CAPE COD GEOLOGY 



223 



Exeter and Slocum, R. I., show that the interlobate axis lay about in longitude 

 71° 32' W. when the ice front had retreated to that locality, and that the axis 

 was but slightly west of that position on the west side of Point Judith when 

 the ice front stood at the Charleston moraine, trending west of south, toward 

 Block Island. Thus the push from the ice lobe moving over the island from 

 Block Island Sound caused the topographic trend noted. The accentuated 

 area lies west of a line skirting from north to south the west coast of Corn Neck, 

 on which there are traces of a topographic trend from northwest to southeast, 

 parallel to the western border of an ice lobe that issued from the Narragansett 

 Bay depression. 



A number of small shoals that lie off the south coast, where the bottom 

 is from 12 to 14 fathoms deep, may be wave-levelled tracts of submerged frontal 

 moraine. A shoal extends southwestward from Southwest Point to Southwest 

 Ledge. It has a minimum depth of 4| fathoms and the sea breaks on it in 

 heavy weather. This shoal is produced by an extension of the glacial formations 

 of the island toward Montauk Point ; but the materials on Southwest Ledge may 

 be largely pre-Wisconsin. 1 



GLACIAL BOULDERS 



The surface of Block Island, according to all accounts, was originally thickly 

 strewn with glacial erratics of all sizes. The largest boulder seen by the writer lies 

 on the northern slope of Pilot Hill, about half a mile from its summit. As meas- 

 ured in 1893 it rose 6 feet above the surface of the ground and was 15 feet 10 

 inches across. About in the middle of the eighteenth century the scarcity of 

 wood for fences, combined with the increase of agriculture, led to the clearing 

 of the fields and the construction of stone fences. Many cartloads of stones that 

 were too small for use in making fences were thrown into the kettles and hol- 

 lows at the bottoms of the hills, so that only a few small tracts were left in 

 something like their natural state. 



The boulders on the surface of Block Island were perhaps brought to their 

 present positions from the mainland by the Wisconsin ice sheet. Although these 

 erratics were derived from the mainland, it is not certain that any particular 

 boulder was carried to the island during the last ice invasion, for the erosion 

 of the older folded boulder beds on the island suggests that boulders from these 

 beds became commingled with the latest drift from the mainland. For this 

 reason particularly it is impossible to determine whether the boulders and 



i See U. S. Coast and Geodetic Survey Chart No. 1211 (Block Island Sound) July, 1912. 



