CAPE COD GEOLOGY 



83 



extended seaward as much as two miles beyond the present coast. If Marindin's 

 determination of the annual rate of recession — a mean rate of 5 feet per annum 

 between Nauset and Highland Light — holds good for an indefinite past, the 

 coast has retreated a mile in about 1,050 years and would have been over two 

 miles farther east when the bars of Provincetown were begun. At the same 

 rate of wear the narrow segment of the Cape between Orleans and Wellfleet 

 will have been consumed by the sea within 3,000 years. 



Independent calculations as to the length of time taken to change existing 

 shore lines at several widely distant points, such as the south coast of England 

 and the coast of Massachusetts, tend to support the idea that about 3,000 years 

 ago certain parts of the North Atlantic shore, after undergoing a positive delevel- 

 ing, were brought to their present level. Reid 1 regards the south coast of England 

 as having been stable for about 3,500 years, or from the beginning of the Bronze 

 Age. Johnson 2 concluded from the accordant level of the successive beach crests 

 at Nantasket that there had been no measurable deleveling during the period 

 of their construction, which he estimated at not less than 1,000 years, and prob- 

 ably 2,000 or 3,000 years. But this conclusion must meet the objections raised 

 by those who hold that submerged peat bogs and the marshes along certain coasts 

 are proofs that a positive deleveling is now in progress. 



SUMMARY OF THE GEOLOGIC HISTORY OF THE REGION 



The rocks of the neighboring mainland afford evidence of some of the con- 

 ditions that existed and the events that occurred during the remote geologic 

 past in this region and indicate the sources of the material that forms Cape Cod 

 and the New England Islands. In late Paleozoic time the land appears to have 

 extended far south and east of the site of the present coast. During that time 

 gravel, sand, and clay that are now changed to conglomerate, sandstone, and 

 shale were brought to the site of the coal beds at Mansfield, Mass., and to the 

 adjoining part of Rhode Island. Prior to the deposition of these coal beds this 

 ancient land had been invaded by magmas that formed granite, on the eroded 

 surface of which the coal beds were laid down. Bordering the granite on the 

 south, beyond the gneiss and diorite of the region about New Bedford, there 

 were areas of Cambrian quartzite carrying marine fossils, the relics of a far 

 earlier invasion of the sea. In Carboniferous time great quantities of these fossil- 



1 Reid, Clement, Submerged forests, The Cambridge Manuals of Science and Literature, New York, 



1913. 



Johnson, D. W., The form of Nantasket Beach, Jour. Geology, 18, pp. 162-189, 1910., 



