ssippi or the
Ohio. At a later stage channels were formed leading from Georgian Bay
to the eastern part of Ontario. Yet later, when the last-named lake
was bared, an ice dam appears to have remained in the St. Lawrence,
which held back the waters to such a height that they discharged
through the valley of the Mohawk into the Hudson. Furthermore, at some
time before the Glacial period, we do not know just when, there
appears to have been an old Niagara River, now filled with drift,
which ran from Lake Erie to Ontario, a different channel from that
occupied by the present stream.

The effects of lakes on the river systems with which they are
connected is in many ways most important. Where they are of
considerable extent, or where even small they are very numerous, they
serve to retain the flood waters, delivering them slowly to the
excurrent streams. In rising one foot a lake may store away more water
than the river by its consequent rise at the point of outflow will
carry away in many months, and this for the simple reason that the
lake may be many hundred or even thousand times as wide as the stream.
Moreover, as before noted, the sediment gathered by the stream above
the level of the lake is deposited in its basin, and does not affect
the lower reaches of the river. The result is that great rivers, such
as drain from the Laurentian Lakes, flow clear water, are exempt from
floods, are essentially without alluvial plains or terraces, and form
no delta deposits. In all these features the St. Lawrence River
affords a wonderful contrast to the Mississippi. Moreover, owing to
the clear waters, though it has flowed for a long time, it has never
been able to cut away the slight obstructions which form its rapids,
barriers which probably would have been removed if its waters had been
charged with sediment.

[Illustration: _Muir Glacier, Alaska, showing crevasses and dust
layer on surface of ice._]

CHAPTER VI.

GLACIERS.

We have already noted the fact that the water in the clouds is very
commonly in the frozen state; a large part of that fluid which is
evaporated from the sea attains the solid form before it returns to
the earth. Nevertheless, in descending, at least nine tenths of the
precipitation returns to the fluid state, and does the kind of work
which we have noted in our account of water. Where, however, the water
arrives on the earth in the frozen condi