the regions north of the Great Lakes, they conveyed quantities of the
_débris_ from that section as far south as the Ohio River. In part
this rubbish was dragged forward by the ice as the sheet advanced; in
part it was urged onward by the streams of liquid water formed by the
ordinary process of ice melting. Such subglacial rivers appear to have
been formed along the margins of all the great glaciers. We can
sometimes trace their course by the excavation which they have made,
but more commonly by the long ridges of stratified sand and gravel
which were packed into the caverns excavated by these subglacial
rivers, which are known to glacialists as _eskers_, or as serpent
kames. In many cases we can trace where these streams flowed up stream
in the old river valleys until they discharged over their head waters.
Thus in the valley of the Genesee, which now flows from Pennsylvania,
where it heads against the tributaries of the Ohio and Susquehanna, to
Lake Ontario, there was during the Glacial epoch a considerable river
which discharged its waters into those of the Ohio and the Susquehanna
over the falls at the head of its course.

[Illustration: _Front of Muir Glacier, showing ice entering the sea;
also small icebergs._]

The effect of widespread glacial action on a country such as North
America appears to have been, in the first place, to disturb the
attitude of the land by bearing down portions of its surface, a
process which led to the uprising of other parts which lay beyond the
realm of the ice. Within the field of glaciation, so far as the ice
rested bodily on the surface, the rocks were rapidly worn away. A
great deal of the _débris_ was ground to fine powder, and went far
with the waters of the under-running streams. A large part was
entangled in the ice, and moved forward toward the front of the