4 Signature hybrid ice margin features in the Winooski basin are mapped in close association with features reported by Larsen and Wright, et al, as a readvance,and in the Vermont Valley in the Bennington to Ruland areas, with evidence consistent with a readvance. Ice Marginal Channels and associated stagnant ice deposits at the late T3 and T4 level and time are likewise identified at many locations elsewhere in the Champlain and Memphremagog Basins. The recognition of this Late T3 and early T4 margin Signature greatly helped and supported ice margin mapping by elevations with the Bath Tub Model.
My early, initial mapping in the Memphremagog Basin, where Ice Marginal Channels are very numerous, then progressed into the Connecticut Basin where Ice Margin Channels are present in the Upper Connecticut but are not found in the Southern and Middle Connecticut Basins. I puzzled about this absence and eventually realized that this is due to en masse stagnation of the ice at two separate times in the Lower and then the Middle Connecticut Basins, as evidenced by Scabby Terrain. Further, recession then led to a third en masse stagnation in the Upper Connecticut Basin.
Obviously, the absence of Ice Marginal Channels in the lower and middle Connecticut Basins likewise was an important clue about Glacial Dynamics and associated deglacial history. Similarly, Ice Marginal Chanels generally are not found in association with T7 ice margin in Champlain Basins. This is believed to indicate the complete warming of the ice in these basins at these times with meltwater penetration to the base of the ice sheet. This observation fits with other evidence suggestive of destabilization and perhaps “Collapse(?)” of these lobes at this time.
The importance of thinking about and using ice margin features , such as in this example for Ice Marginal Channels, in terms of Glacial Dynamics and Styles, together in a holistic way in systems approach for deciphering deglacial history, is obvious.