﻿I58 NEW YORK STATE MUSEUM 



older beds of somewhat different quality, more or less crushed by 

 overriding of the ice; and tills interbedded between the older and 

 the newer clays. No such features have been seen. In the few 

 sections observed reaching to rock the clay reposes directly on the 

 smoothed rock, and the deposit is similar and homogeneous from 

 bottom upward, and very finely laminated. The cases of crump- 

 ling which have been noted are probably explicable by the ground- 

 ing of icebergs, or perhaps by the thrust of the accumulating weight 

 of clay on weaker borders of the deposit. 



An explanation of the large volume and extent of the clay seems 

 to lie in the consideration of the lake conditions at the front of 

 the waning ice sheet and the mechanical factors working there. 

 In ordinary glacial drift or till the coarse materials remain in mix- 

 ture with the clay (rock flour) matrix. But the agitated deep 

 water in which all the deposits of our area have been laid down 

 have screened out the coarse from the fine, dropping the coarse 

 near the ice front, and have carried the fine material away by itself 

 farther from the ice front into the more quiet water. It should 

 be understood that the deposits as a whole were accumulated from 

 south to north, following the departing ice front. In other words 

 they grew backward. It is possible that either by lifting or by 

 toppling the breaking ice kept the water agitated and so facilitated 

 its sifting action. The materials contributed by the glacial streams 

 were already under assorting action. Lack of strong, continuous 

 currents, as rivers, or as in tidal seas, prevented the far removal of 

 the silt, and the muddy waters dropped their clay burden over the 

 bottom not far in advance of the ice front. Subsequently the low- 

 ering waters scraped the silt which had been dropped on the higher 

 surfaces clown into the lower grounds and hollows. As there was no 

 break in the existence of the standing and lowering waters, and 

 consequently no pause in the depositional process, so we find con- 

 tinuity and uniformity in the deposits. 



Pitted clays. In the hollows or basins of the Alexandria Bay dis- 

 trict [pi. 47] are found deposits of clay which are pitted with 

 basins or kettles. In some instances the silt forms merely mounds 

 and ridges with intervening swales and swamps, a good example 

 being seen 3 miles north of Redwood. 



These pitted clay fillings blend on the one hand into till, and on 

 the other into the smooth or merely eroded clay plains. The ex- 

 planation of their origin seems to be the deposition of the silts over 

 grounded ice or anchored ice blocks. Apparently the ice masses 

 were not melted until the silt deposition was ended. 



