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 _debris_ 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
glacier, where it was either dropped at the margin or, during the
recession of the glacier, was laid upon the surface as the ice melted
away. The result of this erosion and transportation has been to change
the conditions of the surface both as regards soil and drainage. As
the reader has doubtless perceived, ordinary soil is, outside of the
river valleys, derived from the rock beneath where it lies. In
glaciated districts the material is commonly brought from a
considerable distance, often from miles away. These ice-made soils are
rarely very fertile, but they commonly have a great endurance for
tillage, and this for the reason that the earth is refreshed by the
decay of the pebbles which they contain. Moreover, while the tillable
earth of other regions usually has a limited depth, verging downward
into the semisoil or subsoil which represent the little changed bed
rocks, glacial deposits can generally be ploughed as deeply as may
prove desirable.

The drainage of a country recently affected by glaciers is always
imperfect. Owing to the irregular erosion of the bed rocks, and to the
yet more irregular deposition of the detritus, there are very numerous
lakes which are only slowly filled up or by erosion provided with
drainage channels. Though several thousand years have passed by since
the ice disappeared from North America, the greater part of the area
of these fresh-water basins remains, the greater number of them,
mostly those of small size, have become closed.

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