excluded. When, therefore, the geologist finds a country abundantly
covered with sand, pebbles, and boulders arranged in an irregular way,
he has generally only to inquire whether the material has been carried
by rivers or by glaciers. This discrimination can be quickly and
critically effected. In the first place, he notes that rivers only in
their torrent sections can carry large fragments of rock, and that in
all cases the fragments move down hill. Further, that where deposits
are formed, they have more or less the form of alluvial deposits. If
now the observations show that the rock waste occupying the surface of
any region has been carried up hill and down, across the valleys,
particularly if there are here and there traces of frontal moraines,
the geologist is entitled to suppose--he may, indeed, be sure--that
the carriage has been effected by a glacial sheet.

Important corroborative evidence of ice action is generally to be
found by inspecting the bed rock below the detritus, which indicates
glacial action. Even if it be somewhat decayed, as is apt to be the
case where the ice sheet long since passed away, the bed rock is
likely to have a warped surface; it is cast into ridges and furrows of
a broad, flowing aspect, such as liquid water never produces, which,
indeed, can only be created by an ice sheet moving over the surface,
cutting its bed in proportion to the hardness of the material.
Furthermore, if the bed rock have a firm texture, and be not too much
decayed, we almost always find upon it grooves or scratches, channels
carved by the stones embedded in the body of the ice, and drawn by its
motion over the fixed material. Thus the proof of glacial extension in
the last ice epoch is made so clear that accurate maps can be prepared
showing the realm of its action. This task is as yet incomplete,
although it is already far advanced.