a. Locale W1
The previous discussion of Locale CB2 included aspects of this Locale, which encompasses the foothills area near the mouth of the Winooski Basin, per the following screenshot:
This is a relatively recent screenshot which is up to date and representative of the deglacial history as presently understood.
As previously, features on the VCGI map are color coded. including the following:
- The heavy black line marks the 2000 foot(610 m) “local” elevation contour.
- The large ochre colored areas in the uplands represent terrain above 1600 feet local elevation.
- Ochre colored deposits in the lowlands indicate deltaic deposits.
- The red dashed lines trace Drainage Lines.
- The orange shaded areas indicate deltaic deposits.
- Gray shaded areas designate stagnant ice deposits.
- The blue, orange, maroon sage, and yellow lines demarcate the T4 1200-1400 foot,T5 1000-1100 foot(305 – 335 m), T6 800-900 foot(244 – 274 m), T7 600-700 foot(183 – 213 m), and T8 500-600 foot(152 -183 m) ice margins respectively.
The T5 and T6 ice margins, as shown on the above map, is marked by a substantial stagnant ice deposit just south of Williston. LiDAR imagery of this deposit includes lobate forms suggestive of progressive ice margin retreat during T5 -T6 times and the formation of this deposit. These features clearly and impressively show the main Champlain lobe at this level extending southward in the Champlain Basin against the foothills, and as well suggest an ice tongue sub-lobe wrapping around the nose of the Winooski Basin mouth, extending eastward into the Winooski Basin. Field examination of this deposit in the vicinity immediately east of Lake Iroquois show impressive, very impressive stagnant ice topography with very pronounced semi-linear ridges. This area shows some of the best, strongest examples of stagnant ice deposit terrain found anywhere in Vermont.
Whereas this deposit represents a stillstand in so far as this is a significantly large deposit which likely required a prolonged time for its formation, the lobate patterns in this deposit shows progressive recession or development during T5 time. This is an example, as found in many locations, indicating that the major ice margins, in this case for T5, represent progressive, incremental recession, even during stillstand times. As discussed in the main text of this report, the T5 and T6 margins extended into the Winooski Basin, forming an ice dam for Lake Mansfield at Gillett Pond, which is marked by a Bedrock Groove. Without repeating all of the details, the Ice Tongue Grooves and the “Wave Washed Till” at Yantz Hill, here interpreted as part of the leakage of Lake Mansfield, are depicted. Again, this VCGI map presents the evidence cited in the text for significant recessional events, including portions of the upper LaPlatte Basin where calving occurred in late T6 time and continuing in T7 time at the Fort Ann level. The T8 ice margin and calving associated with the Champlain Sea in the LaPlatte Basin is just off the map area to the west.
As discussed above, the recession of the ice margin and in particular the lowering of Lake Vermont from the Coveville to the Fort Ann level was a significant event related to the development of a calving ice margin in the Middlebury Bench and the opening of the Winooski Basin. The following examines the progressive recession of the ice margins related to the opening of the Winooski Basin.
A starting point is with the T4 ice margin. As discussed in Section III and depicted by the Statewide map the ice margin in T4 time extended far into the uplands in the Winooski and Lamoille Basins and was associated with the development of proglacial Lakes Winooski and Mansfield.
This map shows the area of Locale W2 at the mouth of the Winooski in T4 time. The T4 margin is designated by the heavy, dark blue line, and the T4 ice sheet by the blue shading. As can be seen, the T4 margin extended into the Winooski Basin through a narrow gap in the mountains, where the T4 margin in the interior uplands was associated with Lake Winooski(east of the above map area). The T4 margin also extended into the Huntington Basin, west of Camels Hump, where the margin is marked by substantial stagnant ice deposits. It then continued around Shaker Mountain into the Champlain Basin, continuing southward along the foothills, beyond the map area along the Champlain lobe margin. Drainage along the foothills in the map area in T4 time, south of the Winooski, is marked by a Bedrock Groove, as marked on this map, as was typical along the foothills to the north.
A sub-lobe developed in T4-T6 time in the LaPlatte re-entrant(located on the above map), with a T4-T6 stagnant ice deposit near Starksboro(immediately south of the map area), from which meltwater led to the formation of the Bristol delta in the narrow Lake Coveville “corridor”(also to the south and not shown). The South Hinesburg delta likely was also forming in T6 time, with Coveville penetration along and through the T4-T6 ice margin “corridor. ”
This map shows the T5 lobe. The ice lobe in the Winooski Basin receded in T5 time, with a sub-lobe into the Huntington Basin to a Bedrock Groove slightly higher than the outlet for Lake Mansfield, possibly representing incipient drainage of Lake Winooski downward to the Lake Mansfield level.
Importantly, the stagnant ice deposit south of Williston in a low saddle area north of the LaPlatte re-entrant began to develop at this time, with Drainage Line evidence of substantial southward drainage within the ice margin. Again, the narrow Coveville corridor in the LaPlatte basin is not shown, but deltaic deposition at the South Hinesburg and Bristol deltas into this corridor continued at this time. The width of the ice dam at T5 time between Lake Mansfield at the ice lobe front in the Winooski basin and the stagnant ice deposit at Williston was about 7 miles.
This map represents T6 time.
In early T6 time, the Bedrock Groove at Gillette Pond developed, initiating Lake Mansfield, with drainage to the South Hinesburg delta at the Coveville level in the narrow Coveville corridor.According to Springston and Wright, Lake Mansfield involved two stages: Lake Mansfield 1 was controlled by the Gillett Pond spillway with drainage into a local proglacial lake in the Huntington Basin, which in turn drained via Hollow Brook to the South Hinesburg delta. Lake Mansfield 2 followed after slight recession, opening the Huntington Basin, with continued drainage at Hollow Brook related to the South Hinesburg delta.
The subsequent opening of the Winooski Basin to Coveville Lake Vermont and the termination of Lake Mansfield, followed by the Fort Ann invasion and the development of Ice Tongue Grooves, and as well the development of a calving ice margin in the Middlebury Bench, are discussed in detail in the body of this report above.
b. Locale W2
This locale is included here primarily to illustrate the outlet area for Lake Winooski south of Williamstown inT4 time in the area of the divide between present day drainage basins of the Stevens Branch to the north and the Second Branch of the White River to the South.
The location of the Lake Winooski outlet at the divide as identified by Wright and others is marked. As can be seen, stagnant ice deposits are mapped on both the north and south side of the divide(marked by the purple line) at the T4 level. A patch of Scabby Terrain is shown to the west of the divide, but occurs more substantially further south along the T2 Ice margin, and further to the west along the perimeter of Lake Winooski. The State Surficial Geology map tab on VCGI indicates lacustrine silt-clay deposits in the lower Stevens Branch basin in the Barre area. These observations are consistent with and lend support to the interpretation given above that the Williamstown outlet for Lake Winooski was initially blocked, with incipient Lake Winooski opening as the ice margin retreated northward from the outlet in T4 time.
The heavy maroon colored line on the above screenshot represents the T2 ice margin(not the T6 margin which elsewhere is also marked by a maroon colored line) associated with the previously described Disconnection of ice mass in the Middle Connecticut Basin. Drainage from Lake Winooski was across the divide between the Winooski and Connecticut Basins. However, this drainage, including into the residual Disconnected ice mass area, has not been studied here.
The mountainous terrain on the western margin of Lakes Winooski, beyond the above map area, is a region with relatively deep valleys draining the eastern flank of the Green Mountain front range. Scabby Terrain patches and stagnant ice deposits are mapped on VCGI in the floors of many of these valleys with drainage extending downslope to deltaic deposits associated with Lakes Winooski and/or Mansfield. These features are interpreted as “Disconnections” of ice masses in these deep valleys, a “Glacier Dynamic” feature whereby the penetration of standing water at the Lake Winooski level along the western perimeter of this water body resulted in separation and isolation of ice masses in these valleys. The T4 and T5 ice margins are discussed in more detail below in regard to the Winooski and Lamoille basins.
VCGI mapping provides evidence marking the receding ice margin of the ice mass in the Winooski Basin headwaters, including the Stevens Branch near the outlet prior to the development of Lake Winooski. The Winooski sublobe abutted against substantial upland terrain. For example, east of Marshfield
The maps below illustrate the T4-T5 time, showing flowlines and associated Disconnections:
This VCGI map illustrates features in this Locale:
Physiographically, this Locale is located in the headwaters area of the Winooski Basin, near the divide with the Lamoille Basin, in a low area between Woodbury Mountain on the west and the more substantial Stannard Mountain on the east(off the map), the latter serving as a major upland between the Connecticut Basin and the Winooski Basin.
Demarcations on the map are as given previously. Again, the heavy blue line represents the approximate the T4 1200-1400 foot(366 – 427 m) ice margin. Stagnant ice deposits(grayed areas on map) at this margin suggest commingling of ice across the divide(marked by the purple line) between the Lamoille and Winooski Basins in this Locale. These deposits represent the ice margin for sub-lobes of the T4 Champlain Lobe, which extended into both the Winooski and Lamoille Basin. The T4 margin is closely associated with kettle hole ponds, many of which have substantial depths, as marked on the VCGI map. For example:
- Nichols Pond 110 feet
- East Long Pond 110 feet
- Greenwood Lake 41 feet
- Valley Lake 70 feet
- Sabin Pond 57 feet
These likely contained longer lasting ice block at T4 time. The occurrence of deep “kettle hole” type basins is typical in the Memphremagog Basin at the T4 to T6 levels, which is part of the evidence which prompted the concept of a “hybrid ice margin” and a Style referred to as “Everything, Everywhere, All at Once, and Continuing.” The T4 “hybrid” ice margin includes both active and a stagnant margins, with the ice masses remaining in deeper kettle holes as the active ice margin receded. Thus, the T4 – T6 ice margins were spatially and temporally overlapping.
The southernmost stagnant ice in this Locale is in the vicinity of Sabin Pond. The State surficial geology map shows a stagnant ice deposit in the Sabin Pond area, which is supported by the soil sewage tab. LiDAR imagery shows very strong kamic topography, including an esker. The elevations of the esker ridge crest indicate a southward gradient, from local elevations of about 1080 feet at the north end to 980 at the south end. These elevations, when adjusted, correspond with the T4 margin.
Immediately south of the Sabin Pond margin is a deltaic deposit(?), as indicated by the State Surficial map and topographic expression. LiDAR imagery indicates mining excavations at the head of the delta, which make geologic interpretation more difficult, but it appears that the esker feature represents a Drainage Line which grades into the deltaic deposit, which likely formed along the ice sheet margin, at an elevation corresponding with Wright’s Lake Winooski. Downgradient from this deltaic deposit to the south, the State Surficial Map shows the presence of lacustrine deposits. Thus, meltwater drainage from stagnant ice in the Sabin Pond area served to feed the Winooski level delta when the area to the south was ice free. This is a significant observation as it indicates that when Lake Winooski developed in T4 time stagnant ice remained along and just above its perimeter. This fits with the progressive recession of the ice sheet, water penetration along its margin, and the long-lasting presence of ice masses left behind, as part of the “hybrid” and “Everything, Everywhere, All at Once and Continuing” Style.
To the northwest of Sabin Pond in the Lamoille/Winooski divide area, above the level of Lake Winooski, Greenwood Lake with a depth of 41 feet and Valley Lake with a depth of 70 feet are both kettle holes. The State surficial map shows an intimate association between stagnant ice deposits and lacustrine deposits in this area. The elevations of these features are near the upper range of the T4 ice margin.
Immediately west of Sabin Pond, downgradient from the Greenwood Lake and Valley Lake areas are Forest Lake and Mirror Lake, with depths of 76 feet and 106 feet respectively. The State map shows a stagnant ice deposit in the close association with Forest Lake and a “moraine” deposit with Mirror Lake. These likewise likely are kettles occupied by stagnant ice, at a lower level, close to the bottom of the bottom of the T4 margin. However, evidence of Lake Winooski deltaic or bottom set deposits are not present, perhaps indicating ice presence at Lake Winooski time.
In general, the evidence suggests commingling of ice across the Lamoille and Winooski divide at T4 time, in close association with Lake Winooski. The large turquoise colored arrows are intended to suggest more substantial ice supply from the north.