According to Meriam-Webster, an “Epiphany is:
“A usually sudden manifestation or perception of the essential nature or meaning of something; an intuitive grasp of reality through something usually simple and striking. An illuminating discovery, realization, or disclosure.”
Near the end of my VCGI mapping I had an “Epiphany,” actually several epiphanies, one leading to another. 1In a sense, – quite simple and striking.. Epiphanies, it seems, for me come at late times in the thinking process, at a time when I have the benefit of retrospective reflection about the many puzzle pieces. In a sense, Epiphanes are part of the maturation process of basic thinking, not unlike paradigm development. The following is a review of several important late epiphanies.
a. Epiphany #1: T3-T4” Signature”
Epiphany #1 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 “streaks” at the mouth of the Ammonoosuc Basin mark an ice margin at the T3 and T4 level and time 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 with regard to Epiphany #1 was the realization that the T3 and T4 margins are mapped and correlated extensively across Vermont, with a recurrent pattern of Ice Marginal Channels on hillsides immediately below which at lower elevations are substantial stagnant ice deposits. This pattern offers a “signature” to greatly aid and support the VCGI mapping throughout Vermont, independent of the usage of the Bath Tub Model. In the Memphremagog Basin, Ice Marginal Channels are especially prominent so as to clearly delineate ice lobes in the major five finger-like tributary basins (including the upper Missisquoi Basin). These features at the T3 and T4 levels are correlative with WMMS features Whereas the T3 margin per se is not mapped in the Memphremagog Basin, this ice margin position, level, and time is substantially and well-marked by numerous Ice Marginal Channels, including above and descending downward to T4 ice margin levels. Further, in the Memphremagog Basin the T4 margin is identified as a “hybrid” type margin, with active ice marked by Ice Marginal Channels at the upper T4 level and stagnant ice deposits at a slightly lower T4 level, with the evidence showing recession of the active ice margin to the next lower and younger T5 position, level and time, while ice remained in the T4 margin stagnant ice deposits, showing continuing drainage graded toward the new T5 ice margin position. This evidence in the Memphremagog Basin indicates overlapping temporal and spatial relationships between the T4 to T5, and the T5 to T6 hybrid margins , referred to as a Style described as “Everything, Everywhere, All at Once , and Continuing,” Further, the T4 margin in the same basin is correlated with the Dixville moraine just across the international border Thus, T3 and T4 times in the Memphremagog Basin are identified as part of a pattern which fits with the White Mountain Moraine System, presumably associated with the readvance as identified by Thompson et al.
A similar pattern is identified in the Champlain Basin, marking the recession of a long convex shaped Champlain ice lobe. Ice Marginal Channels and stagnant ice deposits in the Vermont Valley at Bennington, and similar features at Rutland, at the T3 and T4 times, levels, and positions, respectively, mark the recession of this lobe tip. As part of the pattern, the evidence in the Vermont Valley indicates a progressive step-down recessional sequence marked by progressively lower and younger ice margin and associated proglacial lake features, giving way in early T6 time to Coveville Lake Vermont at the mouth of the Vermont Valley. Again this pattern fits with the step-down recessional pattern identified at the WMMS by Thompson et al. Further, as discussed elsewhere in this report, evidence is found at both the Bennington and Rutland vicinities, which would be consistent with a readvance as at the WMMS. Bennington and Rutland are about 50 miles apart, which would indicate a very substantial readvance. Of course, the low gradient of the Vermont Valley floor might serve to distend the ice margin recessional pattern as compared to the relatively steep slopes at the WMMS, but a readvance of this magnitude is not plausible and deserves further study.
Similarly, the T3 and T4 margins are marked by Ice Margin Channels and associated stagnant ice deposits with a similar step-down recessional pattern for conjoined ice tongues associated with the Champlain lobe in the headwater areas of the Winooski and Lamoille Basins. As noted above, T3 time, level, and margin features, including Ice Marginal Channels and stagnant ice deposits in these basins show a step-down sequence from cols along the divides with the neighboring Connecticut Basin. This pattern gives way first to Lake Winooski and subsequently Lake Mansfield, as mapped by Wright et al, who likewise present evidence of a readvance. VCGI mapping in this area shows that the readvance evidence lies mostly within the footprint of Lake Winooski which is here mapped as marking T4 time, with some readvance features at a higher, earlier level, corresponding with T3 time. As reported elsewhere herein, continued recession in the Winooski Basin is marked by T3 to T7 features in the Winooski and Lamoille Basins, giving way first to Coveville and then Fort Ann Lake Vermont in the former and to Fort Ann in the latter.
Further to the north in the main Champlain Basin is a similar pattern of ice margin recession marked by Ice Margin Channels, Bedrock Grooves, stagnant ice deposits, and associated step-down ice margin and proglacial lake features from the T4 to T7 level.
Finally, a similar recessional pattern is mapped on VCGI, in the Missisquoi Basin, giving way first to Fort Ann Lake Vermont and then the Champlain Sea, with evidence that the T8 margin represents a readvance of the ice sheet of the Champlain lobe in Champlain Sea time. The margin of the T8 lobe extended southward in the main Champlain Basin as a long, convex lobe, with readvance evidence as reported by Wright in the LaPlatte Basin and by Connally in the Bridport vicinity.
Thus, “Epiphany #1,” which again occurred near the end of the writing of this report, recognized the presence of a widespread, recurring pattern, marked by Ice Margin Channels and associated stagnant ice features in many places across Vermont consistent with a readvance associated with T3 and T4 time. Whereas in the Connecticut Basin at the end of T4 time the Connecticut lobe stagnated en masse, elsewhere throughtout Vermont the ice sheet proceeded in a step-down recession. In essence, the WMMS readvance represents the earliest time in the Lobate Phase in Vermont, which was then followed by a step-down recession This “signature” ice margin pattern represents the beginning of this recession across Vermont which can be readily identified by the combination of the lowest Ice Marginal Channels lying immediately above substantial stagnant ice deposits. This pattern can and has been mapped, essentially independently, both with and without reference to elevations in the Bath Tub Model. Of course, this logic may entail a degree of circular reasoning, in that T3 and T4 ice margins identified by usage of the Bath Tub Model are now being said to independently confirm the Model. However, the fact remains that T3 and T4 and subsequent recessional pattern is so recurrent, with Ice Marginal Channels so obvious and prevalent at these levels, that the Signature concept has merit. This raises a question about the nature, formation, and history of the T3 and T4 Ice Marginal Channels, which segues into Epiphany #2.
b. Epiphany #2 – Ice Marginal Channels formed by and associated with a Readvance
Epiphany #2 is more in the form of a hypothesis deserving further study. It has to do with the nature and formation of Ice Marginal Channels themselves at T3 and T4 times, and the recognition that these formed immediately after the readvance as just described. At an early point in my VCGI mapping I recognized Ice Marginal Channels as important deglacial history ice margin markers and as well indicators of ice margin dynamics and associated meltwater drainage. Ice Marginal Channels are perhaps the most common, numerous, and striking of all ice margin features found in this VCGI mapping of Vermont. These features are described in the section of this report above dealing with ice margin features.
Obviously I am not the first to discover and puzzle about Ice Marginal Channels, though different names are used and different theories about their nature, origin, and mode of formation are suggested in the literature. A very substantial literature about such channels and the larger subject of glacial hydrology exists, both within glaciology for present day glaciers and ice sheets, and as well for Pleistocene history studies. A full exploration of this literature is far beyond the scope here.
Beyond their identification and mapping, I puzzled about how such features formed, which thanks to my revisit to the WMMS suggests a possible explanation. One of the questions I pondered was why these Channels are preferentially located on sloping hillsides, usually on the south and east sides of ice lobes, and especially at prominent convex terrain protuberances? This pattern of occurrence is so common that as my VCGI mapping progressed I began to instinctively look preferentially in such places for the presence of Channels, and in many cases in fact identified Ice Marginal Channels in these locations. And why are Ice Marginal Channels always relatively pronounced, but always and only short segments. And why are they characteristically sinuous, with a distinctive wavy form? If these are erosional in nature where is the eroded material? I also recognized that whereas these Channels are the most common and obvious features in such locations, in many places LiDAR imagery also shows associated “streak-like” tonal patterns, which are as yet less well understood. These “streaks” seem to be related to meltwater drainage at the ice margin, but the details about such features are not at this point fully understood. My sense is that such “streaks” deserve further study.
It seems that convex hillslopes in plan view, especially “corners” where ice lobes rounded such corners or passed from a larger basin into a smaller tributary basin, including very small scale topographic irregularities, were especially important. Shilts, who worked and reported on the deglacial history of the Lac Megantic area of Quebec, refers to “pivot points” which may relate to this issue. Ice Marginal Channels are distinctly different from “Ice Tongue Grooves,” the former marking contours and the latter extending down slope, but both may be meltwater features related to such pivot points (if the latter are in fact bona fide features).
As discussed above in the Ice Margin Feature section of this report, I believe Ice Marginal Channels formed along ice margins, specifically beneath the ice, close to the margin, by the action of meltwater, and in association with active ice. To expand on this:
- That they are related to, and mark ice margins seems obvious to me, especially in the Memphremagog Basin where, as discussed in this report, they are so numerous as to give a clear delineation of ice lobe footprints. However, lobate shapes are also numerous and obvious in other Basins.
- That they formed by meltwater erosion to me seems obvious by the very shape or form of the channels – they can not possibly have formed depositionally, or by mechanisms other than meltwater erosion.
- That they relate to active ice and not stagnant ice margins also seems obvious to me. In my first two years of graduate school I had an experience when I was stuck in the “quagmire” of the Kaskawulsh glacier’s stagnant ice margin. As is well known, stagnant ice margins are complex features with a haphazard array of deposits with a complexity, basically a mess of ice, water, mud, and crevasses, that is hard to exaggerate. The terms “kame and kettle topography” or “hummocky” hardly do justice to the irregular and chaotic geomorphic character of stagnant ice deposits. In my opinion, Ice Marginal Channels are far too uniform in shape, and could not possibly have formed in association with stagnant ice.
- That they formed beneath ice margins and not alongside margins likewise seems obvious. Surface water streams have characteristic geomorphic forms and patterns. Ice Marginal Channels always and only are short sinuous segments which in this respect are unlike free-flowing surface water features.
- That they formed close to the margin, so as to mark the margin, and did not form deeper within the ice, closer to the interior, more central basin floors of the ice lobes, is likewise obvious. Ice Margin Channels only formed on hillsides, and never on basin floors. They descend down hillslopes in multiple nested patterns, in many cases approaching basin floors and associated stagnant ice deposits, but never are mixed with stagnant ice deposits on basin hillsides or floors.
- That they only formed parallel or subparallel to topographic contours is likewise obvious, which is important as they must therefore mark lateral ice lobe margins. In many places Ice Margin Channels can be seen to have gradients indicating drainage in directions as would be expected along ice lobe margins, toward lobe tips.
- That these formed predominantly at the south or southeast sides of basins suggest meltwater directed on the ice sheet surface and/or within or below the ice sheet, directed toward downgradient locations by hydraulic head distributions within and/or beneath the ice, toward terrain barriers.
- That these Channels are short segments, with characteristic wavy sinuosity, almost always on localized topographic convexities is taken as indicating confined conduit flow at the base of the ice sheet where such terrain convexities projected upward, as obstructions.
- That they occur almost always in nested series on hillsides across a range of elevations is again significant, indicative of progressive ice margin recession and ice surface lowering.
Taken together these observations lead to a hypothesis (such hypothesizing admittedly being a reasoned form of speculation), that Ice Marginal Channels represent confined flow within and beneath the ice sheet at receding ice margins in reverse gradient settings, with polythermal conditions, where ponded water drainage formed erosional channels on the terrain at the base of warmed ice. Further, that such features represent “outlet channels” for the impounded and confined subglacial waters, in a sense analogous to channels formed at outlets for proglacial lakes. As noted above, this suggests that the receding ice sheet at this early time likely was both a polythermal “sandwich” with warm ice above and below an intervening cold ice layer, – and as well polythermal warm ice at the margin, with cold ice laterally in the interior. Further, with time as deglaciation progresses, again as noted above, the warming of the ice sheet apparently became complete, which explains the absence of Ice Marginal Channels at ice margins formed late in the deglacial history.
As part of my “Epiphanization,” I thus concluded that that Ice Marginal Channels represent “inverted polythermal” ice environments, with the base of a “cold” ice sheet becoming warmed along its margin. This thought lingered and festered. The recognition that the Ice Marginal Channels formed shortly after the WMMS readvance , led to the heart of Epiphany #2, that Ice Marginal Channels may have been favored by the WMMS readvance, whereby the readvancing cold ice sheet over previously deglaciated and warmed terrain may have served to warm the basal portion of the ice sheet in a manner favoring Ice Margin Channel formation. This is not to suggest that all Ice Marginal Channels necessarily formed in this manner. Some Ice Marginal Channels occur above the T3/T4 level which is correlated with the WMMS and its readvance. It is possible that the higher and earlier Ice Marginal Channels were formed by an early readvance, earlier than the moraines at the WMMS, or that the readvance itself was earlier than T3/T4 time. Alternatively, perhaps the nature of the Laurentide ice sheet recession in this region was generally oscillatory in nature, whereby oscillations which are less substantial than a bona fide readvance may have served to warm basal ice for the formation of earlier and higher Ice Marginal Channels. Clearly, this concept needs further study.
c. Epiphany #3 – Vermont Deglacial History here is late glacial time
Epiphany #3 came from piecing together the evidence that a) the T3/T4 margins in Vermont correlate with the WMMS and a readvance which Thompson et al, report as taking place in Older Dryas time, which is dated as about 14.0- 13.8 K.A., and b) that the T8 ice margin likewise marks a readvance that took place after the initial development of the Champlain Sea at the marine limit. 2The dating of T8 as being at a later Champlain Sea time, after the initial opening of the Sea, is documented by till and ponded water sediment on the upper surface of Champlain Sea deltas in the Missisquoi Basin. Whereas the Champlain Sea strandline is diachronic, representing different times at different places, in general the age of this time, as reported by Cronin et al,3 Cronin T., et al, 2008, Impacts of post-glacial drainage events and revised chronology of the Champlain Sea Episode 13-9 KA; Paleogeography, ,Paleoclimatology, Paleoecology ; V. 262 Issues 1-2, PP. 46-60 is approximately 13 K.A.. It is inferrred that the T8 readvance may correspond with the Younger Dryas cooling time. Taken together, therefore , Epiphany #3 is the realization that the totality of the recessional record mapped here on VCGI represents a very late glacial time span, and further that the recession of the ice margin from T3/T4 to T8 time was brief and therefore that recession must have occurred quite rapidly. As stated elsewhere this observation is quite surprising, but is supported by evidence for calving ice margins, Disconnections, Disaggregated ice margins, and other evidence of destabilization of the ice sheet associated with meltwater in a reverse gradient setting, all of which is generally reecognized as being associated with rapid ice margin recession.
d. Epiphany #4 – Evidence related to older deglacial history before T1 – T4 time
Epiphany #4 stemmed from a recent chance encounter with Paul Bierman, who is on the UVM faculty and in a sense succeeded me when I left UVM. Paul provided me with a copy of a paper he recently co-authored with others. 4 This is a pre-print paper, dated July 29, 2024, entitled: In situ Cosmogenic 10Be and 26Al in Deglacial Sediment Reveals Interglacial Exposure, Burial, and Limited Erosion Under the Quebec-Labrador Ice Dome, which is available at the following online url address: https://doi.org/10.5194/egusphere-2024-2233 In their report, the authors present findings from a study of beryllium and aluminum isotopes, documenting multiple Pleistocene glacial events associated with the Laurentide ice sheet over long period of time. According to Paul, he and others are now planning a similar study of the Vermont Pleistocene. Obviously, as has been long and well known, the glaciation of southern New England entailed earlier times, but the Bierman paper suggest multiple glaciations. Thus, Vermont represents the tip of the iceberg, so to speak.
This present VCGI study focuses on ice margin features which as it turns out, as just underscored in the preceding Epiphanies, surprisingly represent only very late glacial time, essentially from Older to Younger Dryas, or very approximately from about 15,000 years BP to 10,000 years BP. Thus, this is an incomplete record of even the last glaciation. T1 and T2 times are associated with Disconnections of the ice mass in the Connecticut Basin, with T3 time representing the first, earliest, highest ice margin for the deglacial history record for the Lobate Phase. The high elevation Ice Marginal Channels identified on VCGI maps mark lowering ice sheet margins in the preceding Nunatak Phase. But obviously, the ice sheet extended further south, prior to these early times, with lobate ice margins only appearing later. Bierman’s paper raises questions for me as to what took place in Vermont prior to the time represented by the findings given here, where is this early record, and what is the nature of this record, if any record exits at all?
The following sidebar represents my thoughts about Bierman’s findings and suggested possible lines of inquiry which may prove suitable for further study:
It is likely that evidence of earlier glacial times in Vermont, prior to the earliest ice margins as recorded by depositional evidence here, may be represented stratigraphically by glacial till, and by erosional evidence, neither of which are examined in this VCGI study. In other words, in early glacial times when the ice lobe margins were still south of Vermont, the ice sheet in Vermont may have been eroding the terrain and depositing till beneath the ice sheet.
With regard to glacial till, this gets to the issue of how and when during glaciation glacial till is formed, specifically lodgement type till, which is a larger topic beyond this VCGI study. However, the evidence as discussed in regard to the Ice Marginal Channels formed in T3/T4 time in association with the White Mountain Moraine System readvance suggests that these features were formed with warm basal ice in an inverted polythermal “sandwich,” and that this is represented by the till identified by Thompson et al as indicative of the readvance. This suggests that glacial till in the region may have been deposited by warm ice, as has been suggested in the literature, as discussed above as one of my “epiphanies.” As such, it is possible, therefore, that the early part of the last glaciation in Vermont is recorded by lodgement-type glacial till. Stated another way, the record of early glacial history associated with the last glaciation of Vermont may be found in such till , and perhaps more specifically in lower, older portions of such deposits.
Whereas, again, neither glacial till nor erosional evidence per se are specifically examined in this VCGI study, the evidence from this present study provides information bearing on earlier glacial times, where further study by isotopic analysis by Bierman’s methodology may be fruitful:
- Overdeepened Basins: As described above, the tributary basins in the Memphremagog Basin are over-deepened and occupied by deep lakes, with ice margins at the T3/T4 level similar to the Valley Heads Moraines similar to the Finger Lakes of New York. The origin or mechanism of over-deepening appears to be related to the ice flow in the Memphremagog lobe becoming constrained by the bedrock topography.
Likewise, the “Deep Lake” portion of the Champlain Basin floor represents an over-deepening, as depicted on a bathymetric map already given and discussed previously above. This over-deepening likewise may be caused or attributable to the constraint of bedrock topography, by bedrock uplands on the Vermont and New York sides of the basin serving to constrain ice flow in the Champlain lobe. To underscore and emphasize the nature and significance of this over-deepening, the following is a portion of the previously shown bathymetric map for Lake Champlain:
The over-deepened basin floor is illustrated by the blue colored shading. With regard to the southern end of the overdeepened trough, fundamentally this corresponds with a “necking” of the physiographic Champlain Basin floor, as can be seen on the following physiographic map, which likewise has been presented previously:
The red line appended to the above map marks the approximate boundary of a low portion of the basement floor which fundamentally represents bedrock controlled terrain with only a relatively thin surficial cover veneer. The apex or “neck” of this basin “funnel” is southwest of Vergennes, and in fact Thompson Point represents a bedrock projection which extends westward toward bedrock uplands across the Lake in New York, effectively representing a bedrock controlled physiographic basin mouth. And likewise, bedrock controlled Adirondack foothills mark the tightening “neck” of this funnel nearby in New York. It is believed that this funnel caused ice flow to accelerate in this funnel, although the over-deepened basin is located upgradient of the “neck,” which suggests complex physical flow dynamics.
Also as presented and discussed above, various researchers have identified and delineated suggested or inferred ice margin positions in the Champlain Basin. For example, the maps below are from two separate reports by Franzi et al, the map on the left from Figure 2 in a report by Franzi et al,5 Franzi, D.A., et al, 2016, Post-Valley Heads Deglaciation of the Adirondack Mountains and Adjacent Lowlands, as modified from a previous report by Ridge , 2003. and the map on the right from Figure 3 in a different report by Franzi et al: 6 Franzi, D.A. et al, 2015, Quaternary Deglaciation of the Champlain Valley woith specific examples from the Ausable River Valley, 29 p.
To be clear, whereas the above maps show a close correspondence between ice margins and physiography, essentially in conformance with a “Bath Tub Model,” it is not suggested here that the ice sheet associated with these ice margins at the time of their formation had anything directly to do with the over-deepening per se. Rather, these margins are shown here only to illustrate the close correspondence of the ice sheet to physiography. It is believed that the over-deepening occurred earlier, perhaps in fact caused by multiple glaciations over a long time such as recognized by Bierman. The above ice margins are seen as draped on pre-existing landscape, which likely formed over a much longer time period associated with the geomorphic history of the Appalachian (Green Mountain) and Adirondack Mountains ,and the intervening Champlain Basin. In fact, the absence of a sediment bulge-like deposit on the basin floor immediately south of the neck on a scale corresponding with the over-deepened portion of the basin floor may be evidence that the over-deepening developed at an earlier time, conceivably with little added deepening associated with the last glaciation.
The intent of the preceding is to make the point that the over-deepened floor of the Champlain Basin is glacial in origin and not fluvial. This observation applies to the entire “ Deep Lake,” including both the northern and southern thresholds. Whereas isostatic rebound has served to flood the southern portion of the Basin, rebound has not fundamentally altered the fact that this “Deep Lake” portion of the Basin floor is a “closed” basin formed by glacial scour, the lower elevations of which are below present day sea level. Physiography provides a reasonable explanation for the formation of the basin by over-deepening related to the constriction of the Basin which again likely resulted in greater ice movement velocity, though the exact physical dynamics of this scour no doubt were complex. In fact, the over-deepenings in the Champlain Basin, the Memphremagog Basin, and as well the Finger Lakes of New York are located north of, which is to say upgradient from the present-day drainage divides, and in that respect are unlike spillways for ponded surface water drainage.
As to the location of the northern end of the over-deepening scour, the pattern of the “Deep Lake” thalweg and its control related to Glacial Dynamics is less clear. An argument can be made that the physiographic constriction of the basin floor is associated with a progressive narrowing of the basin in the latitude of the Shelburne, Charlotte Middlebury areas, or in other words in conjunction with the “Middlebury Bench.” As discussed above, this Bench was significant during deglacial history in regard to calving, but may as well have been important during glacial advance (again, perhaps in the early times of the advance when the base of the ice sheet may have been warm-type ice) when basal ice erosion may have been favored. In theory, perhaps, associated age differences may be recorded isotopically by older erratics in the lower, older portions of the till sheet located downgradient from the over-deepened basin, essentially in New York, or perhaps as well in the Vermont Valley.
Whereas published reports about the Finger Lakes suggest alternative explanations for the glacial formation of these overdeepened basins, these likely were over-deepened by ice scour, perhaps at multiple times in the past as suggested implicitly by the Bierman paper. Thus, these over-deepened basins might be favorable locations for further isotopic study by Bierman and his team. For example, the age of surface of the bedrock would reasonably seem to possibly vary with elevations in the floors and walls of these basins. Further, the till deposited to the south of these basins might provide a “shadow” enriched by “older bedrock” stones becoming progressively diluted in a downgradient direction, again in New York or the Vermont Valley. Conceivably this shadow may extend a long distance in the downgradient direction.
2. Roches Moutonees: In general, the larger topographic, mountain top scale elements of the terrain in both Vermont and New York show markedly asymmetric stoss and lee side topographic differences related to ice movement for both north-south and east-west profiles. For example, the profile of Adirondack terrain when viewed from the Vermont side of the Basin is distinctly asymmetric. My mapping in the 1960s and 1970s and again here with the VCGI mapping suggests that the Champlain lobe developed a rampart of till on the western slopes of the Green Mountains, , in effect as a way for the ice sheet to more easily cross over the mountains in a southeasterly direction. And likewise, the eastern lee side of these mountains tend to be areas of shallow bedrock suggestive of plucking. For example, my older mapping thick showed this till “rampart” is on the order of 100 feet thick or more on the western flank of Mount Mansfield. Accordingly, this area may be a location where the Bierman team might explore isotopic age differences in the bedrock and till reflective of this history.
3. Scabby Terrain Patches:As described elsewhere, isolated patches of such Scabby Terrain , particularly in the Memphremagog Basin, may represent areas of localized erosion associated with ice falls on the lee sides of bedrock knobs, as described by Davies and others in regard to “Disconnections,” as described previously above. Again, these might be areas having age differences, particularly in bedrock and till on stoss versus lee sides of such knobs.
4. Step-down sequence of ice margins: This VCGI study indicates that the deglacial history of Vermont occurred in a progressive step-down sequence, with Ice Marginal Channels in the Nunatak Phase and progressively younger features at lower elevations including glacial and proglacial deposits associated with the Lobate Phase. This pattern of deglaciation corresponds with the observations reported by others, as for example Koteff and Pessel for southern New England, and has been observed by studies as referred to above in New Hampshire, Quebec, and New York. Such step-down deposits and features , which are very numerous in Vermont as identified on VCGI maps, might as well be locations where further isotope study might be fruitful, depending on the sensitivity of the methodology to short time differences.
Footnotes: