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CAPE COD GEOLOGY 



the whole valley, as the streams of group 2, working back from the coast, found 

 shorter courses to the sea, and intercepted the older stream, diverting its water. 

 The second valley of group 1 here considered runs parallel to the first and in- 

 cludes the country along the Middle Road. It starts south of Peaked Hill and 

 is now occupied in its lower part by the Tiasquam River, west of West Tisbury. 

 This old valley is lost under the outwash in the same way as the other valley. 



Fig. 7. — Map showing location of pre- Wisconsin valleys on Marthas Vineyard. 



Whether it once extended farther northeast, toward Lagoon Pond, or joined 

 the north valley cannot be told. The headwaters of this valley were inter- 

 cepted by a brook that flows into Chilmark Pond. There is a sharp elbow of 

 capture and a very considerable deepening of the original valley. 



To this same group, and perhaps to the same drainage system, belong the 

 now submerged and glaciated valley of Lagoon Pond (Plate 18, fig. 1) and Vineyard 

 Haven Harbor. The upper parts of this old river course are now obliterated by 

 the outwash plain. The lower part has been much changed by glacial scour, 

 for its direction is nearly parallel to the movement of the ice of the Cape Cod 

 Bay lobe. Later wave action built a bar that nearly separates the lower part 



