GEOLOGY OF LAKE PLEASANT QUADRANGLE 65 



southwest except in the southeastern portion where it was more 

 nearly southwest, thus harmonizing with the northern portion of 

 the Broadalbin quadrangle. It is also worthy of note that instead 

 of the strongly predominant southward ice movement across the 

 North Creek quadrangle we have here a predominating southwest- 

 ward movement. 



As regards depth of ice, the most significant striae are those on 

 Wallace mountain which lie at an altitude of from 2100 to 2200 feet 

 and, since the glaciation here was vigorous, there can be no doubt 

 but that the ice was deep enough to cover the mountain. The striae 

 on West hill are at 1360 feet so that, at the very lowest calculation, 

 the ice through the W^ells valley was 500 feet deep. The other 

 striae are too near valley bottoms to be especially significant. The 

 presence of glacial boulders and lakes, however, on some of the 

 highest mountains of the quadrangle, together with the general con- 

 sideration of all northern New York, leaves no doubt but that the 

 great ice sheet completely buried the whole quadrangle. 



No positive evidence for any important ice erosion was discovered 

 within the region. The throughly glaciated surfaces here described 

 do not necessarily imply deep cutting by the ice. In its passage 

 through the Wells valley the ice must have plucked off many large 

 blocks of the Cambric rocks as proved by the presence of so many 

 such blocks in the w^est Sacandaga valley in the vicinity of Black- 

 bridge. It is more than probable that the Sacandaga lake and Lake 

 Pleasant basins were scoured and somewhat deepened by the ero- 

 sion, but how much can not be said. At any rate, ice erosion was 

 sufficiently effective to scrape ofif practically all Preglacial soils 

 and decomposed rocks so that good examples of rotten rock in situ 

 are of very uncommon occurrence. 



Ice deposits 

 Erratics. Glacial boulders or erratics are of course scattered 

 over all portions of the quadrangle, and as usual most of them are 

 within or not far to the south or southwest of the rock areas from 

 which they were derived. W^here exposures are scarce, the great 

 predominance of a certain kind of rock among the boulders gives a 

 fair idea of the underlying rock mass. More or less angular 

 erratics, often several feet across, occur even on high mountains, 

 thus proving great depth of ice. The only noteworthy erratics ap- 

 parently derived from without the quadrangle are of typical gabbro 

 in the northwestern portion in the vicinity of the two large lakes. 



