But there are two circumstances connected with these knolls
deserving special notice. They frequently present the glacial marks only
on one side, while the opposite side has all the irregularities and
roughness of a hill-slope not acted upon by ice. It is evident that the
polished side was the one turned towards the advancing glacier, the side
against which the ice pressed in its onward movement,--while it passed
over the other side, the lee side as we may call it, without coming in
immediate contact with it, bridging the depression, and touching bottom
again a little farther on. As an additional evidence of this fact, we
frequently find on the lee side of such knolls accumulations of the
loose materials which the glacier carries with it. It is only, however,
when the knolls are quite high, and abrupt enough to allow any rigid
substance to bridge over the space in its descent from the summit to the
surface below, that we find these conditions; when the knolls are low
and slope gently downward in every direction, they present the
characteristic glacier-surfaces equally on all sides. This circumstance
should be borne in mind by all who investigate the traces of
glacier-action; for this inequality in the surfaces presented by the
opposite sides of any obstacle in the path of the ice is often an
important means of determining the direction of its motion.
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