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.