42 NEW YORK STATE MUSEUM 



The effects of a general ice sheet moving in a north-south direc- 

 tion, whether representing only the last invasion of the ice or the 

 last, together with previous invasions, are recorded on the area of 

 the Cohoes quadrangle in the smoothed surfaces of rock hills and 

 in glacial scratches. The drumlinized forms of many of the rock 

 hills are conspicuous to the eye in the field and are also clearly 

 indicated by the contour lines of the topographic sheet. The char- 

 acter of the rocks (indurated shales and sandstones) and their 

 steeply inclined planes of stratification are unfavorable for register- 

 ing the ice movements by glacial scratches and they were observed 

 only in two localities, as follows: about i^ miles northwest of 

 Stillwater, on the surface of a projecting mass of rock in a field 

 south of the highway (see map), io° west of south; about i^ 

 miles south of Valley Falls on rock at roadside, 5° east of south. 



The close of the latest period of general prevalence of ice, marked 

 by the retreat of the ice sheet to the north, is recorded in the 

 materials left from the melting ice and forming the till sheet, or 

 ground moraine of the uplands portions of the quadrangle. The 

 recession of the ice front appears to have been relatively steady 

 and uninterrupted. This is inferred from the fact that there is no 

 recessional moraine of any considerable continuity. In a few local- 

 ities on the eastern uplands areas, groups of hills of definite mor- 

 ainic character occur, evidently marking temporary and local stand- 

 ings of the ice front. 



In the gradual process of the melting of the ice sheet the uplands 

 were bared before the thicker ice filling the broad valley of the 

 Hudson had wasted. A broad and deep lobe of ice thus lingered 

 in the valley, probably for a long time after the upland areas were 

 ice-free. South of the southern limit of the valley ice, and con- 

 stantly extending northward with the retreat of the ice lobe, as 

 well as spreading laterally over depressed areas adjacent to the 

 Hudson valley, gathered and for a long time stood at a permanent 

 level the body of waters which formed Lake Albany. 



Due to the more rapid melting of the ice in its lateral portions 

 than in the thicker middle portion, the ice lobe narrowed gradually 

 toward its southern end and thus were formed two embayments 

 of the lake waters, lying on either side of the elongated wedge of 

 ice. These embayments received sediments derived from the melt- 

 ing ice and especially from streams from the north which developed 

 in the depressions between the lateral margins of the lobe of ice 

 and the bared land slopes. The coarser sediments were deposited 

 in the upper and narrower parts of the embayments while the finer 



