GLACIAL GEOLOGY OF THE SCHENECTADY QUADRANGLE 39 



REVIEW AND SUMMARY 



The Pleistocene history of the area oi the Schenectady quadrangle 

 begins with that condition of glaciation which produced the striae 

 now observable on the surface of the bedrock. We start, then, with 

 the conception of a sheet of ice overspreading the country and 

 moving in a general direction of some 30 degrees west of south. 

 The number of localities where striae were observed is insufficient 

 to determine differential movements due to the adjustment of the 

 flow of the ice to local irregularities of surface. A comparison with 

 similar observations for adjoining territory would also be necessary 

 to determine whether the direction of movement was influenced by 

 the regional topography which, as suggested by Chamberlain,^ 

 caused a westward movement of ice in the eastern portion of the 

 Mohawk valley. It may be noted, however, that at the western edge 

 of the sheet striae showing a direction of 57 degrees west of south 

 were observed in one locality. 



With the change to climatic conditions that resulted in the gen- 

 eral retreat of the ice sheet to the north, we conceive that the south- 

 ern portion of the area was first freed of ice. Its lower altitude as 

 compared with the northern portion of the sheet strengthens the 

 probability of this inference. The withdrawing ice sheet left in its 

 wake the materials of the ground moraine together with the debris 

 dropped by the melting ice, forming the sheet of till that now over- 

 lies the bedrock. 



As the channel of the Mohawk became uncovered it seems prob- 

 able that glacial waters filled the rock depression underlying the 

 present basin west of Schenectady. Rocks lie at a considerable, 

 though unknown, depth below the present surface of the basin ; we 

 also lack knowledge of the present extent of this rock basin toward 

 the south. We may, however, with considerable confidence assume 

 the accumulation of glacial waters in this locality before the 

 Mohawk valley to the west was opened. It is likewise probable 

 that the rock basin below Vischer Ferry filled in with waters 

 derived from the melting ice. 



How soon after the withdrawal of the ice from the southern 

 portion of the sheet the Lake Albany waters spread over the bared 

 area could be determined only by a study of the general conditions 

 involved in the origin and development of Lake Albany. It is even 



1 U. S. Geol. Survey, 3d An. Report, p. 361-65. 



