100 



CAPE COD GEOLOGY 



This section shows the same succession of beds that is seen on the islands on 

 the west, except that the Herod gravel member, below the Montauk till, is not 

 represented. The blue clay in this section may be interpreted as the lost bed of 

 clay at Sankaty Head, formerly seen in Anthony's Nose; the yellowish sand 

 may be the equivalent of the fossiliferous sand, and the pebbly clay or till may be 

 the same bed that lies above the shell beds on the east coast, but that interpreta- 

 tion is not made here. 



In the low headland west of Copaum Pond there is a bed of pebbly clay or 

 till carrying rounded pebbles of white quartz, probably derived from Cretaceous 

 gravel. The base of this bed is not exposed. It is overlain by a bed of gravel, a 

 foot thick, containing pebbles, the largest 4 inches in diameter, upon which lies 

 a bed of fine, buff clayey sand. Upon all these beds lies the universal coastwise 

 recent wind-blown sand, here 3 feet thick. Other exposures show what appear 

 to be parts of the same set of beds. In the best of these sections gravelly till at 

 the base is succeeded by grayish buff clay, which is in turn overlain by sand 

 containing scattered pebbles. 



At the southern limit of the town of Nantucket, southwest of Windmill 

 Hill, beds of rusty clay are exposed. These beds pass southwestward under 

 fine yellowish sand, evidently repeating the section at Sachem Spring. It is prob- 

 ably one of these same beds of clay that is brought up by a fold in the hill on 

 which the water tower stands, just east of Washing Pond. 



East of the town, in the Wier valley, glacial gravel is struck in shallow wells 

 apparently at a horizon above that of the beds west of the town. 



Only one bed of pre-Wisconsin till appears to be exposed on the island, and 

 this probably lies at the horizon of the bed of stony blue clay at Gay Head, 

 which is referred to the Mannetto stage. It nowhere carries large boulders, and 

 at many places it passes laterally into water-laid drift. The section exposed is 

 not as a whole exactly comparable with that on the islands to the west and 

 lacks the apparent persistent parallelism of the beds that enabled Fuller to trace 

 the Pleistocene above the basal beds from Long Island eastward to Marthas 

 Vineyard. If the bed of till is at the horizon of the stony blue clay at Gay Head, 

 the Gardiners clay proper and higher beds are not represented, probably because 

 they have been eroded away. The island is subject to great erosion, and erosion 

 may have taken place at one of the stages of the Vineyard interval marked by 

 now partly masked terraces on Marthas Vineyard. The island was not only sub- 

 ject to erosion by the sea but was crumpled and deformed by the Wisconsin ice. 



