132 



CAPE COD GEOLOGY 



CHANNELS 



The numerous channels or creases on the outwash plain are among the 

 most conspicuous features of the island. Their form and distribution show that 

 they were once occupied by streams. They now contain no running water, but 

 their lower parts are occupied by ponds, which stand practically at sea level. 

 (Plate 19, fig. 1). These channels form a branching network that extends over 

 the greater part of the plain. The only part of it in which they are not found 

 extends along its northern margin. Some of the larger channels were evidently 

 master streams to which the smaller ones were tributaries. The bottoms of 

 these channels are fairly broad and flat and have an even grade from their, heads 

 down to the ponds. The banks of most of them are less than 20 feet high. The 

 east and the west banks of many of them differ greatly in steepness, the west 

 bank being almost invariably the steeper. This difference has been noted in 

 many of the large northward and southward-flowing streams of the world and 

 was long the subject of speculation. The work of G. K. Gilbert 1 and others 

 shows that it is possibly due to the deflection of the streams by the rotation of 

 the earth. When the forward motion of the Wisconsin ice was about balanced 

 by the melting, a large volume of water must have been poured over what is 

 now the outwash plain, sufficient to form streams that produced the now de- 

 serted channels and to carry material far from the melting ice. Most of these 

 channels, even those in the western part of the plain, with the exception of Tis- 

 bury Great Pond, point toward the area occupied by the Cape Cod Bay lobe, 

 indicating that most of the water was derived from the Cape Cod Bay lobe 

 and that this lobe was the last one to retreat from the region. The heads of 

 Edgartown Great Pond and Tisbury Great Pond alone suggest drainage from 

 the Buzzards Bay lobe. 



SHORE FEATURES 



Marthas Vineyard, being an island and being made of unconsolidated 

 material, has naturally many shore features that have been formed by the 

 action of the waves, currents, and tides since Pleistocene time. The action 



1 Gilbert, G. K., The sufficiency of terrestrial rotation for the deflection of streams, Am. Jour. Sci., 

 3d ser., 27, pp. 427-432, 1884. See also Lewis, E., Certain features of the valleys or watercourses of 

 southern Long Island, Am. Jour. Sci., 3d ser., 13, pp. 215-216, 1887. 



For additional consideration of this question the reader is referred to the following articles : 



Von Baer, Karl E., Ueber ein allgemeines Gesetz in der Gestaltung der Flussbetten, Bull. Imp. 

 Acad. Sci., St. Petersburg, 2, pp. 218-250, 353-382, 1860. 



Reclus, Elisee, The earth, pp. 372-377, New York, 1872, or 2, pp. 433-439, New York, 1871. 



Bryson, John, The geological formation of Long Island, New York, with a description of the older 

 watercourses, New York, 1885. 



Cobb, Collier, Note on the deflective effect of the earth's rotation as shown in streams, Elisha 

 Mitchell, Sci. Soc. Jour., pp. 26-32, 1893. 



