4 



CAPE COD GEOLOGY 



though Cape Cod at first sight appears to be quite unlike the islands, a close exami- 

 nation shows that it has certain features in common with Marthas Vineyard and 

 Nantucket, and that it is composed of deposits of similar character and origin. The 

 islands and sounds were formed by glacial action in Pleistocene time, when beds 

 of unconsolidated gravel, sand, and clay were laid down by the continental glacier, 

 which then advanced over this region from the north, and by the erosive work of 

 streams. The outer shores of Cape Cod and of Nantucket show where the great 

 frontal moraines of that glacier pass seaward to the submerged fishing banks off 

 the coast. The maximum extension of the great sheets of ice that produced these 

 features is shown in Figure 1. 



Some of the formations belonging to the older part of the Pleistocene series 

 are greatly disturbed, and the work of Veatch l and of Fuller 2 on Long Island 

 first made known the sequence and the geologic age of certain beds of blue clay 

 in the New England Islands, which can now be separated from beds of dark 

 Cretaceous clay, so that the older Pleistocene deposits can be correlated. In this 

 report Dr. Wigglesworth has made the proper correlation of these beds of clay 

 and the associated beds of sand on Marthas Vineyard. Except for local breaks 

 in their basal part, the Pleistocene beds extend almost continuously from 

 Long Island eastward to Marthas Vineyard. 



The topography of Cape Cod and of the New England Islands is predomi- 

 nantly glacial, but some tracts display forms shaped by the action of running 

 water on unconsolidated clay and sand and modified by the action of moving ice. 

 Certain plains and high tracts or ridges, both on Cape Cod and on the islands, 

 present a morainal configuration, but much of the general form of Marthas 

 Vineyard is not distinctly morainal. At many places morainal features are 

 imposed upon an older surface that was not shaped by glacial or stream action. 

 The appearance of morainal origin is due in part to the numerous ice-borne 

 boulders that strew the surface. 



Throughout large tracts on Block Island and No Mans Land and in the 

 northern part of Nantucket there is a pseudo-morainal topography due to the 

 kneading and deformation of old stratified glacial deposits by the last ice sheet. 

 No geologic sections are available to show the relation of the form of the surface 

 to the details of the structure beneath, and it is therefore difficult to decide 

 whether the configuration at many places is due to morainal accumulations 

 laid down by the last ice sheet or to the form of older deposits that are covered 



1 Veatch, A. C, and others, Underground water resources of Long Island, N. Y., U. S. Geol. Survey 

 Prof. Paper 44, 1906. 



2 Fuller, M. L., The geology of Long Island, N. Y., IT. S. Geol. Survey Prof. Paper 82, 1914. 



