200 GEOLOGY OF NEW YORK CITY
Drumlins and Karnes are names given to certain heaps of drift material, characteristic of drift territory. Drumlins are elongated hills made up of till (a
clayey mixture of stone fragments, boulders, gravel, sand, the filched
detritus of all sorts, of the rocks over which the glacier has passed),
their longer axes lying in the direction of the ice movement. They may
have been made in the ice sheet, and contributions to their increase
may have been brought by the water currents of the glacier. These water
currents were of large volume, and accomplished work both as carriers
and sculpturing and reasserting agents of the drift. Osars, eskers, are
ridges of drift-stuff formed on or in the ice by glacial streams, and
finally deposited underneath the glacier or left bodily by the melting
away of the surrounding ice. Karnes are hills of drift material which have become stratified; they
have resulted from water action; they are apt to lie across the
direction of the ice sheet, rather than, as with drumlins, in
conformity with it; they are associated usually with the terminal
moraine, and the lines of stratification are often undulating; they are
less like ridges than the osars, less confused in composition and
smaller than the drumlins.
The
moraines, and specifically the terminal moraines, reflect more
cohesively the nature of the ice action than anything else. The
terminal moraine is that unassorted barrow of stones, till, gravels,
sand, boulders, small and large, which is interpreted as marking the
extreme advance of the glacier, and which, in loops, angles, sweeping
lines, and straight frontiers, crosses the United States. It is
composite in construction and in age and in distribution. It may mark
numerous advances of the ice; it is unquestionably a conglomerate of
mineral constituents, and it varies enormously in its depth and
development at different points, here covering subadjacent formations
with a heap of debris hundreds of feet thick, and there just thinly
veiling the older rocks. This disparity arises, of course, from
dissimilar conditions in the path of the glacier's course; where there
was an abundance of decayed or fragmental rock this
