CAPE COD GEOLOGY 9 



The lakes or natural ponds on Cape Cod lie in depressions left in glacial 

 deposits by the melting out of large blocks of ice. From their shapes, dimensions, 

 and alignment it is possible to restore a picture of the ice as the glacier ap- 

 proached its final stage. These ponds are particularly numerous in the plains 

 on the south side of the morainal belt on Cape Cod. They are enclosed by steep- 

 sided banks of sand and form great natural wells in the enclosing glacial material. 

 Numerous shallower depressions of like origin also occur in the region, but these 

 are dry, because their bottoms stand above the level of ground water. The 

 depth of the deepest of these ponds, as shown by soundings, is 81 feet. Many 

 small, shallow ponds that formerly appeared on maps have been drained or 

 filled and converted into cranberry bogs. 



The coast line of Cape Cod and of the islands has for long stretches been 

 straightened by the attack of the sea, but some of the indentations of the coast 

 near headlands have been protected from such attack by the natural construction 

 of offshore bars and beaches, which are attached at one end to the cliffs that 

 afford the gravel and sand for their construction and maintenance. The indenta- 

 tions of the coast line, the larger of which form small harbors or ports of refuge 

 against certain winds, are nearly all of glacial origin. 



Along most parts of the Atlantic coast tidal currents sweep away rapidly 

 the gravel, sand, and clay that are loosened from the cliffs by the waves of storms. 

 The shoals about Nantucket and southeast of Cape Cod are famous for their 

 ever-shifting sand. 



GENERAL GEOLOGY 



Some account of the geologic formations of the mainland of Massachusetts 

 and Rhode Island is necessary to show the source of the boulders and other 

 glacial erratics that are so widely distributed on the islands off the south coast of 

 these states. For the same reason the boulders on Cape Cod require an account 

 of the geology of the east coast of Massachusetts and the country farther north- 

 east. Much of the gravel, sand, and clay that constitute the deeper formations 

 of the islands and of Cape Cod has been derived from the rocks of the mainland. 

 The geological formations underlying the Upper Cretaceous beds that form the 

 lowest stratum found on the islands and on Cape Cod are here called the pre- 

 Cretaceous basement. 



Pre-Cretaceous Basement 

 Throughout the mainland of southern New England the geologic formations 

 that lie beneath the glacial drift and beneath a few exceptional areas of soft 



