it drags, and thus forces the stones on one level over those below.
Where the waters of the subglacial stream have swept the bed rock
clean of _débris_ its surface is scored, grooved, and here and there
polished in a manner which is accomplished only by ice action, though
some likeness to it is afforded where stones have been swept over for
ages by blowing sand. Here and there, often in a way which interrupts
the cavern journey, the shrunken stream, unable to carry forward the
_débris_, deposits the material in the chamber, sometimes filling the
arch so completely that the waters are forced to make a detour. This
action is particularly interesting, for the reason that in regions
whence glaciers have disappeared the deposits formed in the old ice
arches often afford singularly perfect moulds of those caverns which
were produced by the ancient subglacial streams. These moulds are
termed _eskers_.

If the observer be attentive, he will note the fact that the waters
emerging from beneath the considerable glacier are very much charged
with mud. If he will take a glass of the water at the point of escape,
he will often find, on permitting it to settle, that the sediment
amounts to as much as one twentieth of the volume. While the greater
part of this detritus will descend to the bottom of the vessel in the
course of a day, a portion of it does not thus fall. He may also note
that this mud is not of the yellowish hue which he is accustomed to
behold in the materials laid down by ordinary rivers, but has a
whitish colour. Further study will reveal the fact that the difference
is due to the lack of oxidation in the case of the glacial detritus.
River muds forming slowly and during long-continued exposure to the
action of the air have their contained iron much oxidized, which gives
them a part of their darkened appearance. Moreover, they are somewhat
coloured with decayed vegetable matter. The waste from beneath the