about Lake Tahoe and Lake Mono ran
out far into the water and formed icebergs I think is
quite certain, and that parallel moraines open below are
characteristic signs of such conditions I also think nearly
certain.

_f. Glacial Erosion_. My observations on glacial pathways
in the High Sierra, and especially about Lake Tahoe, have
greatly modified my views as to the nature of glacial erosion.
Writers on this subject seem to regard glacial erosion as
mostly, if not wholly, a _grinding_ and _scoring_;
the debris of this erosion as rock-meal; the great bowlders,
which are found in such immense quantities in the terminal
deposit, as derived wholly from the crumbling cliffs above the
glacial surface; the _rounded_ bowlders, which are often
the most numerous, as derived in precisely the same way, only
they have been engulfed by crevasses, or between the sides of
the glacier and the bounding wall, and thus carried between
the moving ice and its rocky bed, as between
the upper and nether millstone. In a word, all bowlders,
whether angular or rounded, are supposed to owe their
_origin_ or _separation_ and _shaping_ to
glacial agency.

Now, if such be the true view of glacial erosion, evidently
its effect in mountain sculpture must be small indeed.
_Roches moutonnees_ are recognized by all as the most
universal and characteristic sign of a glacial bed. Sometimes
these beds are only imperfect _moutonnees_, i.e., they
are composed of _broken angular surface with only the points
and edges planed off_. Now, _moutonnees_ surfaces
always, and especially angular surfaces with only points and
edges beveled, show that the erosion by grinding has been only
very superficial. They show that if the usual view of glacial
erosion be correct, the great canyons, so far from being
_formed_, were only very _slightly modified_
by glacial agency. But I am quite satisfied from my
own observations, that this is not the only _nor the
principal_ mode of glacial erosion. I am convinced that
a glacier, by its enormous pressure and resistless onward
movement, is _constantly breaking off large blocks_ from
its bed and bounding walls. Its erosion is not only a grinding
and scoring, but also a _crushing and breaking_. It
makes by its erosion not only rock-meal, but also large
_rock-chips_.