NORTH AMERICA IN THE ICE PERIOD. 239 



the upper till, as would be expected, considering that the fragments in 

 front of the glacier principally consist of extensive tracts of that de- 

 posit crowded into a small compass. In connection with it one often 

 sees sloping plains of gravel and sand deposited by streams of melted 

 ice acting upon the moraine, or, if the supply of water has been co- 

 pious, the unstratified dx*ift has all been modified. Some authors think 

 the upper till is only the surface portion of the ground-moraine acted 

 upon and oxidized by atmospheric agencies. If the difference in the 

 angularity, roughness, and distance traveled of the stones is not suffi- 

 cient to justify our definitions, then these writers must explain why 

 our terminal moraines should show the oxidation throughout their 

 whole mass, hundreds of feet thick, while the upper till is usually very 

 thin, often no deeper than the roots of large trees. 



There are two lines of terminal moraines more conformable to each 

 other than to the extreme southern limit of the glacier. East of Cape 

 Cod this line is supposed to have passed over the St. George's and 

 Great Newfoundland Banks, while the icebergs carried debris from 

 the land to unknown distances southerly over the Atlantic Ocean. 

 The outermost series of terminal moraines commences upon the Isl- 

 and of Nantucket, and is traceable thence across Martha's Vineyard, 

 Block, Long, and Staten Islands ; whence, according to Professor 

 Cook, " the whole line of the moraine [across New Jersey] is remark- 

 ably plain and well defined." The line across Pennsylvania has been 

 traced out recently by Mr. H. C. Lewis. This whole series, as far 

 west as Ohio, occupies the outer margin of the glaciated area ; and 

 Professor Cook thinks the same moraines will be found conterminous 

 with the extreme southern limit of the ice-sheet to its remotest bound 

 in Montana. 



The inner line of moraines starts at the middle of the east coast of 

 Cape Cod, follows the curved shore to old Plymouth, thence south to 

 the Elizabeth Islands on the border of Buzzard's Bay. After passing 

 under water for several miles, it comes to the surface along the south 

 shore of Rhode Island west of Narragansett Bay, touches Plum and 

 Fisher's Islands, continues to the northeast angle of Long Island, passes 

 through the greater part of Brookhaven, Riverhead, and Southold, 

 where it disappears. 



The outer moraine determines the topography of Long Island, as it 

 constitutes a marked ridge, or " backbone," as sometimes called, from 

 Montauk Point to Fort Hamilton. The highest point is three hundred 

 and eighty-four feet, and the base of the moraines is usually more 

 than fifty feet above tide-water. The southern slope is a gently in- 

 clined sandy plain, made of the ruins of the terminals ; the northern 

 slope terminates in cliffs, because so largely consisting of cretaceous 

 clays. The eastern and middle portions of this ridge consist of modi- 

 fied drift, containing few or rare bowlders. The western portions 

 represent the typical constitution of terminal moraines, and are well 



