The lower LaPlatte Basin is a particularly important location, especially given the report by Wright identifying evidence he interpreted as marking a readvance in this area. As noted above, in my previous report I correlated his readvance evidence with the T8 ice margin in Champlain Sea time, and in turn with T8 margins in the Missisquoi Basin to the north and the West Bridport area far to the south. This indicated a long convex ice lobe in T8 time, which is the issue now being re-examined in this Addendum.
To give a larger overview introductory perspective, setting the stage for features identified on LiDAR in this basin and further studied in the field, the screen shot below is taken from my VCGI map at a regional scale showing a substantial portion of the LaPlatte Basin:
The LaPlatte River on this map extends northwestward from the South Hinesburg area to Lake Champlain near Shelburne, with its headwaters just off the map to the south near the village of Starksboro. In my previous report, prior to this amendment, based on my study of LiDAR imagery and information from my field work in the 1960s, I identified multiple ice margins in the LaPlatte Basin. These included the T6 margin as marked by the maroon colored lines, and the T4 margin by the dark green line.
As a side bar note, as previously noted, again in my previous report, ice margins in Vermont were complex, and as a result tell a complex history:
In my early mapping in the Memphremagog Basin I recognized that ice margins were “hybrid,” with both active and stagnant ice margin components, for example with the T4 stagnant ice margin persisting while the active ice margin receded to a new, lower, T5 level. This led to the concept of a “Style described as, “Everything, Everywhere, All at Once, and Continuing.” As also described previously, Hybrid ice margins were as well identified in the Champlain Basin, as for example with the T4-T6 step-down ice margin sequence with a) the active ice margin associated with Bedrock Grooves on the western flank of the Green Mountains, including at Gillette Pond as an outlet channel for Lake Mansfield, which drained via Hollow Brook to the Coveville level of a major multi-tiered delta complex at South Hinesburg, and b) the T4-T6 stagnant ice margins along the foothills. These ice margins in the LaPlatte and nearby areas tell the story about ice recession of both active and stagnant ice margins and as well the presence of local proglacial lakes along the ice margins which progressively lowered and coalesced. This coalescence was significant, with Coveville Lake Vermont developing in T6 time as a regional water body, along a narrow, more or less open “Disaggregated ice margin in T6 time. Changes in the levels of large proglacial water bodies, especially when substantial and sudden, as when the Coveville level lowered to the Fort Ann level, owing to changes in the outlets as an externality, have the capability of dramatically altering the ice sheet. In the Champlain Basin, this lowering in T7 time initiated calving in both the multiple re-entrant tributary basins in the Middlebury Bench, such as the LaPlatte, and as well along the south facing lobe tips of the Champlain lobe in the main basin. The evidence for calving in the LaPlatte includes Ribbed Lacustrine deposits, Headless Deltas, and Thickened Bouldery Lacustrine deposits. These deposits were mapped, again in my previous report, at the head of the LaPlatte Basin and as well along its sidewalls, indicating progressive westward, down-basin, diachronic recession of the LaPlatte ice stream tip in T7 time. However, only the early T7 margin at the beginning of the progression in T7 time was identified and mapped at that time.
As just stated, these hybrid ice margins were also as well diachronic. For example, the T6 stagnant ice margin stood at the head of the LaPlatte Basin at the Starksboro vicinity (immediately south of the above map area) marked by a major stagnant ice deposit, with an outwash plain leading southward to a major delta at Bristol at the Coveville level. This was followed, still in T6 time by ice margin recession to allow for the development of the South Hinesburg Coveville delta, with the late T6 stagnant ice margin marked by a major deposit at the mouth of the Winooski associated with the ice dam for Lake Mansfield in the interior uplands. This deposit persisted with drainage features extending southward into the LaPlatte Basin at the Fort Ann level as marked by Headless Deltas, which in turn were associated with the beginning of calving of the LaPlatte ice stream in early T7 time, and the draining of Lake Mansfield. The recession of the T7 calving margin in the LaPlatte, as just described, thus was diachronic. Again, whereas the initiation of calving in early T7 time at the head of the LaPlatte Basin was marked by a major Ribbed Lacustrine deposit and multiple Headless deltas at the Fort Ann level, additional Ribbed Lacustrine and Headless Delta deposits were mapped on VCGI further westward and downgradient on the LaPlatte basin walls, and Thickened Bouldery Lacustrine deposits on the LaPlatte Basin floor, marking the recession of the head of the calving ice stream westward down the basin to a position at the mouth of the LaPlatte along the margin of the foothills, all in late t7 time. To avoid clutter these deposits are not shown on the above map.
As a consequence of this temporal and spatial complexity it is challenging to represent this deglacial history on simple maps for the LaPlatte and other basins.
As can be seen on the above map:
- The T6 (maroon) ice margin extended along the foothills near South Hinesburg, as part of a sublobe in the LaPlatte, which again extended southward to the Starksboro vicinity (off the map to the south) at the head of the LaPlatte Basin, where the margin is marked by a substantial stagnant ice complex, with an outwash plain graded to the major Coveville delta at Bristol. Lake Coveville at this time occupied the narrow, more or less open water corridor associate with the “Disaggregated” T6 margin. As just stated the T6 margin was diachronic. Further recession in T6 time led to the incursion of Coveville waters into the South Hinesburg vicinity. The major delta at South Hinesburg is multi-leveled with two upper levels marking local step-down proglacial lakes, the third level marking Coveville and the fourth Fort Ann Lake Vermont. The lowering of Lake Vermont to the Fort Ann level at early T7 time initiated calving with the development of the LaPlatte ice stream in the South Hinesburg vicinity.
- The early T7 (dark green) margin indicates the ice sheet at that time in the LaPlatte Basin occupying a relatively broad area across the Middlebury Bench. In general, the Middlebury Bench is low, rolling, uneven terrain west of the Green Mountain foothills. Many bedrock outcroppings show that this terrain is bedrock controlled, generally with thin, patchy till cover which is thicker in lower portions of the Bench. This is in contrast to the re-entrant basins, such as the LaPlatte, in which Ribbed Lacustrine, Headless Delta, and Thickened Boulder Lacustrine soils are found, and as well scattered stagnant ice deposits.
- A late T7 margin (as newly mapped in this Addendum) marked by the light yellow green margin on the above map, indicates a distinct lobe in the LaPlatte Basin, with schematic successive lobe tip positions showing progressive recession in T7 time. This recession is again marked by Ribbed Lacustrine and Headless delta deposits along the LaPlatte basin side walls, and by Thickened Bouldery Lacustrine deposits on the Basin floor, but to minimize clutter these individual deposits are not shown. As just indicated, this margin was not delineated in my previous VCGI mapping. The blue arrows are intended to schematically indicate the ice stream in the LaPlatte and the multiple dashed yellow green arcuate lines schematically suggesting the progressive recession of the ice stream lobe tip.
The following is a second screen shot at a larger scale on which details at the “mouth” of the LaPlatte basin in the Middlebury Bench, along the eastern edge of the “Trough,” are depicted:
This screen shot is with the VCGI tab for Stewart and MacClintock’s surficial geology map turned on, with the purple shading indicating bedrock controlled uplands, thin ground moraine as the olive shading, and the pale red shading as S & Ms’ stagnant ice kame terrace deposits. My VCGI mapping shows a) the Champlain Sea marine limit by the orange line, b) the LaPlatte Champlain Sea delta by the ochre shading, which was associated with T8 time, and c) the late T7 margin as marked by the pale yellow green line. Also shown on this map is the location of Wright’s readvance position, a nearby exposure at the LaBerge farm in the Champlain Sea deltaic deposit, and another location at a Scarp at the LeClaire farm.
In my previous January 2026 report I identified a significant Ribbed Lacustrine deposit near the Town of Charlotte Old Town Dump, which is also marked on the map. This deposit has characteristic ribbing of the terrain, with an upper layer of silt and clay, but a buried stagnant ice core of sand and gravel as identified in a well log and an exposure on the LaPlatte. Stewart and MacClintock mapped this deposit as a kame terrace. This deposit was mapped by me at the T7 level and was thought to represent the end of Fort Ann time. The pattern of the LaPlatte makes a sudden, sharp turn to the west near this deposit, which was thought to indicate drainage control along the receding ice margin. Immediately to the west is the Champlain Sea delta, as mapped by me in my 1972 report, also shown on the above map.
At the LaBerge farm, just a short distance to the west, is an old mostly slumped pit in the deltaic deposit. Stewart and MacClintock mapped this area as part of their kame terrace. This pit was revisited in 2025, prior to this Addendum, and found to be deltaic, but with coarse pockets of gravel suggestive of the possible close proximity of an ice margin. Further, the upper stratum is a laminated silt-clay layer, which is similar to the veneer on Champlain Sea deltas in the Missisquoi Basin where glacial till was also identified by Cannon (1964) and Wagner (1972), interbedded in the Champlain Sea deposits, representing a readvance in that basin in T8 time. Thus, I interpreted the LaPlatte evidence as indicating a T8 margin in Champlain Sea time. This interpretation, along with readvance evidence at West Bridport, also in Champlain Sea time, was one of the stepping stones leading to the concept of a long convex Champlain lobe in T8 time.
Also as part of the long convex T8 lobe, Wright identified an exposure in a deposit along the lower LaPlatte, also marked on the above map, which he interpreted as indicating a significant readvance. The description given by Wright suggests that this deposit is in materials resembling deltaic deposits, at an elevation close to the Champlain Sea level. Thus, in my previous report I interpreted this readvance as correlating with the readvance evidence in the Missisquoi and West Bridport areas, again at T8 time, with this margin drawn as a heavy yellow line on my previous VCGI maps.
Again, as part of the Addendum mapping I identified and mapped the late T7 margin from the LaPlatte Basin southward to the Orwell area marking the southern tip of the Champlain lobe. This identification and mapping of the late T7 margin proved to be fortuitously fortunate, because closer examination of this late T7 margin led to the discovery of Scarps, which came to be understood, as indicated in the preceding discussion above, as marking the lateral shear margin of calving ice streams, and other calving features.
Whereas the presence of a calving late T7 margin recession as just described in the upper LaPlatte Basin, was recognized and delineated in 2025, this present 2026 examination of VCGI and the discovery of Scarps suggests that the late T7 ice margin likely extended as a lateral shear margin eastward along the dashed light yellow green line into the LaPlatte basin in the Bench and as well southward along sloping western flank of the Trough. This shear margin is marked by many Scarps throughout the Addendum map area, which formed in late T7 and Lake Fort Ann time. Whereas the evidence indicates that the T7 lobe in the LaPlatte Basin was an ice stream extending up basin with a receding calving lobe tip, and as well the T7 lobe in the Trough likewise marked a south flowing ice stream, at this point it is not possible to determine if the LaPlatte ice stream preceded the Trough ice stream or alternatively may have formed simultaneously at the same time. The Trough ice stream likely lacked buttressing support of the Bench terrain which may have accelerated flow of the Trough ice stream. Again, blue colored arrows have been added to the map to indicate the main Basin ice stream versus the upper LaPlatte ice stream.
Several Scarps are identified on LiDAR imagery on the above map. Only the westernmost Scarp at the mouth of the LaPlatte Basin on the Patrick LeClaire farm along the frontal slope of the “Trough” has been examined in the field. The following map is a close-up from VCGI of this Scarp with LiDAR imagery turned on:
This is associated with and on a suspected Ribbed Lacustrine deposit, with the ribbing strongly evident, and with the Scarp marking the western margin of the deposit. According to the land owner the surface soils in the vicinity are entirely silt-clay. However, excavations for a new septic system showed that these silt-clay soils in the vicinity of the farm house are about 4-5 feet (1.2 -1.5 m) thick, beneath which excavations showed a sandy or perhaps sand and gravel type soil. This finding is consistent with and supports the interpretation that this is a Ribbed Lacustrine deposit, in this area marking the late T7 margin. Whereas LiDAR imagery gives a sense of a possible Ribbed Lacustrine deposit with slight kamic topography, in the field the terrain is non-descript, relatively flat, and gently sloping to the west, perhaps slightly rolling or undulatory but with no suggestion of kamic topography. The Ribbed Lacustrine deposit, like many of the locations identified elsewhere with Scarps, is a definite, pronounced bench, the Scarp being on the frontal edge of the bench. Again, the terrain is slightly undulatory, with drainage-like depressions associated with the Ribbing, but these are not marked by soil differences, being part of the silt-clay cover. It is possible that the terrain is very slightly constructional with kamic “dimples” but this would not ordinarily be recognized as associated with stagnant ice.
The frontal face of the Ribbed Lacustrine deposit, immediately west of the farm house, corresponds with the Scarp identified on LiDAR imagery. The terrain on the Scarp slopes steeply downward to the west, generally with no topographic evidence suggestive of ice presence. The base of the Scarp, which corresponds with the red dashed line on the above map, likewise corresponds with the juncture between the Ribbed Lacustrine frontal slope and the floor of the small basin occupied by the north-flowing LaPlatte tributary. The terrain west of this break in slope is relatively flat. Postglacial erosion associated with this small tributary has incised a shallow flood plain in the basin floor as can be seen on the LiDAR imagery, with the incision slightly lower and west of the Scarp base. To the north, the base of the Scarp, specifically along the red-dashed line from A to B, is a 30-40 foot high bedrock cliff face (Monkton Quartzite), at the foot of which is a large mass of boulders and cliff talus. In one location is a small linear ridge, about 100-150 feet (30 – 46 m) long, and perhaps 5-10 feet (1.5 – 3 m) high and 30-50 feet (9 – 15 m) wide, but whether or not this ridge may be a Lateral Shear Moraine is uncertain. No exposures of subsurface soils were found anywhere along the Scarp base but the ridge appears to be composed of till.
Based on this field study, in conjunction with the literature regarding shear zones, it is believed that this Scarp formed along a Lateral Shear Zone associated with the main Champlain Basin Trough ice stream. Whereas the evidence as summarized above indicates the presence of a calving ice stream extending eastward, up the LaPLatte Basin, as noted previously the absence of Scarps upstream along the LaPlatte ice stream is taken as indicating less vigorous streaming related to the orientation of the LaPlatte as compared to the main basin Trough.
Wright’s exposure was revisited in 2026 as part of the field work associated with this Addendum. The exposure is located on a steep embankment on the north side of the LaPlatte River. Whereas his exposure is about 10 meters above the River, there are numerous smaller exposures along the sloping terrain, at different elevations. These exposures show the surficial geologic stratigraphy in this area. The lowest exposures show a distinctive, very compact and dense glacial till which is heavily laden with a colorful variety of rounded pebble size stones, and as well larger boulders This till is very similar to the till identified by Wright as a “Lodgement Till.” The hillside above this stratum consists predominantly of bottom-set lacustrine material, with intermixed sand and silt layers, and with interbedded pocket-like deposits of other material, including large heavily cemented blocks of sand and gravel. Wright’s deformation evidence is in the upper portion of this stratum.
Exposures near the upper elevations of this same hillside indicate an uppermost third stratum of silt-clay at the terrain surface, which is also shown by Soil Conservation Service soil maps. At the top of the hillside, the terrain flattens, forming a bench. The terrain of this bench is irregular, perhaps slightly kamic, and is crossed by several swales which resemble those found on Ribbed Lacustrine deposits. The topography of this bench is not like a deltaic deposit, and is at an elevation slightly above the Champlain Sea delta nearby to the west. Bedrock outcroppings further north indicate this bench is constructed of the above described materials, as a Ribbed Lacustrine deposit, against a bedrock upland.
Owing to slumping, the exposure of the deformed strata identified by Wright is no longer visible. The following is a screen shot of this exposure, taken from Figure 6 of his report:
Whereas Wright interpreted this exposure as representative of a “Deformation Till,” which he regards as evidence of a readvance, neither the material in the photo in Wright’s report, nor the material observed in the exposure at this level, can be characterized as a “till,” but instead is consistent with deformation of strata with in the Ribbed Lacustrine deposit, at a lateral shear margin. In fact, Wright’s caption identifies this as “Intensely folded and faulted ice-proximal lacustrine sediments.”
Wright’s exposure is now believed to be part of a Ribbed Lacustrine deposit which correlates with the similar nearby deposit along the LaPlatte near the old Dump, associated with the late T7 margin. The elevation of the stratum in which the deformation is exposed is above the level of the Champlain Sea. Further, there is no topographic evidence in this Locale which would support the presence of the Champlain Sea. Most importantly, the deformation shown in Wright’s Figure 6 is consistent with the description given in the literature of Lateral Shear Zones. The deformation shown in Wrights Figure 6 is at a small scale as can be seen by the centimeter ruler, within the stratification of the lacustrine material. Whereas the three dimensional shape of the deformed strata can not be exactly determined from the two dimensional exposure, in general the deformation suggests shear from west to east, as would be expected in a lateral shear zone.
Thus, it is concluded, based on the entirety of the evidence from previous VCGI study and as well more recent study of the LaPlatte Basin that the Wright “Deformation Till” exposure on the LaPlatte is part of the lateral shear margin in late T7 time, and not evidence for a T8 margin in Champlain Sea time. This interpretation eliminates a major argument for ice presence in this area in T8 and Champlain Sea time, and as well fits with the findings based on many observations as presented in this Addendum for associated ice streaming as part of the collapse of the Champlain lobe triggered by the lowering of Lake Vermont to the Fort Ann level, again all in late T7 time.