CAPE COD GEOLOGY 



105 



glomerates in the Carboniferous areas of southeastern Massachusetts. Reddish-brown Cre- 

 taceous sandstone identical with fragments and nodules found in the drift on Marthas Vine- 

 yard, Block Island, and Long Island. Lignitiferous sandstones of Cretaceous age. 



Metamorphic rocks: Gneiss and amphibolite schist; rocks like those found on the south 

 shore of Massachusetts and Rhode Island from Newport eastward. 



The fragments of Cretaceous sandstone were probably of local derivation; 

 the other fragments may have traveled from the east coast of Massachusetts 

 or from points now under the neighboring waters on the east. 



POST-GLACIAL OR RECENT CHANGES 



Since the Wisconsin ice sheet melted on Nantucket the recorded changes 

 are few, consisting of the establishment of vegetation on the glacial gravel and 

 sand, a slight weathering of the mainly siliceous rocks of the drift, and the results 

 of the dominant action of the sea. Owing to the nature of the subsoil, rainfall 

 has had little effect in sculpturing the surface, particularly as the drainage area 

 is too small to permit the formation of rivers. The chief postglacial features 

 are the swamps and marshes, the beaches, and the dunes along the shore. A 

 minor feature consists of thin heaps of shells along the coast, accumulated by the 

 aboriginal inhabitants of the island. Of some scientific interest are the sand- 

 blasted pebbles or glyptoliths which abound in the areas of "lag gravel" on or 

 near the brows of wind-swept cliffs, particularly along the north coast of the 

 island. 



NATURAL CHANGES 



The sea is slowly wearing away the south and east coasts of Nantucket, 



but the points of its attack have shifted somewhat, so that places which were 



formerly receding are now fronted by broad sand beaches. Many years ago the 



cliffs at Sankaty Head and Siasconset were freed from talus by the waves, but 



now a broad beach encircles this part of the coast and the sea rolls in against 



the cliffs only in high storms. Between Siasconset and Tom Nevers Head a wide 



foreland of sand has been built out and, farther west, near Surfside, at a jog 



in the coast line, another foreland has in recent years been formed in front of 



the coast guard station. Between these forelands the sea has eaten away the 



cliffs rapidly, and at Wauwinet also it is even more rapidly changing the form 



of the coast line. 1 Still greater changes have modified the forms of the beach 



bars thrown across depressions, such as that at the head of Haulover Break, 



between Wauwinet and Coskata Island, and that at the west end of Nantucket. 



1 According to Mr. W. H. Codd, the coast at Wauwinet retreated 400 feet between 1882 and October 

 3, 1893. 



