74 



CAPE COD GEOLOGY 



glacial action into their present positions they may not be detached from the 

 terrane of which they are a part, and they thus differ from boulders and from 

 those large shoved blocks of strata that were surrounded and underlain by 

 glacial drift. 



Most of the glacial deposits considered in this report fall into two great 

 classes — ice-laid and water-laid drift. No drumlins or eskers are described, 

 although certain narrow and relatively short ridges of gravel and sand that rise 

 between two pits, kettle holes, or ice block holes in the plains and kame tracts, 

 may be likened to eskers. These interkettle and interlake ridges are, however, 

 quite distinct from those serpentine ridges in central and northern New England 

 which clearly represent the aggraded courses of streams flowing toward the 

 ice front under the ice or between walls of ice. 



The classification above presented takes account only of forms produced by 

 the action of ice and water during the glacial period. The drift deposits display 

 depressions, unfilled areas, and eroded tracts or channels which may be classified. 

 These fall under the following headings : 



1. Depressions due to the melting out of ice 



a. In till, giving rise to pits and kettle holes 



b. In sand plains, giving rise to kettle holes and ice block holes. 



2. Channels eroded by water escaping from the ice or flowing within the ice 



c. Stream channels cut in drumlins and ground moraine or across a frontal moraine 



d. Stream channels cut across an outwash plain (creases) 



The numerous lakelets and ponds in the Plymouth woods and on Cape Cod 

 are depressions formed either in the frontal moraine or in outwash plains. The 

 outwash plains on Nantucket and Marthas Vineyard bear channels cut by 

 streams flowing in open air from the ice front. 



THE LARGER GLACIAL ERRATICS OF THE DISTRICT 

 Cape Cod and the adjacent islands are noted for their glacial boulders. Most 

 of these boulders are on the uplands, in areas displaying the knob-and-basin top- 

 ography of unassorted drift, commonly spoken of as morainal topography and 

 interpreted as due to the direct deposition of material from the ice. This in- 

 terpretation is sustained by the absence of boulders from the sand plains, outwash 

 plains, or fields of water-laid drift that lay outside of the ice sheet. Here and 

 there, as on Cape Cod and Marthas Vineyard, boulders as much as 3 feet in diam- 

 eter may be seen on the plains near broken ground having a morainal topography, 

 in places where the ice advanced upon gravel already washed out from the 



