Following the Late T3 readvance, the T4 ice margin stood at the divide between the Connecticut Basin and the Memphremagog Basin, marking the recession of the ice margin in T4 time. The T4 margin was a hybrid type margin, marked by both Ice Marginal Channels and stagnant ice deposits, with T4 Ice Marginal Channels correlated with the Dixville moraine in Quebec, based on their close proximity, alignment, and similar elevations.
The following map represents T4 time, marked by the darker blue shading :
In the Upper Connecticut Basin, the disconnected ice mass and en masse stagnation in the Upper Connecticut Basin is marked by the orange colored shading. The T4 Scabby Terrain tract is interpreted as indicating a third Disconnection, associated with feeder ice streams in the narrow Memphremagog tributary basins, resulting in en masse stagnation of the ice in the Upper Connecticut Basin, again as shown by the orange shading. This Disconnection, like those in T1 and T2 time occurred as a result of ice thinning across divides, specifically divides with the Barton, Willoughby, and Clyde Basins, which previously had served to supply active ice to the Upper Connecticut Basin in T3 time. These basins include “over-deepened lakes,” resembling the Finger Lakes in terms of the over-deepening and as well stagnant ice deposits at valley heads. 1 This over-deepening is fascinating in and of itself and deserves further study. It is believed that the over-deepening may somehow reflect the ice sheet’s “struggle” to supply ice to the Connecticut Basin. The suggestion given below that the T4 margin is related to the Bethlehem-Littleton readvance raises intriguing questions as to the history of over-deepening. Ice blocks in these and many other depressions in the Memphremagog Basin remained as part of the T4 stagnant ice margin and persisted while the active ice margin receded northward to the next T5 position, again in the Style referred to as “Everything, Everywhere, All at Once, Progressive, and Continuing.”
As discussed above and in Appendix C, the lower, younger portion of the “White Mountain Morainic System” identified by Thompson et al in the Ammonoosic Basin in New Hampshire likely correlates with the T4 margin, as depicted by the blue colored ice margin line as shown by the appendage added to the above map. Also as discussed above and in Appendix C, Thompson et al indicate that their morainic system marks a readvance of the ice sheet. As noted, this readvance likely occurred in Late T3 time immediately preceding the Disconnection which took place in T4 time.
Ice blocks in these and many other depressions in the Memphremagog Basin remained as part of the T4 stagnant ice margin and persisted while the active ice margin receded northward to the next T5 position, again in the Style referred to as “Everything, Everywhere, All at Once, Progressive, and Continuing.”
Stewart and MacClintock identify the “St Johnsbury moraine” in the Upper Connecticut Basin, said to be the best such feature known by them in New England. This “moraine” corresponds with a Scabby Terrain Tract marking the upper boundary of en masse stagnant ice (much of the orange-colored area on the map) closely associated with, but at slightly higher levels than the stagnant ice deposits. As stated above, the upper elevation boundary of the Scabby Terrain Tract is marked by the lavender colored line on the above T4 map, extending around the northern and western perimeter of the Upper Connecticut Basin as far south as the Bradford vicinity, where outwash plain terraces are graded southward into Lake Hitchcock level deltaic deposits. The Scabby Terrain and associated stagnant ice deposits in the Upper Connecticut Basin occur at elevations below the divides with the Memphremagog Basin, and well below the uplands in the northern Connecticut Basin near the Quebec border. T4 stagnant ice deposits in the Upper Connecticut Basin occur at and just below the 1200 foot(366 m) level which is close to the Basin floor. This includes many eskers as evident on LiDAR imagery, including the “Passumpsic esker” identified by Stewart and MacClintock as a remarkably major feature. The number of eskers in many of the Connecticut sub-basins is very large, seemingly in every sub-basin, indicating widespread stagnation, which is consistent with and indicative of en masse stagnation.
The T4 hybrid margin is well marked in the Memphremagog Basin and Champlain Basin by stagnant ice deposits, Ice Marginal Channels, and Bedrock Grooves, this last in the northern Champlain Basin along the western flank of the Green Mountains and the foothills. The Bedrock Grooves represent substantial meltwater along Cold, active ice margins. T4 time is correlated with the Dixville moraine.
A Drainage Line on and adjacent to T4 level kame terraces (not shown on this map) extended southward down the Black River Basin, one of the Memphremagog sub-basins, across the divide with the Lamoille Basin. This Drainage Line correlates with kame delta deposits near East Hardwick and the western flank of the Worcester Range at the Lake Winooski level. The correlation of the T4 ice margin in the Memphremagog Basin with Lake Winooski serves multiple benefits in regard to not just documenting history but as well helping to validate the Bath Tub Model methodology and the T4 margin for northern and central Vermont as a bonafide ice margin for the area extending from the Quebec border southward over a distance of nearly 50 miles. Ice margin features, including both stagnant ice deposits and kame deltas, which correlate with Lake Winooski, extend around its perimeter, further supporting and extending the T4 ice margin. Interestingly, as noted above, the local elevation of the outlet for Lake Winooski near Williamstown as compared to the local elevations of ice margin and associated Lake Winooski strandline features along the northern reaches of the Lake indicate an isostatic surface gradient of about 5.7 feet per mile (1.1 m/km), which is close to but slightly higher than the 5 feet per mile (0.96 m/km) factor used in this report for making isostatic adjustments.
On the eastern margin of Lake Winooski its penetration along the T4 ice margin is marked by recession of a hybrid type margin between longer lasting ice in stagnant ice deposits with graded relationships downward to Lake Winooski kame deltas. And on the western side of the Lake Winooski basin, similarly rapid penetration of Lake Winooski waters severed and isolated, possibly still active ice masses in deep valleys on the eastern flank of the Green Mountains.
Wright et al reported evidence of a readvance in the Winooski Basin. This evidence is at multiple locations, generally within the footprint of Lake Winooski, but additional readvance sites are also mapped by Wright et al at locations above the T4 margin . It is here believed that this evidence corresponds with the readvance associated with the WMMS, albeit as marked by a highly convoluted margin.
In the northern Champlain Basin, the T4 margin is well marked, again by numerous substantial stagnant ice deposits and by Bedrock Grooves (representing a hybrid active and stagnant ice margin), delineating the T4 eastern margin of the Champlain lobe against the Green Mountain western flank. Ice margin features at the T4 levels/times for the Champlain lobe are mapped southward along the eastern margin of the Basin along the foothills, into the mouth of the Vermont Valley near Rutland, where the margin is marked by a substantial stagnant ice deposits as mapped by Stewart and MacClintock, who for good reason identified these as especially remarkable ice margin features which they considered a “moraine.” LiDAR imagery along this margin clearly shows recessional markings indicating both westward downgradient recession of the lateral margin and northward recession of the frontal tip. Portions of this margin include both stagnant ice deposits and kame delta deposits showing the close relationship between the ice margin and standing water related to the step-down sequence of local water bodies, giving direct evidence of the close Glacial Dynamic interplay between standing water in proglacial water bodies and the ice margin.
In the main Champlain basin floor, ice margin features likewise trace the T4 margin around the nose of the Taconics to near Benson Landing, beyond which the ice margin for this lobe at T4 time was in New York. Lake Vermont post-dated T4 time in Vermont, but likely occurred earlier in New York near the Hudson/Champlain divide. As discussed in Appendix C, the T4 margin is suggested as possibly correlative with Franzi et al’s ice margin position 7 and De Simone and La Fleur ice margin position 12, but this merits further study.
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