10 CAPE COD GEOLOGY 



beds of Miocene age, such as those at Gay Head and in the town of Marshfield, 

 Mass., are ancient deformed and displaced sediments into which magmas that 

 formed granite and other eruptive rocks have been thrust by several intrusions. 

 The last cycle of geologic change, the effects of which are still evident, was char- 

 acterized by erosion which produced widespread base leveling of the hard ancient 

 rocks of this part of the continent. The best defined surface formed by this 

 erosion is the beveled slope of the harder rocks in this region, whose extension 

 beneath the New England Islands forms the floor upon which the later beds rest 

 in this part of the Atlantic coastal plain. This surface was formed, locally, at 

 least, before the Upper Cretaceous deposits were laid down. The dissected 

 remnants of this surface along the south coast of Massachusetts, Rhode Island, 

 and Connecticut have a seaward mean dip of about 50 feet to the mile between the 

 100-foot contour line and the 300-foot or 400-foot contour line. The dip increases 

 from the southern coast of Massachusetts progressively westward to the southern 

 coast of western Connecticut, as shown below: 



100-foot Dip in feet 



Quadrangle contour to per mile 



Massachusetts 



New Bedford ........ 200 22? 



Fall River . 300 20? 



Rhode Island 



Newport . . - 



Charlestown ......••• 300 32 



Connecticut 



Stonington ......-•• 300 33 



New London 400 33 



Saybrook ....-•••• 



Guilford • • 



New Haven • 400 50 



Bridgeport 400 60 



Norwalk 300 60 



Stamford 500E 45 



Stamford 500W 72 



The space between the shore and the mean position of the restored 100-foot 

 contour line is too much obscured by morainal deposits or by inlets of the sea 

 to be considered in estimating the slope of rock surface in the greater part of 

 the district. The increasing inclination toward the west of part of the theoretical 

 old floor on which the Upper Cretaceous beds are believed to have overlapped 

 upon the mainland for at least from 5 to 10 miles inside of the present shore 

 line indicates a steepening warp of the old surface westward. The inclination 

 on the east is nearly that at which strata are laid down on a plain. Farther 



