GLACIAL DRIFT. 303 



sand, with occasional layers of gravel containing pebbles up to six inches in diameter, 

 exposed for 20 feet, and probably extending as much farther to the sea-level. 



The accumulation of these thick deposits of modified drift, occupying an area more 

 than forty miles long, with an average breadth of five miles, remote from any large 

 river, and bordered on each side by the sea, seems capable of explanation only by sup- 

 posing the material to have been held in an ice-sheet, which extended to the line that 

 we have indicated, covering the Vineyard sound, Cape Cod and Massachusetts bays, 

 and thence reaching to the north-east over a large part of the Gulf of Maine. When 

 the return of a warmer climate drove back the front of these ice-fields to the long ter- 

 minal moraine of the Elizabeth islands, Falmouth, Sandwich, and Plymouth, the rivers 

 which flowed from their melting surface were principally discharged at two points, those 

 at the south-west converging towards Barnstable, while those which descended from 

 the glacial sheet over Massachusetts bay had their mouth in Wellfieet and Truro. The 

 bordering walls and irregular masses and ridges of ice, which beset these rivers at their 

 points of escape from the ice-sheet, caused their deposits over these areas to be massed 

 in kames. The ocean at this period stood 150 feet or more above its present height; 

 and the part of the burden of these glacial rivers, which was carried beyond their 

 mouths, was spread by marine currents in nearly level plains, bordering the front of the 

 ice-sheet. The true terminal moraine of till, formed by the ice at this bound of its 

 greatest extent, is covered by the sea or by these beds of modified drift. 



The north end of these Champlain deposits is at High Head. The whole of Prov- 

 incetown consists of sea-sand, with no pebbles. This sand has come from the erosion 

 by the sea of the east shore of the cape ; has been swept north and west to its present 

 place in the lee of this breakwater ; lifted by the waves into beach-ridges ; and further 

 raised by the wind into hills a hundred feet in height. 



On Long Island the farthest limit attained by the ice-sheet is probably indicated by 

 a series of drift hills, which is commonly known as the "backbone of the island." 

 These hills are well exposed along the south shore for about ten miles west from Mon- 

 tauk point, forming cliffs from 20 to 140 feet high. Westward, they extend through the 

 north part of East Hampton, and from Sag Harbor south-west to the Shinnecock hills 

 and Canoe place. Thence they continue in a nearly west course, including Osborn's 

 hill, a few miles south-west of Riverhead ; Terry's hill, south of Manor ; Holman's hill, 

 north of Yaphank; the Coram, Seldon, and Bald hills; Mount Pleasant, west of Ron- 

 konkoma lake ; Pine hill ; the Commac, Dix, and West hills ; Spring, Wheatly, and 

 Harbor hills, the last of which, near Roslyn, is the highest point on this island. Far- 

 ther west, this series of hills trends a little more to the south, passing near Lakeville, 

 and close north of Creedmoor, Jamaica, and East New York. Thence it nearly co- 

 incides with the south-east boundary of Brooklyn, and reaches to the Narrows, forming 

 the sites of Cypress Hill cemetery, Ridgewood reservoir, and the cemetery of the Ever- 

 greens, of the highest portions of Prospect park and Greenwood cemetery, and of Fort 

 Hamilton. 



The length of this range from Montauk point to the Narrows is about 115 miles. It 



