grooves
elsewhere. Here the direction is much more northern (Figs. 57 and 58).
The ice foot has struck this inclined face of gneiss and, being lifted
up by its resistance, has gouged out these undulating gutters. In the
photographs reproduced here, the boys limit on either side the width of
the deep channels, and the urchin in the center is seated on the high
medial ridge or wave. The grooves in the second are almost two feet
across from crest to crest, and are themselves inclosed on the floor of
what appears to be a very wide, gigantic glacial cut or tunnel. On this
spot, or from the text it so appropriately furnishes, the teacher could
make his or her pupils realize very quickly what the ice has done in
planing and chiseling the rocks, and near at hand, on the west of the
meadow, are also some more " wanderers," as a further reinforcement of
the story.
If
the teacher will visit Mt. Tom (Fig. 59), at the foot of 83d Street and
North River, in the Riverside Park, the northern exposure shows a
group of furrows near the grassy embedment of the rock, passing over
it, while broad, plate-like depressions, obscurely recognized, perhaps,
have been excavated over its surface. Immediately south of it is a
companion knob, which has lost its rounded symmetry from blasting, but
which is also scored and unevenly impressed (Fig. 60).
There
need be no confusion in the mind of the observer between the glacial
grooves, striae impressions, etc., and the deceptive linings of the
almost vertical gneiss sheets. The cavities, hollows, long gutters,
etc., which frequently run for considerable distances over the face of
the gneiss, arise from unequal weathering, and have no relation to the
glacial grooves. This is seen quickly when the difference in direction
is taken into account, the glacial grooves and wearings sweeping over
the gneiss almost at right angles to the latter. Fig. 60 shows this
distinction most plainly.
Besides
such impressions as the rocks show, the stones taken from the drift are
frequently finely scored, the scratches passing usually from end to
end along the longer axis of the stone.
