Page:Popular Science Monthly Volume 20.djvu/251

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239
NORTH AMERICA IN THE ICE PERIOD.

the upper till, as would be expected, considering that the fragments in front of the glacier principally consist of extensive tracts of that deposit 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 copious, the unstratified drift 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 sufficient 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 débris from the land to unknown distances southerly over the Atlantic Ocean. The outermost series of terminal moraines commences upon the Island 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 remarkably 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 inclined 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 modified drift, containing few or rare bowlders. The western portions represent the typical constitution of terminal moraines, and are well