14 NEW YORK STATE MUSEUM 



we do not know at just what point to put the curb on our imagi- 

 nation. 



The ^manner of progress of the wear on this land surface has 

 depended on the rock characters and structures and must be left 

 for later consideration. During this long time interval, the region 

 has experienced changes of altitude, in part because of wear, in 

 part because of up or down movements of the earth's crust. We 

 are as yet far from being able to trace out these movements. 



In times very recent comparatively, namely only some thou- 

 sands of years ago, the district was covered by the ice sheet of the 

 Glacial period. How long this condition persisted, how many 

 times the ice came and went over the immediate region, we do 

 not know. The advancing ice sheet removed the soil and loose 

 rock from the surface and scoured away at the rock ledges be- 

 neath. The retreating ice sheet spread a heavy mantle of deposit 

 over the surface. The melting ice gave rise to streams and lakes 

 which rehandled some of the glacially deposited material. The 

 larger preglacial topographic features were little changed and 

 remain today substantially as they were before the onset of the 

 ice. The minor irregularities of the surface were largely obliter- 

 ated however, mainly by the deposits laid down during retreat. 

 The stream valleys were filled nearly or quite to the brim, and 

 the modern streams are largely in new courses therefore, specially 

 the smaller ones. Unequal valley filling, and unequal deposit 

 elsewhere left hollows in the surface in which lakes now nestle, 

 lakes which had no existence before the advent of the ice. The 

 Mohawk valley lowland is a preglacial feature, but the preglacial 

 divide between the east and west flowing streams in this lowland 

 seems to have been at Little Falls, and was certainly not at Rome, 

 its present position. After the ice, on its last northerly retreat, 

 had uncovered the Mohawk valley but still lay across that of the 

 St Lawi^nce, the drainage of the Great lakes passed to the sea 

 by way of the Mohawk, the eastern end of the lake in the Ontario 

 basin being at Rome. The present Mohawk is an insignificant 

 stream as compared with its great predecessor, which has of 

 course left its mark on the vallev. 



