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CAPE COD GEOLOGY 



Pre-Cretaceous Basement 



The thickness of the unconsolidated material that lies upon the crystalline 

 basement rocks is unknown. The only available method of determining the 

 depth of this basement below the level of the sea is to project beneath the island 

 the average slope of the remnants of an old surface of these rocks from the 

 south coast of the mainland. This surface dips about 20° south of east at the 

 rate of about 30 feet to the mile, whence it is presumed that the bedrock lies 

 about 540 feet below sea level under the north end of the island, and about 810 

 feet below it under the south end. This estimate, however, takes no account 

 of valleys in the surface of the bedrock; it indicates only the general depth. 

 Faulting along the coast or warping of the ancient rock surface may increase 

 considerably this estimate of the depth. 



The rocks of this crystalline basement, so far as they can be determined 

 by observing the crystalline rocks on the neighboring mainland, consist of 

 gneisses, schists, and later granitic intrusives of Carboniferous age or older. 

 The coal beds of the southern extension of the Narragansett coal field should 

 lie at similar depths between 10 and 30 miles east of Block Island, under a cover 

 of Cretaceous sand and clay, upon which lie glacial deposits. 



Upper Cretaceous Deposits 



CHARACTER AND DISTRIBUTION 



The beds of light clay and of dark lignitic Upper Cretaceous clay, found 

 under most of the islands off the south coast of New England, crop out in small 

 exposures as beds of white clay on the east coast of the island and at Clay 

 Head, on the north coast, and as dark lignitic clay at a place back of the beach 

 near Old Harbor, in the southern part of the island. Nodules containing the 

 leaves of Cretaceous plants have been found in the glacial drift, showing that 

 they were derived from Cretaceous beds that lay far north of the site of the 

 island. 



THE WHITE CLAY 



The beds of kaolinite, or white clay, and the associated beds of white quartz 

 gravel that are folded in with beds of glacial sand and gravel in the cliffs at Clay 

 Head contain no fossils but are presumably like the neighboring beds of lignitic 

 clay found at the horizon of the Magothy formation in the lower part of the 

 Upper Cretaceous series. These beds are found at but few places, and as all the 



