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CAPE COD GEOLOGY 



135 



first lighthouse on Gay Head had to be replaced by the present one in 1858, 

 for the encroachment of the sea made it no longer safe. A shoal extends north- 

 westward into the sound from the cliffs. This shoal is not made of shifting 

 sand but is underlain by the same material that forms the cliffs. On this shoal 

 there are many large boulders, such as cover the surface of Gay Head and are 

 seen in some of the Pleistocene deposits. These boulders were probably too 

 large for the waves to remove at the time the sand and gravel originally asso- 

 ciated with them were removed and were left as monuments marking the former 

 extent of these deposits. The name "Devil's Bridge" has been given to this 

 boulder-strewn shoal. 



The Nashaquitsa cliffs, although not so strikingly colored as the Gay 

 Head cliffs, except in small patches at their east end, are very picturesque. 

 (Plate 20, fig. 1). They are considerably longer, being two miles in length, and 

 they are somewhat higher in their highest part. Their lack of the vivid colors 

 of Gay Head cliffs is due to the absence of the Cretaceous clay and the abun- 

 dance of glacial gravel and clay. The clay found here is the blue-gray Pleistocene 

 clay. These cliffs begin at a small point at the east end of Stone Wall Beach 

 and rise abruptly to a height of 150 feet. The cliffs remain fairly high for about 

 three-quarters of a mile and then fall to a height of about 50 feet. From that 

 height, with some variations, they slope down to a height of about 20 feet 

 at their east end. These cliffs are also being cut back rapidly. According to an 

 estimate made by the United States Coast and Geodetic Survey 1 in 1853 they 

 are being cut back about 3 feet a year. 



Of the cliffs along the northwest coast east of Gay Head, those north of 

 Prospect Hill are the most conspicuous. They are about 120 feet high and are 

 composed largely of beds of loose gravel and, at the base, of some Cretaceous 

 beds and glacial clay. The differences in the character of these beds causes 

 differences in the forms produced by the erosion of the cliffs, the Cretaceous 

 beds eroding in ridges and gullies, with an occasional pinnacle; the beds of 

 gravel eroding in smooth slopes at the angle of repose; and the beds of glacial 

 clay forming vertical walls. These different materials also give different colors 

 to the cliffs, among them notably the white of the Cretaceous beds and the 

 blue-gray of the glacial clay. At Cape Higgon and Norton Point there are 

 cliffs about 80 feet high, composed largely of beds of loose gravel, which are 

 continually creeping down as the waves attack them at the base. 



1 Rept. Supt. Coast and Geodetic Survey for 1853, p. 30, 1854. Statement by Assistant H. L. Whiting. 



