124 



CAPE COD GEOLOGY 



INFLUENCE OF THE OLDER TOPOGRAPHY 



The attitude and the elevation of the older beds determined to a considerable 

 extent the type of topography that was developed on them by normal stream 

 action during the interglacial period preceding the advance of the Wisconsin 

 ice. The surface of the low-lying plain and of the northeastern area, in both of 

 which the beds are but little or not at all disturbed, was not much altered by the 

 erosion, but the western area was considerably altered, for it stood high enough 

 to allow the streams to cut rather deeply and the structure of its beds controlled 

 to some extent the courses of the streams. 



THE BUZZARDS BAY LOBE 



The topography of the western area must be ascribed mainly to the action 

 of the Buzzards Bay lobe of the Wisconsin ice sheet. On reaching Marthas 

 Vineyard the ice encountered rather high elevations formed by deposits laid 



FlG. 6. — a. Generalized profile across the northeastern area and the great plain, b. Generalized profile across 

 the western area and the great plain. 



down by earlier glaciers on beds of clay and sand. On these beds the ice deposited 

 morainal ridges of gravel and a thin coating of till. The ice moved forward long 

 and in great volume, covering the high elevations and passing down on to the 

 plain, but it probably did not reach the limit of its advance until near the end of 

 this substage of the Wisconsin ice sheet. In the meantime it melted almost 

 rapidly enough to counteract its forward motion, and an immense quantity of 

 sand and gravel was washed out beyond its edge. As the ice melted, it deposited 

 as a sheet of till the part of its load that was not thus washed out, and this till 



