on a spread of glacial drift left by the retreat of continental
ice sheets, such as covers much of Canada and the northeastern
parts of the United States, its infantile stage presents the same
characteristic features,--a narrow and shallow valley, with
undeveloped tributaries and undrained interstream areas. Ground
water stands high, and, exuding in the undrained initial
depressions, forms marshes and lakes.

LAKES. Lakes are perhaps the most obvious of these fleeting
features of infancy. They are short-lived, for their destruction
is soon accomplished by several means. As a river system advances
toward maturity the deepening and extending valleys of the
tributaries lower the ground-water surface and invade the
undrained depressions of the region. Lakes having outlets are
drained away as their basin rims are cut down by the outflowing
streams,--a slow process where the rim is of hard rock, but a
rapid one where it is of soft material such as glacial drift.

Lakes are effaced also by the filling of their basins. Inflowing
streams and the wash of rains bring in waste. Waves abrade the
shore and strew the debris worn from it over the lake bed. Shallow
lakes are often filled with organic matter from decaying
vegetation.

Does the outflowing stream, from a lake carry sediment? How does
this fact affect its erosive power on hard rock? on loose
material?

Lake Geneva is a well-known example of a lake in process of
obliteration. The inflowing Rhone has already displaced the waters