8. Ice Tongue Grooves - vermontdeglacialhistory.org

a. General

In three locations, all in similar physiographic settings near the mouths of the major Champlain Basin tributaries, namely the Lower Missisquoi, Lamoille, and Winooski Basins, are sets of somewhat arcuate, nested grooves in the bedrock terrain, suggestive of lobe-shaped ice tongue margins for sub-lobes projecting up these basins, at the junctions with the main Champlain Basin. Interestingly, at a late time in the writing of this report, as part of an “Epiphany,” as discussed further below, it was realized that linear groove-like features which I had previously identified on the eastern flank of Snake Mountain as “Ice Marginal Lines” may instead be part of multiple, detached, remnant portions of Ice Tongue Grooves at a similar setting at the mouth of the Otter Creek Basin. In the following, each of these four locations are described in detail

Ice Tongue Grooves features are believed to be indicative of destabilization of the Champlain lobe eastern ice margin, perhaps beyond a “tipping point,” caused by the rapid penetration of standing regional waters, first for Coveville, then Fort Ann, and finally the Champlain Sea, along the eastern margin of the Champlain lobe where ice sheet tongues extended up major tributary basins. It is hypothesized that the Ice Tongue Grooves were formed by erosion associated with drainage along steepened ice tongue margins, related to the transformation of the eastern margin from a lateral toward a frontal margin, with re-orientation of ice sheet flow lines and changes in other Glacial Dynamics leading to the final exit of the ice sheet from Vermont.

Whereas considerable information is available for these features, especially for the Missisquoi and Winooski ice tongues, further detailed field study is needed for their confirmation, and therefore the findings presented here are regarded as tentative.

As discussed in Locale CV6, the surficial geology associated with the Missisquoi features is relatively well documented. These Grooves are interpreted to be active ice margin features capable of restraining meltwater, with meltwater drainage extending down the Grooves along a steepened margin, graded to Champlain Sea deltaic sands at the marine limit. In addition, these Grooves include a kame terrace and kettle hole at the head of one of the Grooves indicating the presence of a stagnant ice margin, with drainage graded down the Grooves. Thus, the Missisquoi Ice Tongue Grooves likely represent a hybrid type margin.

The Lamoille and Winooski Ice Tongue Grooves occur in similar physiographic settings, with drainage gradients suggesting a relationship to the Fort Ann Lake Vermont level for the former and both Coveville and Fort Ann for the latter. Whereas limited information about the Lamoille features is available from VCGI study here, in contrast substantial information about the Winooski features and the surrounding area is available. The Lamoille and Winooski Basin features are similar, though different ages, with the Lamoille features formed at Fort Ann time and the Winooski features formed at the Coveville to Fort Ann transition time. The Otter Creek features differ from the Missisquoi, Lamoille, and Winooski features in that, if in fact these features are Ice Tongue Grooves, they occur as multiple, separate pieces across a larger reach of the Otter Creek Basin mouth, perhaps caused by the low slope of the Otter Creek Basin floor which may have served to distend the ice margin across a larger extent of the Basin floor. They may have formed in earlier Coveville time.

Ice Tongue Grooves, if valid, would have major significance bearing on deglacial history, Styles, and Glacial Dynamics. In order to fully introduce such features requires substantial discussion, including advance preview of deglacial history interpretation. In the following, a regional overview perspective is first given, followed by sections dealing with deglacial histories for each of these three sets of Ice Tongue Grooves.

b. Regional Perspective

In general, the eastern margin of the Champlain lobe is marked along the foothills by a closely associated step-down sequence of ice margin and standing water bodies features, indicating the progressive recession of the lobe and coalescence of these standing water bodies into regional water bodies. These step-down features give way at the base of the sequence to deposits associated with regional water bodies associated with Lake Vermont and the Champlain Sea. The temporal step-down sequence varies spatially, or geographically. South of the Lincoln area, which is just south of Middlebury, the sequence of ice margin features represent the T3 -T6 levels and time. T3 margins (but not levels) are absent north of Lincoln due to the fact that the elevation of the T3 ice margin north of the Lincoln area was in the Nunatak Phase. Southward from and including the Winooski Basin, the T3 -T6 step-down sequence gives way to Coveville Lake Vermont deposits. The mouth of the Winooski marks a transition, including limited Coveville and more substantial Fort Ann features. The opening of the Winooski also is related to the draining of Lake Mansfield. North of the Winooski to the Quebec border the step-down sequence is associated with the T4-T7 margins giving way to the Fort Ann level of Lake Vermont. In the Missisquoi Basin the step-down sequence includes the T8 margin, giving way to the Champlain Sea at the marine limit. However, the T7 and T8 margins are also identified at various locations to the south on the floor of the Basin, marking a long, convex lobe in T7 and T8 times. As discussed elsewhere, the low physiographic terrain on the Basin floor served to both promote the southward extension of the ice lobe and as well the development of calving along the south facing margin at multiple ice streams in the Middlebury Bench, and a narrow, more or less open water, Disaggregated ice margin and “Corridor” along the eastern margin. In general, the Champlain Basin lobe developed progressively as a long convex lobe in the Basin, projecting far southward in T7 and T8 time, even at the time of the Champlain Sea “invasion.”

In general, the evidence as just described indicates a south to north ice margin recession of the Champlain lobe frontal tip as deglaciation progressed, increasingly so by the development of a calving ice margin in Coveville trough Champlain Sea times. However, the evidence also indicates that the Champlain lobe eastern ice margin likewise included a progressively enlarging, long narrow, more or less open water “Corridor” which expanded both to the north and to the west with time, reaching the Quebec border and beyond in Fort Ann time.

This understanding of the configuration and recession of the Champlain lobe relates to and departs from the conventional way we tend to think about ice margin recession, as discussed below in the section on “Paradigm Traps.” In general, the step-down sequence of ice margins suggest a long convex shape for the Champlain lobe throughout much of its recessional history in Vermont. At an early time this lobe extended southward into the Vermont Valley in the Proctor and Rutland area, and southward around the nose of the Taconics toward the head of the basin in New York State, giving way in early T6 time to Coveville Lake Vermont. Progressive northward and westward recession of this lobe continued, giving way to Fort Ann in late T6 time, continuing in T7 and Fort Ann time, and finally to Champlain Sea in T8 time, with Champlain lobe in T8 and Champlain Sea time still extending southward in the Basin as a long, convex ice mass.

The lowering of Lake Vermont from the Coveville to the Fort Ann level took place as the result of an “externality” related to the opening of a lower outlet for Lake Vermont. This was a substantial water level lowering on the order of 100 feet (30 m) which likely occurred relatively suddenly. Also, as discussed by Franzi et al, ¹ the Coveville to Fort Ann transition was associated with or was closely followed by the “catastrophic” breakout of Lake Iroquois in the Ontario Basin into the Champlain Basin. Further, this lowering was associated with the draining of Lake Mansfield which was a major proglacial lake in the interior uplands of the Lamoille and Winooski basins. The evidence indicates that Lake Mansfield drained gradually, not catastrophically, but this likewise was a major lowering on the order of 160 feet(49 m). The lowering from Fort Ann to the Champlain Sea levels was associated with the ice margin recession into Quebec, eventually leading to sea water invasion into the Basin. This Fort Ann to Champlain Sea lowering was also substantial, on the order of 200 feet(61 m), which according to Wright in Vermont occurred suddenly, which is suggestive of an ice dam failure in Quebec, likely along the extended more or less open water corridor by which Fort Ann level lowered to the Champlain Sea level, ending Lake Vermont.

The deglacial history involving the step-down recession in a reverse gradient setting, with progressive coalescence of proglacial water bodies from local to regional scale, and as well the suddenness of such water level changes, had a significant impact on the Champlain lobe and the manner and speed of its recession in the Champlain Basin.

This deglacial history likely provides important information bearing on present day global warming, in particular for the Greenland Ice sheet and other ice caps and glaciers, as discussed elsewhere herein. The coalescing and sudden water level lowerings associated with the step-down recession of the Champlain lobe affected the Glacial Dynamics of the Champlain lobe, including:

The development of calving ice margins on the frontal tips of ice streams in the “Middlebury Bench,” as discussed in a subsequent section below. Calving was triggered by the lowering of Lake Vermont from the Coveville to the Fort Ann levels, but it is likely that calving may have begun at an earlier Coveville time caused by locally deeper Coveville waters attributed to the physiography of the Basin floor in the “Deep Lake” area (the evidence for this, if any, is likely to be predominantly in New York). Calving of the south-facing margin likely persisted from late T6 through T8 times.
The progressive development of Ice Tongue Grooves at the mouths of the three Basins, as discussed here. These features are interpreted as indicating destabilization of the eastern margin of the Champlain lobe, associated with reorientation of its streamlines by which the eastern margin transformed from a lateral to a frontal margin. The evidence indicates that water levels and depths in the Winooski, Lamoille, and Missisquoi Basins, east of the Ice Tongue Grooves, was insufficient for calving.
The possible formation of the Shattuck Mountain Potholes, which as discussed separately below represent a remarkable tract with alternative interpretations given by Cannon versus Wright but in either case indicating substantial meltwater drainage volumes and velocities, likely associated with the opening of the Lamoille Basin in conjunction with the formation of the Ice Tongue Grooves in that Basin.

It is likely that this Dynamic was associated with the rapid and accelerated recession of the Champlain lobe, beyond its “tipping point” as reflected by the complete recession of the lobe from Vermont, albeit accompanied by multiple oscillations, including two readvances. The Ice Tongue Grooves are part of the deglacial history ice recession story, but as well tell the story of a Glacial Dynamic related to the demise of the Champlain lobe, which again has significance with regard to present day global warming concerns.

The step-down sequence, again along the foothills from the Winooski Basin southward, giving way to and closely associated with Coveville Lake Vermont deposits, at the base of the sequence, led to the concept of Coveville Lake Vermont in T6 time as occupying a narrow, more or less open water “Corridor” between the ice margin and the foothill terrain. As discussed below in this section and as well elsewhere, for example in the section on “Paradigm Traps,” Coveville waters likely occupied a “Disaggregated Margin” formed by a labyrinth of interconnected crevasses with patches of standing open water in this narrow Corridor. This has to do with our conceptual understanding of the ice margins, again as discussed below. Based on field reconnaissance of the Rutland-Proctor-Brandon area in 2025 it appears likely that more detailed field study of this area may provide further information about the nature of this corridor/labyrinth. The thrust of the Paradigm Trap discussion in a separate section below relates to the configuration of the Champlain lobe as a convex ice mass with recession of the ice margin, from both south to north and east to west.

A distinction is made between stagnant ice margin deposits as found all along much of the foothills in the step-down sequence, with larger, more substantial deposits in the tributary basins .Three distinct settings and ice margin Styles are identified:

Ordinary lateral margin: Along much of the foothills the stepdown sequence is a fringe along the foothills, interpreted as marking a lateral margin, with the distinction of T4 to T6 or T7 margins difficult to make.
Indentations of the ice margin in small tributary basins: Tributary basins such as the New Haven River, the Middlebury River, the Neshobe River, and perhaps Otter Creek.The evidence indicates that these larger tributary basins were occupied by sub-lobes, with the T3 or T4 to T6 ice margin deposits interpreted as having formed along the frontal margin tips of these sub-lobes.
Large tributary basins such as the Missisquoi, Lamoille, Winooski, and Otter Creek: These basins were occupied by substantial ice tongues , marked by features as described here

The evidence indicates that these larger tributary basins were occupied by sub-lobes, with the T3 or T4 to T6 ice margin deposits interpreted as having formed along the frontal margin tips of these sub-lobes. This is in contrast to much of the foothills where similar but generally smaller deposits are regarded as having formed along the lateral margin. The point being made here is that this evidence suggests that the eastern lateral margin of the Champlain lobe was predominantly a lateral margin in Coveville and late T6 times, becoming a frontal margin associated with the lowering to the Fort Ann and then Champlain Sea levels, reflecting the progressive destabilization of the eastern margin of the Champlain lobe in T7 and T8 times. Again, this transition is part of the story associated with Ice Tongue Grooves.

The issue of the nature of the eastern margin of the Champlain lobe as a frontal versus lateral ice margin has to do with the pattern of ice flow within the lobe. The proposition regarding the ice margin transformation is supported by information reported in the literature about striations as indicators of ice movement direction. Such information is not given by VCGI mapping, but reports in the literature bear on this matter. The striation directions reported by Stewart and MacClintock suggest a radial pattern of ice movement in the Basin, with both southerly and easterly components, with for example striations near the floor of the Winooski Basin indicating easterly flow in conformance with the Basin physiography. Cannon, who was a field assistant in the S & M team mapping the State, in his report on the Enosburg Falls area, suggests that the pattern of striations in the northern part of the Champlain Basin indicates that ice flow adjusted during recession of the lobe, becoming more radial, with more easterly ice flow as the Champlain lobe ice thickness diminished and the margin receded. Similarly, Wright’s report on the northern Charlotte area likewise indicates crosscutting striations, with a shift from earlier flow from northwest to southeast to later flow from north to south. This change in flow direction in northern Charlotte likely relates to the development of a calving ice margin in the Middlebury Bench, as discussed below.

As well, this striation story may relate to the thermal conditions of the ice sheet associated with the ice sheet recession, possibly including a shift from cold based to warm based ice conditions. The thermal condition of the Champlain lobe ,initially likely was entirely cold based, shifting to polythermal with warm basal ice, then becoming entirely warm,is an intriguing aspect which deserves further study. For example, this relates to the formation of Ice Marginal Channels, which as suggested above, may indicate polythermal conditions, giving way to warm-based conditions. Ice Marginal Channels generally are not found in the low foothills and on the topographic irregularities of the Champlain Basin floor, perhaps reflecting the development of warm-based conditions. But again, this is a matter for further study.

Finally, as part of this introductional regional perspective overview it is noted that north of the Winooski Basin the step-down sequence of ice margins along the foothills is from T4 to T7, with the T7 ice margin features being closely associated with Fort Ann strandline features, suggesting that the eastern open water corridor/labyrinth progressed northward in T7 and Fort Ann time, first at the Winooski Basin, then the Lamoille, and finally the Missisquoi Basin where and when the transition from Fort Ann to Champlain Sea time occurred, followed by a readvance in T8 time. The presence of Fort Ann equivalent level deposits in Quebec (as “Lake Candona”) indicates this progression extended beyond Vermont into Quebec. Whereas the shape of the Champlain lobe has been reported and is conventionally thought of as having been flattened by calving, with predominantly south to north recession of the ice margin, the evidence here indicates a more convex lobe with flattened tips of multiple ice streams associated with calving on the Basin floor in physiographic “re-entrants” in the “Middlebury Bench,” with both a south to north and east to west recessional pattern of the lobe margins. As explained, this may or may not have involved active ice but may instead have been part of the Champlain lobe collapse.

As a reminder, this is a recessional story, recording the progressive lowering of the ice sheet levels and the progressive recession of its margins, in accordance with the physiography as expressed by the “Bath Tub Model.” As indicated in the preceding, in T3 time, the ice sheet had already thinned substantially, with lobes developing in southern Vermont in the Vermont Valley. However, to the north, in the vicinity of the Winooski, whereas the ice sheet was greatly influenced by physiography, with southward flow in the Champlain Basin as part of the developing Champlain lobe, in T3 time the level of the ice sheet was still in the Nunatak Phase. However, by T4 time the configuration of the ice sheet margins in this area became well defined as discrete lobes, in accordance with the physiography. Thus, we can identify both a main Champlain Basin lobe and as well a Winooski lobe in T4 time. Physiography was a major controlling element, determining the configuration of the ice margins and meltwater drainage.

The role of physiography in the recession of the ice margins is made more evident in this story about the development of the Ice Tongue Grooves and associated history at the mouth of the Winooski. The terrain, or landscape, meaning its physiography, controlled the configuration of the Champlain lobe, again as part of the recessional story, with the main ice lobe in the main basin, and as well sub-lobes in the major tributary basins, specifically the Winooski, Lamoille, and Missisquoi Basins. As recession progressed, physiographic differences associated with progressively smaller terrain elements became increasingly important. This is evident here in the Ice Tongue Groove story, and as well in regard to the calving ice margin story which is described in a subsequent section below. In fact, the calving margin story involves relatively small scale elements of the physiography. Suffice it to say here, that the terrain at the mouth of the Winooski Basin includes the major elements, meaning the main Champlain Basin occupied by the Champlain lobe, but as well the Winooski Basin and its Winooski sub-lobe of this ice sheet. As recession progressed, from T4 to T8 time, the smaller or finer elements of the terrain became increasingly important and actually played a controlling role in ice margin recession and as well the associated management of meltwater, in both flowing and standing forms.

As a side bar:

It is possible to give a sense of the increasing importance of smaller scale physiographic differences, and therefore the function and validity of the Bath Tub Model in the vicinity of the mouth of the Winooski, and thereby to better understand the deglacial history associated with this area. To help explain the physiography at the mouth of the Winooski, it is helpful to have a discussion from the vantage point of present day residents and travelers, and to present a physiographic map that was developed for the Champlain Basin in regard to discussion about calving ice margins. The map shown here was borrowed from the discussion in a subsequent section below about calving ice margins.

Starting from the University of Vermont (UVM), which sits on a hilltop on the Champlain Basin floor in Burlington (near the northern margin of the “Middlebury Bench” which is more substantially south of the Shelburne vicinity), the traveler can look westward to see the lowland of the Champlain Basin with present day Lake Champlain on the floor of the Basin, northward and see the lower terrain of the Winooski Valley, eastward to the opening of the Winooski Basin in the foothills with the Green Mountains as a backdrop, and southward and see the continuing hilly terrain of the Middlebury Bench on the Champlain Basin floor.

The “Middlebury Bench,” as an element of the physiography, is shown below to have played an important role in the development of calving ice margins. A traveler, for example, by car or bicycle on Route 7, or Spear Street, or Dorset Street, can fully appreciate that these routes give excellent views of important physiographic differences that affected the ice sheet, and can appreciate that these physiographic differences are real, not just map elements, and quite substantial

As an aside, the road, hiking, and biking routes southward from Burlington reflect the historical development of Vermont; these routes followed and were essentially dictated historically by the physiography and in a a sense reflect the associated deglacial history. In the mid 1800s these routes were stagecoach lines connecting villages in the Champlain Valley from Burlington to Shelburne, Charlotte, Ferrisburg, Vergennes, Middlebury, and beyond. Interestingly, one could digress and take this further as an anthropomorphic exploration of the colonial development of Vermont, controlled by physiographic elements of the terrain which themselves reflect deglacial history. Thus, for example, it is no accident that the city of Burlington is located at the Winooski Falls, and the cities of Vergennes and Middlebury are located at falls on the Otter Creek, as were various other villages. These falls were important commercial and industrial parts of the history and development of Vermont, having their origins rooted in deglacial history. Similarly, the pathways of, for example, the LaPlatte River, Little Otter Creek, and the New Haven River all can be seen to have reaches which were controlled by the receding ice sheet margins, with major deflections of certain reaches along former ice margin positions. Obviously, this anthropomorphic part of the story is an interesting but quite different tale than the focus here. In the mid 1800s stagecoach lines connected villages in the Champlain Valley from Burlington to Shelburne, Charlotte, Ferrisburg, Vergennes, Middlebury, and beyond. Interestingly, one could digress and take this further as an anthropomorphic exploration of the colonial development of Vermont, controlled by physiographic elements of the terrain which themselves reflect deglacial history. Thus, for example, it is no accident that the city Burlington is located at the Winooski Falls, and the cities of Vergennes and Middlebury are located at falls on the Otter Creek, as were various other villages. These falls were important commercial and industrial parts of the history and development of Vermont, having their origins rooted in deglacial history. Similarly, the pathways of, for example, the LaPlatte River, Little Otter Creek, and the New Haven River all can be seen to have reaches which were controlled by the receding ice sheet margins, with major deflections of certain reaches along former ice margin positions. Obviously, this anthropomorphic part of the story is an interesting but quite different tale than the focus here.

This physiography established and defined the Champlain lobe and the Winooski sub-lobe, controlling not only the ice sheet margin, but as well the disposition of meltwater. Because this recession was everywhere in this area in a “reverse gradient” setting, this necessarily resulted in the meltwater being confined by the terrain as proglacial lakes, per the “Bath Tub Model.”

In a sense the Middlebury Bench represented both a critical buttress support for the ice sheet and its “Achilles Heel.” As a buttress, once recession of the ice sheet “cleared” the Bench, the lower elevations of the eastern Basin floor to the north promoted a more rapid, complete, and final recession. As a weakness, the Bench was at a critical elevation whereby the ice sheet was penetrated by crevasses to the Bench, with re-entrant draIn a sense the Middlebury Bench represented both a critical buttress support for the ice sheet and its “Achilles Heel.” As a buttress, once recession of the ice sheet “cleared” the Bench, the lower elevations of the eastern Basin floor to the north promoted a more rapid, complete, and final recession. As a weakness, the Bench was at a critical elevation whereby the ice sheet was penetrated by crevasses to the Bench, with re-entrant drainage basins within the Bench becoming occupied by calving ice streams. Further, the Bench, as a buttress support, in contrast to the lower Basin floor to the west, served to sustain the Champlain lobe in the Middlebury Bench portion in contrast to the broad spanse of open water in the lowland to the west. This resulted in an asymmetry of the Champlain lobe which, as discussed below resulted in a very different deglacial history on the New York side of the basin.inage basins within the Bench becoming occupied by calving ice streams.

From the vantage point of our traveler, the southeastern corner of the mouth of the Winooski Basin is where one can see the low and wide opening of the Winooski Basin to the east. The scale of this low is impressive, making it easier to appreciate the development of a Winooski sub-lobe as the ice sheet thinned. Similarly, even smaller scale physiographic differences as depicted on physiographic maps can be seen by the traveler to in fact be impressive terrain differences. For example, from the vantage point of Dorset Street, just south of UVM, the Muddy Brook basin, which is a northward draining tributary to the Winooski in the Williston area, can be seen to be an impressive low basin, making its occupation by a sub-tributary lobe of the Winooski sub-lobe easy to envision. For the receding ice margin, this small and seemingly minor basin was important, being occupied by an appendage sub-lobe of the Winooski lobe, marked by features which have been identified by Springston and De Simone ² along Sucker Brook on the eastern flank of the Muddy Brook Basin as ice proximal and which are here interpreted as small Coveville and Fort Ann kame deltas which are part of the record relating to the development of Ice Tongue Grooves. These deposits are evidence of the close proximity between the ice margin and ponded waters in Fort Ann and Coveville times, giving one of many clues relating to the opening of the Winooski Basin, the draining of Lake Mansfield, and the development of the Ice Tongue Grooves.

Springston and De Simone also mapped the surficial geology of Yantz Hill in the Williston area, where the Ice Tongue Grooves at the mouth of the Winooski Basin are located. Their map suggests that several of the Groove floors were flushed clean of unconsolidated material, with floors on bedrock, which is suggestive of and consistent with drainage along the ice margin.

Furthermore, substantial additional information in the vicinity of the Winooski Ice Tongue Grooves exists including “Wave Washed Till” as mapped by S & M, which is here interpreted as marking drainage of Lake Mansfield beneath the ice, suggesting these were related to drainage of Lake Mansfield and the invasion of the Winooski Basin first briefly by Coveville Lake Vermont, and subsequently more substantially by Fort Ann Lake Vermont. This history is discussed in more detail below.

Again, even small physiographic elements had importance bearing on the deglacial history of interest here, which is both an indication of, and a testament to, the validity of the Bath Tub Model. This reflects the fact that as suggested previously that the ice sheet at the time of interest here was sufficiently thin to be very significantly affected by and indeed largely controlled by the physiography, per the “Bath Tub Model.”

c. Winooski Basin Ice Tongue Grooves

As just noted, the information available pertaining to the Winooski Basin Ice Tongue Grooves is substantial, and thus is organized into the following subsections, beginning with a map of the area of interest, followed by a discussion of the deglacial history.

Map of the Winooski Basin Ice Tongue Grooves and Environs

The map below is taken from the VCGI map. For locational purposes:

The Winooski River is labeled, with the Colchester and Essex vicinities to the north.
The villages of Williston and Richmond are close to the present day Winooski River.
The villages of South Hinesburg and Bristol are likewise identified.
The LaPLatte Basin, which was a significant physiographic re-entrant in the Middlebury Bench related to an ice stream and the development of a calving margin, is labeled. The present day 400 foot(122 m) contour marked by the pale yellow line gives a sense of the LaPlatte Basin physiography. The neon blue line marks the 500 foot (152 m) contour along a northern portion of the LaPlatte Basin; this elevation is close to the Fort Ann Lake Vermont level, as marked by multiple caving margin features, including different types of deltaic deposits. Again the calving ice margin is discussed in detail in a later section below.

Geologically, in terms of ice margins, recessional history, and associated Lake Vermont:

The Ice Tongue Grooves, as marked by red arrows, are located, on the northeast flank of Yantz Hill, just south of the Winooski River, east of Williston.
The heavy blue, orange, maroon, sage and yellow colored lines on the map below demarcate the T4, T5,T6, T7, and T8 ice margins, respectively.
The gray colored area, south of Williston, is a major stagnant ice deposit. This deposit includes remarkable LiDAR stagnant ice patterns ( these features in the field are also remarkable) and an associated esker, indicating progressive ice margin recession and substantial drainage to the south from the Winooski Basin via the Muddy Brook Basin, across a divide into the LaPlatte Basin in T6 time, at first into Coveville and then Fort Ann waters in the LaPlatte Basin, and as well a later and younger phase of drainage westward into the Sucker Brook Basin, south of Oak Hill, again first into Coveville and then Fort Ann waters. Field examination of this area in the 1960s and again in 2024 document the presence of substantial, strong stagnant ice deposits.
On the floor of the Winooski Basin at its mouth in the North Williston and Kirby Corners vicinities is an ochre colored sprawling type Fort Ann deltaic deposit. This is the Winooski River Fort Ann delta.
Portions of higher elevation, shallow bedrock terrain above about 1600 feet(488 m) are also marked to give a sense of the physiography by ochre colored areas, although not all such areas are so marked.
Major Gilbert-type deltas at South Hinesburg and Bristol are marked by ochre colored terrain.The former include four levels, including two upper local water bodies, a third at the Coveville level, and the lowest fourth level marking Lake Fort Ann. The village of Bristol is located on a Gilbert-type delta at the Coveville level, with a lower sprawling delta to the west at the Fort Ann level. These Fort Ann deltas mark the transition from Coveville to Fort Ann and the development of calving for ice streams in the Middlebury Bench, in this case for the LaPlatte ice stream and the Little Otter Creek/New Haven River ice stream.
Bedrock Grooves are marked by red arrows in the uplands in the eastern part of the map area. A Bedrock Groove at the T4 level near Shaker Mountain corresponds with drainage toward the Coveville Bristol delta. However, this delta was constructed primarily by drainage from a T4-T6 stagnant ice margin in the Starksboro vicinity for a tributary lobe occupying the headwaters of the La Platte and Lewis Creek Basins. The presence of both active and stagnant ice margin features in this area represents a hybrid type margin.
Another set of Bedrock Grooves at multiple levels at the T5 and T6 margins are in the Gillett Pond vicinity. Springston et al, identified Gillett Pond as the outlet for their Lake Mansfield 1. A Drainage Line(red arrow) marks the drainage for both Lake Mansfield 1 and 2 levels, again per Springston et al, leading to the Coveville delta at South Hinesburg.
Bright green and blue dotted lines extend along LiDAR identified strandlines for Coveville and Fort Ann waters along the north flanks of Oak Hill and Yantz Hill.

The following map gives an enlarged view of the Yantz Hill Ice Tongue Grooves (red colored dashed line arrows) and adjacent features as just referred to:

The Ice Tongue Grooves are on the northeast flank of Yantz Hill. These are associated with the T6 margin(maroon colored line), and are interpreted as indicating their formation in late T6 time along a receding active ice margin. The easternmost Grooves are graded to deltaic deposits at the Coveville level and the westernmost Grooves to the Fort Ann level, associated with the opening of the ice dam for Lake Mansfield. The “Wave Washed Till” deposit as mapped by Stewart and MacClintock on the north flank of Yantz Hill (yellow pattern on the above map) is regarded here as evidence of Lake Mansfield leakage beneath active ice, with drainage into a stagnant ice margin at the T6 level, as marked by the gray colored deposit on the above map, with LiDAR markings and an esker indicating southward drainage into the LaPlatte Basin associated with that basin’s calving ice stream. Thus, the ice recession is marked in this area by “hybrid” type margin features, with recession of the active margin taking place while stagnant ice persisted, in the Style referred to herein as “Everything, Everywhere, All at Once and Continuing .” This evidence underscores and signifies the recession of a temporally and spatially complex ice margin, not a simple line on a map as Vermont ice margins conventionally tend to be regarded. As shown on the above map, LiDAR features on the north side of Oak Hill mark the Coveville and Fort Ann levels.

2) Deglacial History

As can be seen on the above maps, the ice margins near the mouth of the Winooski include the T4, T5,T6, T7, and T8 ice margins, showing the progressive recession of the eastern margin of the Champlain lobe. In T4-T6 time the Champlain lobe extended into the Winooski Basin with recession giving way to first Lake Winooski in T4 time, followed by Lakes Mansfield 1 and 2 in T5-T6 time. As such the Winooski ice lobe served as an ice dam for these upland water bodies. This ice dam required active ice in order to impound the water bodies, with the recession indicated by multiple Bedrock Grooves, as identified in the preceding and depicted on the above VCGI map, while at the same time stagnant ice deposits were developing along the eastern frontal and lateral margins of the Champlain lobe at favorable physiographic locations along the foothills and at the tips of the ice lobes extending upward into tributary basins. This recession led to the opening of a drainage outlet at Gillett Pond for Lakes Mansfield 1 followed by Lake Mansfield 2, with drainage leading to the South Hinesburg Coveville delta, along with the formation of the Bristol delta by meltwaters from a stagnant ice deposit near Starksboro. Together these features represent the aforementioned “hybrid-type” ice margin, similar to that described in the Memphremagog Basin. The major stagnant ice deposit immediately south of Williston represents a part of this T6 hybrid margin.

The T6 ice margin demarcates an ice lobe extending up the Winooski Basin, forming an ice dam in the Winooski Basin for Lake Mansfield. The frontal position of this margin is drawn in the Jonesville vicinity but this position is approximate and uncertain.

In late T6 time extending into T7 time, the evidence indicates that the thinning ice dam in the Winooski Basin began leaking, leading to the lowering of standing waters in the Winooski Basin to the Coveville level, via the standing water penetration of a narrow, Disaggregated ice margin labyrinth corridor, thereby briefly establishing Coveville waters in the Winooski, shortly followed thereafter by lowering of Lake Vermont to the Fort Ann level. This transition is marked by many features, including, for example, the stepped strandline Coveville and Fort Ann features on the north side of Oak Hill, the Ice Tongue Grooves and “Wave Washed Till” feature (the latter of which is here interpreted as subglacial drainage of Lake Mansfield waters into the stagnant ice deposit south of Williston, which transported sediment into the La Platte basin as part of the Thickened Lacustrine sediment on the floor of the basin), and a Ribbed Lacustrine deposit in close association with a Fort Ann deltaic deposit south of Richmond.

The LiDAR revealed patterns in the major stagnant ice deposit south of Williston, and as well an associated esker in this deposit indicate southward flow of meltwater into the LaPlatte Basin associated with the opening of that basin to standing waters associated first with Coveville waters in late T6 time, followed by Fort Ann waters in T7 time. Again, the evidence described subsequently below for the deglacial history of the LaPlatte Basin indicates that this T7 margin was a calving type. In addition to southward flow of Lake Mansfield waters into the LaPlatte Basin via the Williston stagnant ice deposit, the features near Oak Hill likewise indicate the opening of drainage to the west into the Sucker Brook Basin, at first the Coveville and then the Fort Ann level in close proximity to the T7 ice margin.

The northwest portion of this stagnant ice deposit, in the vicinity of Oak Hill, has been examined and reported on by multiple investigators:

Stewart and MacClintock mapped this deposit as a stagnant ice feature, including an appendage extending toward the west along Sucker Brook and Old Creamery Road.
Wagner(1972) likewise identified a portion of this as a stagnant ice deposit; Mud Pond was interpreted as a kettle hole. ³ The western appendage portion was interpreted as a spillway-like channel for a local proglacial water body, with drainage to the west to a delta deposit at the Fort Ann level.
As indicated above, Springston and DeSimone interpreted the channel as outwash from an ice margin at Mud Pond, leading to a deposit along Sucker Brook with evidence that this deposit formed against an ice margin.
In the present investigation using VCGI, this area has been re-examined. LiDAR imagery shows a pronounced channel extending from the stagnant ice margin at Mud Pond, extending to and across the drainage divide into Sucker Brook Basin as a possible spillway or ice marginal drainage channel. The spillway/channel grades westward into deposit remnants with sand and gravel soils, and relatively flat topography, which are interpreted as deltaic at the Coveville level. This deltaic deposit has been eroded along Sucker Brook with a steepening gradient extending southwesterly to another, lower semi-flat deposit interpreted as a delta at the Fort Ann level. This deposit corresponds with the feature reported by Springston and DeSimone on Sucker Brook as indicative of having formed along an ice margin, and is here interpreted as a kame delta at the Fort Ann level. Further to the southwest is a large area mapped by Stewart and MacClintock as “pebbly sand.” This deposit may as well be deltaic at the Fort Ann level. A geomorphic trough associated with the deposit may represent its formation along an ice margin associated with Lake Fort Ann.

To recapitulate:

In essence, the Ice Tongue Grooves at the mouth of the Winooski indicate destabilization of the eastern margin in late T6 time from what formerly was more broadly and predominately a lateral type ice margin, trnsitioning toward a frontal margin, with standing waters in the Winooski Basin falling briefly to the Coveville level shortly followed by the Fort Ann level, including as described subsequently below a calving margin in the deeper Fort Ann waters in the re-entrants in the Middlebury Bench in T7 time. As suggested, the lowering of Lake Vermont from the Coveville to the Fort Ann level likely was a triggering event for calving and destabilization, coinciding in close association in time and space with the opening of the Winooski Basin.
On the northwest flank of Oak Hill the dotted bright blue Coveville margin and dotted bright green Fort Ann lines on the above map are interpreted as marking the receding ice margin and narrow labyrinthian Lake Vermont semi-open water Disaggregated corridor between the higher terrain and the ice margin. The ice margin recession occurred in step-down progression from slightly above the Coveville level to slightly below the Fort Ann level, from the late T6 to early T7 levels and times. These strandlines are believed to represent the progressive northward encroachment and advance of Coveville and Fort Ann waters along the receding ice margin into the Winooski Basin.
Again, the feature mapped as “Wave Washed Till” on the north flank of Yantz Hill by Stewart and MacClintock is here interpreted as having formed by leakage of Lake Mansfield waters beneath the receding late T6 active ice margin. LiDAR imagery shows no geomorphic evidence which would be suggestive of a standing water strandline origin of this feature as interpreted by S & M as being “wave washed till.” It is believed that subglacial water served to winnow fine-grained silt and clay from the till ground moraine in this area, perhaps coincident with ground moraine formation or postdating its formation, with these fine-grained materials being transported southward toward the LaPlatte Basin contributing to the “Thickened Bouldery silt-clay features in that area, as described below in regard to calving ice margins. If the S & M mapping of this deposit (but not their interpretation) is correct it would appear that leakage may have begun at an early time and level, perhaps correlating with the Lake Mansfield level which is at a local elevation of about 660 feet(201 m)and adjusted elevation of 910 feet(277 m), or even earlier and higher, which corresponds with T6 time.
Further to the east, on the northeast flank of Yantz Hill, are the Ice Tongue Grooves, which, as indicated previously, are regarded as tentative and as yet uncertain. The topography of these Grooves as viewed on VCGI topographic maps and LiDAR imagery suggest that a series of eastern grooves may be graded to the Coveville level, as distinct from a series of western Grooves which may be graded to the Fort Ann level. These are interpreted as indicating the presence of the ice tongue ice margin extending eastward into and up the Winooski Basin. The Ice Tongue Grooves are taken as possible evidence of ice margin destabilization, as previously discussed.
On the southeast side of Yantz Hill are: a) A long narrow feature at the Coveville level mapped by Stewart and MacClintock as “Beach Gravel, which again lacks any geomorphic evidence on LiDAR imagery which would correspond with such an interpretation; b) Deltaic deposits and a “Ribbed Lacustrine” deposit at the Fort Ann level. These features and deposits are regarded as further evidence of penetration of Coveville and Fort Ann waters into the Winooski Basin, with formation in close proximity to the ice margin.

The T7 margin is identified in the Essex and Colchester areas as marked by eskers which are graded to a Winooski basin Fort Ann deltaic deposit. This margin extended southward, across the Winooski Basin to the Sucker Brook deposit identified by Springston and De Simone at the Fort Ann level. To the south, the T7 margin is drawn on VCGI as a lobe extending into the LaPLatte Basin, corresponding with a Ribbed Lacustrine deposit in the lower LaPlatte close to but just above the Champlain Sea level. It is likely that this deposit, and others like it to the south, were shortly followed by lowering to the Champlain Sea level at the T8 level. The T7 and T8 margins are traced further south into the Little Otter Creek/New Haven River re-entrant, beyond the map area shown here, likewise marked by Ribbed Lacustrine deposits, in close association with Thickened Bouldery lacustrine deposits and Headless Deltas as noted below, related to a calving ice margin.

d. . Lamoille Basin Ice Tongue Grooves

Ice Tongue Grooves at the mouth of the Lamoille Basin are marked by red dashed lines, per the following VCGI map screen shot:

The location is above Jeffersonville, where the Lamoille Basin transitions from the Champlain Basin floor and the foothills into the Green Mountains. As can be seen on the above map, the Grooves extend down the hillside on the south side of the Lamoille valley from the T4 margin(blue line), through the T5 and T6 margins(orange and maroon lines) to near the valley floor in conjunction with the sage colored T7 margin. In general, the T4-T7 margins in the Lamoille Basin, including the above map area and as well the area to the east, mark the progressive step-down recession like the oft stated “multiple rings on a slowly draining bath tub.”

As discussed below in the section of this report dealing with deglacial history, without delving into details here, the margin of the Champlain lobe stood at the mouth of the Lamoille and along the foothills to the north and south, as marked by the blue line T4 ice margin, the orange line T 5 ice margin, the maroon line T6 ice margin, and the sage colored T7 ice margin. These ice margins correspond with numerous ice margin features both to the north and south along the foothills, and as well within the ice tongue in the Lamoille Basin, again marking the progressive step-down recession of the ice sheet in the main basin and the Lamoille basin. As can be seen, Ice Tongue Grooves descend from the T4 margin high on the hillside to the T7 ice margin on the Lamoille Basin floor in close association with a substantial kame delta deposit at the Fort Ann level on the basin floor, likewise with ice margin recessional markings.

The T4, T5, and T6 margins extend up-valley in the Lamoille Basin, marking the Lamoille ice tongue, where they are associated with kame deltas marking the strandlines of Lake Winooski and Lake Mansfield, and thus the Lamoille lobe of the ice sheet served as an ice dam for these water bodies, as in the Winooski Basin. Recessional ice margin features on the floor of the Lamoille Basin mark the progressive down-valley recession of the Lamoille lobe ice margin in T6 time, suggestive of a relatively rapid recession of the ice margin. The above map shows the westernmost T6 and T7 lobe tips associated with recession west of the village of Johnson.

Also as can be seen on the above map, the T7 margin lobe tip is marked by a deposit mapped as a kame terrace on the State surficial map, and a nearby deltaic deposit at the Fort Ann level, which is on the floor of the Basin just below the Ice Tongue Grooves. The step-down ice margins on the hillsides to the north and south of the Lamoille Basin likewise show a progression downward toward the Fort Ann level. Again, the Ice Tongue Grooves appear to be graded to the deltaic deposits at the Fort Ann level.

Thus, the Ice Tongue Grooves appear to be related to the recession of the Lamoille lobe from an early time when it served as an ice dam for local upland lakes into the time when the basin opened for the invasion of Lake Vermont at the Fort Ann level. If, as suggested here, the Ice Tongue Grooves formed at the margin of the Lamoille lobe ice tongue as it receded westerly, the question arises as to the formation of these features in the context of the “Bath Tub Model.” The Ice Tongue Grooves suggest relatively steep gradients of the lobe tip, with drainage from relatively high levels at 1200-1400 feet(366 – 427m) at T4 time or T5 levels at 1000-1100 feet(305-335 m), downward to the T6 and T 7 levels at 800-900 feet(244 – 274 m) and 600-700 feet(183 – 213m), whereas the Bath Tub Model operates under the presumption of relatively low ice surface gradients at each ice margin position and time. Thus, the relatively steep gradient of the Ice Tongue Grooves is problematical. This is as well the case with the other Ice Tongue Grooves.

The answer to this apparent conundrum is suggested by relatively recent glaciological literature, as discussed below, which indicates that ice sheet margins, including ice tongues fronted by standing water, tend to be unstable, all the more so for reverse bed gradient conditions, leading to destabilization, sometimes beyond a “tipping point” whereby destabilization may become self-reinforcing and thus continuing. These conditions tend to cause the destabilization of at least that portion of the ice sheet, as a Glacial Dynamic, with lowering of ice sheet margins, steepening of ice surface gradients and adjustment of ice stream flow directions, with accelerated steaming toward the water bodies, with increased ice margin recession, with or without calving depending on the local conditions. These conditions persist as the ice sheet attempts to adjust back toward a “normal,” glacial dynamic equilibrium condition. In this case, restabilization would have involved the transformation of the eastern margin of the Champlain lobe from a lateral margin to a frontal margin, at a new, lower frontal ice margin level. This interpretation applies to the Missisquoi and Winooski Basin Ice Tongue Grooves, and perhaps as well to possible Ice Tongue Grooves at the mouth of the Otter Creek Basin.

e. . Missisquoi Basin Ice Tongue Grooves

These features generally are similar to the features at the Lamoille and Winooski Basin, except that they mark the T8 margin in close association with the Champlain Sea. These are not discussed separately here as these features are discussed in detail below in conjunction with Locale CV6. However, the nature and historical significance of these features is similar to the discussion above for the other Ice Tongue Grooves.

f. Otter Creek Ice Tongue Grooves

After I had written much of this report, my continued thinking led me to reflect about Ice Tongue Grooves I had identified at the mouths of the Winooski, Lamoille, and Missisquoi Basins. As discussed below as one of my “Epiphanies,” it seems to me that if my interpretation of these features as Ice Tongue Grooves is correct is correct, then an examination of other basin mouths would be warranted. This led me to the mouth of the Otter Creek Basin, near Snake Mountain. In fact features identified in that area would seem to fit with the hypothetical interpretation, or model, for such features and associated deglacial history.

The following map shows the features identified as possible Ice Tongue Grooves at the mouth of the Otter Creek Basin:

These are located on the eastern flank of Snake Mountain, above and successively down to the T6 level, which along with Buck Mountain to the north, mark the Basin mouth opening. These are located on the eastern flank of Snake Mountain, above and successively down to the T6 level, which along with Buck Mountain to the north, mark the Basin mouth opening. The thin black dashed lines on the eastern flank of Snake Mountain represent the suspected Ice Tongue Grooves.

Whereas these features likely reflect bedrock fabric, it is possible that drainage along the receding ice margin enhanced the linear depressions. Unlike the Ice Tongue Grooves mapped elsewhere these linears are not so markedly graded downward to a common level. However, this may reflect the low gradient of the Otter Creek Basin whichmay have distended the ice margin. Evidence for the close presence of an ice margin in the area is given by the Stewart and MacClintock map which shows a kame moraine deposit nearby to the south at DeLong Hill at an elevation of about 440 feet (134 m). This deposit is part of the evidence cited here, along with other evidence such as Headless Delta deposits, in support of a calving ice margin in close association with Fort Ann Lake Vermont. LiDAR imagery shows a moraine-like ridge nearby at West Cornwall at a similar elevation, in close association with a possible Ribbed Lacustrine deposit. LiDAR imagery of the Snake Mountain area shows a gravel pit on the south side of Snake Mountain at about this level, and a bench-like feature on the north side of Snake Mountain also at a similar level; these features may be Fort Ann strandline features but circumstantially the evidence raises the possibility of a close proximity of an ice margin at the T6 level, whereby Snake Mountain would have stood as a nunatak above the ice sheet at this level. Accordingly, the linears on the east flake of Snake Mountain can reasonably be interpreted as marking the recession of an ice margin, again at the mouth of Otter Creek Basin, in a manner consistent with Ice Tongue Grooves.

The difference between the mouth of the Otter Creek Basin versus other major Basins where Ice Tongue Grooves are identified is believed to be fundamentally attributable to physiographic differences in the configuration of the funnel- like openings of tributary basins. The gradient of the destabilized lateral ice margin stood against Basin mouth “funnels,” which especially for the Winooski and Lamoille Basins were relatively steep, sharp, and strong for both the ice sheet gradient, in conjunction with standing water at the ice margins. In contrast, the Otter Creek Basin mouth “funnel” is relatively less well defined, with a lower gradient, and at a position closer to the frontal lobe tip. As a consequence, the Champlain lobe ice tongue margin at T6 to T7 times at the Otter Creek Basin mouth, in association with and in response to the Coveville to Fort Ann transition, became much more distended in contrast to other Basin mouths and associated Ice Tongue Grooves.

g. . Published Literature Relevant to Ice Tongue Grooves

Grooves formed at the margins of ice tongues are identified in the literature, much of which pertains more generally to calving ice margins. A thorough review of this literature is beyond the scope of this present report. A report by Quiquet et al(2021) ⁴ cited here as examples of this literature. The following web site from the National Snow and Ice Data Center provides a diagram and description related to calving ice shelves which is especially applicable here: https://nsidc.org/learn/parts-cryosphere/ice-shelves/quick-facts-about-ice-shelves#anchor-how-do-ice-shelves-form-

The web site states: “Two types of events occurring on ice shelves have attracted the attention of scientists. One kind is iceberg calving, a natural event. The other kind is disintegration, a newly recognized phenomenon associated with climate change.” Further, “Warm ocean water can break up an ice shelf into many pieces. The scientific community is adopting the term “disaggregation” in the most extreme cases where an ice shelf gets to the point that it looks like spilled Chiclets, a brand of candy-coated chewing gum.”

The reference to disaggregation with a “Chicklet” type ice margin is conceptually consistent with the penetration of standing water along an ice margin which is highly crevassed as suggested here for the step-down margin of the Champlain lobe giving way to standing water bodies, as described above. This concept, specifically “disaggregation” and associated destabilization, is believed to be pertinent here. The evidence indicates, again as discussed below, that calving of the frontal tips of ice streams occurred in the re-entrants in the Middlebury Bench, but that calving of the Winooski, Lamoille, Missisquoi and Otter Creek basins ice lobes generally did not occur, presumably owing to the relatively shallow depths of Lakes Coveville and Fort Ann in these basins.

The above quoted Web text illustrates and underscore the effectiveness of meltwater and standing water, especially with changes in standing water levels, as an accelerant for water penetration along and through fracture networks in the ice margin, resulting in destabilization. The reference to “Disaggregation” suggests a highly fractured, labyrinthian ice margin by which Fort Ann waters were able to penetrate the “Chicklet-like,” fragmented narrow corridor along the ice margin, first into the Winooski, and then northward from the Winooski basin to the Lamoille Basin and continuing northward to the Quebec border and beyond, leading to the opening of the ice barrier with the incursion of the Champlain Sea, which is likewise marked by Ice Tongue Grooves in the lower Missisquoi Basin. Again, this is discussed in more detail in the Deglacial History Section below.

In essence, the observation that the Ice Tongue Grooves span a range of elevations is consistent with the steepening of the ice sheet surface gradient toward the east, thus resolving the aforementioned “conundrum.” Whereas the ice sheet surface, by its steepening departed from the Bath Tub Model, this explanation for Ice Tongue Grooves is compatible with and actually strengthens the Model as the Grooves mark the ice sheet transition toward a lowered eastern margin in transition from a lateral to a frontal margin. Destabilization dynamics are the ice sheets way of shifting toward and re-establishing “normalcy,” speaking to the power of standing and flowing water as an accelerant for ice recession and control of ice margins The ice sheet surface at the time of the formation of the Ice Tongue Grooves time had a relatively steep lateral gradient toward a base level dictated by the standing water body at the ice margin in the corridor.

It is believed that Ice Tongue Grooves and associated corridor represent a unique, distinct and very important “Glacier Dynamic.” The evidence indicates, again as presented and discussed in the Deglacial History section below, that the ice margin was grounded in the foothills, but that low, standing water areas in the foothills became progressively interconnected forming very irregular pathways within a narrow corridor for the northward advance of local proglacial lake waters within and between the grounded ice mass and the mountain front in early T6 time, and that lowering of Lake Vermont from the Coveville to Fort Ann levels caused instability resulting in opening or widening of the narrow corridor associated with the draining Lake Mansfield, and development of a calving ice shelf on the south facing front of the Champlain lobe.

Of course, again, all of this is predicated on the premise that Ice Tongue Grooves are valid features, in a manner consistent with this explanation, which is a matter for future investigation, especially by field study. If this explanation is correct it suggests that the recession of the Champlain lobe was controlled and accelerated by the action of standing water from an early time. Because the chronologic information contained in the geologic record given here is all in relative and not in absolute terms, it is not possible to establish the actual speed or duration of the recession, but the evidence suggests that this recession was relatively fast and brief, which is supported by evidence related to calving ice margin features as discussed below.

As just explained, the explanation for Ice Tongue Grooves given above represents a model of considerable importance for the deglacial history of Vermont, and by implication may as well bear on modern day ice sheets and the issue of global warming. It appears that destabilization occurred during deglaciation of the Champlain Basin, especially at later times when meltwater and proglacial standing waters were substantial and had coalesced into regional water bodies. The extent to which this was local versus having more regional impact on the ice sheet beyond Vermont is uncertain, but given that Ice Tongue Grooves are identified in multiple locations in the Champlain Basin, from Coveville and early Fort Ann to late Fort Ann time transitioning into Champlain Sea time, suggests that this represents a major Glacial Dynamic representing a period of recession induced by the effects of accelerated standing water penetration along lateral ice margins.

Footnotes:

1
Recession of the ice margin opened an interior lowland area portion of the Winooski and Lamoille Basins, east of the Green Mountains, where the step-down sequence gave way to proglacial water bodies associated with large proglacial water bodies extending across both basins, as identified by, for example, Stewart and MacClintock in their Statewide mapping, and Wagner(1972). These lakes have been depicted and named Lake Winooski and Lakes Mansfield 1 and 2 by Springston et al. The older, higher Lake Winooski drained across an outlet near Williamston, into the Connecticut Basin. Springston et al have identified an outlet for Lake Mansfield 1 at Gillett Pond, east of South Hinesburg and a lower drainage outlet for Lake Mansfield 2 at Hollow Brook, both draining to a major delta at South Hinesburg at the Coveville level. In the VCGI mapping here, ice recession at the T4 level gave way to Lake Winooski, and at the T5-T6 levels to Lakes Mansfield 1 and 2.
2
Springston, G. and DeSimone, D., 2007, Surficial geologic map of the Town of Williston, Vermont; VT. Geol. Survey Open File Report VG07-5.
3
In the 1970s, this kettle hole was probed, finding its depth to be about 50 feet. A C 14 date from the base of the peat gave an age which is too young, likely representing cross-contamination
4
Aurélien Quiquet, Christophe Dumas, Didier Paillard, Gilles Ramstein, Catherine Ritz, Didier M. Roche 2021Geophysical Research Letters Volume 48, Issue 9 e2020GL092141