DAVID P. STEWART Department of Geology, Miami University, Oxford, Ohio 45056

Pleistocene Mountain Glaciation, Northern Vermont: Discussion

In his recent report concerning Mountain Outwash plains are described by Wagner Glaciation in Vermont, Wagner (1970) noted (1970) as sand and gravel deposits with terrace- that the latest comprehensive accounts of the like form; however, he does not describe the glacial geology of that state by Stewart (1961) structure of the gravel within the deposit. All of and Stewart and MacClintock (1969) made no the deposits that Wagner describes as outwash references to local glaciation. The omission of plains have been classified on the Surficial Geo- this subject from these reports was not an over- logic Map of Vermont (Stewart and MacClin- sight inasmuch as no conclusive evidence of tock, 1970) as kame gravel, the majority of Mountain Glaciations was found during the 11- which are kame terraces. They are so desig- yr survey of the state completed in 1966. The nated because of their topographic form, their participants in the mapping program were cog- ice-contact (slumping) structures, and their nizant of the earlier reports (Flint, 1951; Christ- unusually high content of large, ice-rafted boul- man, 1956) that had recorded evidence of local ders. These features can be observed in stream glaciation. The evidences cited in these reports valleys, highway cuts, and in numerous gravel were studied, but were found to be inconclu- pits excavated into them. It is assumed that sive. Wagner interprets these deposits as valley The assumption by Wagner (1970) that Lau- trains that were formed by water from melting rentide ice activity could not have occurred on glaciers upstream. Our interpretation contends the east side of the Green Mountains after the that the deposits were formed between the ice active portion of the glacier had receded to the and the valley wall when ice still occupied the west cannot be substantiated. The last recorded valley. glaciation to move across the Green Mountains Wagner stated that he identifies deltas as invaded from the northwest (Stewart and Mac- bench-like forms composed of sand and gravel Clintock, 1969). The ice piled up on the west overlying lake silts and clays. The most com- side of the mountains until its thickness was mon lake sequence in north-central Vermont great enough for it to cross the summits. Un- contains clay or silty clay, or both, at the bottom doubtedly the first ice to cross the mountains with sand above the silty clay and gravel com- moved down the eastern slopes in lobes that monly at the top. The thickness of the clay or were probably confined to the valleys. More silty clay, or both, is often about equal to the importantly, however, insofar as erosion and sand, but the gravel thickness is much less. The deposition are concerned, was the waning of sand and gravel are horizontally bedded, and the glacier. Active recession by ablation thin- they may contain cross-bedding within the lay- ning could occur only in the upper, thicker por- ers, but no long, gently dipping beds that char- tion of the glacier. The lower ice, however, acterize a delta. Beach gravel is sometimes could not retreat because it was trapped be- found above the sand along the valley walls. tween the mountains (Stewart, 1961, p. 25-26). The classification of many of the deltas by Wag- The ice that remained between the mountains ner is, therefore, questioned. after the upper portion had receded was thick Lake Mansfield does occur in a cirquelike enough to produce independent motion wher- depression at the head of Miller Brook. The ever movement was possible. The last remnants shape of the basin, however, does not conclu- of these ice masses were confined to the valleys, sively prove that it is a cirque. Investigations and they no doubt moved down and receded made during the surficial geology mapping pro- up the valleys similar to valley glaciers. The gram showed that the walls of most of the basin valley remnants could have produced mo- were joint faces and no evidence of glacial abra- raines, but they could not have eroded cirques. sion was evident.

Geological Society of America Bulletin, v. 82, p. 1759-1760, June 1971 1759

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It is not the intent of this paper to imply that Stewart, D. P. The glacial geology of Vermont: the writer is certain that local glaciation did not Vermont Geol. Surv., Bull. 19, 124 p., 1961. occur in Vermont. The possibility is a most in- Stewart, D. P.: and MacClintock, Paul. The surfi- teresting problem. It is believed, however, that cial geology and Pleistocene history of Ver- mont: Vermont Geol. Surv., Bull. 31, 251 p., the evidences cited by Wagner do not conclu- 1969. sively prove Mountain Glaciation. Stewart, D. P.; and MacClintock, Paul. Surficial REFERENCES CITED geologic map of Vermont: Vermont Geol. Surv., 1970. Christman. R. A. The geology of Mount Mansfield Wagner, W. P. Pleistocene Mountain Glaciation, State Forest: Vermont Dept. Forest and Parks, Northern Vermont: Geol. Soc. Amer., Bull., Vermont Development Comm. and Vermont Vol. 81, No. 8, p. 2465-2470, 1970. Geol. Survey, 26 p., 1956. Flint, R. F. Highland centers of former glacial out- flow in Northeastern North America: Geol. MANUSCRIPT RECEIVED BY THE SOCIETY NOVEMBER 2, Soc. Amer., Bull., Vol. 62, p. 21-38, 1951. 1970

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