The key to this beautiful harmony is the ancient glaciers; where they flowed the trees followed, tracing their wavering courses along canons, over ridges, and over high, rolling plateaus.

The Cedars of Lebanon, says Hooker, are growing upon one of the moraines of an ancient glacier.

All the forests of the Sierra are growing upon moraines.

But moraines vanish like the glaciers that make them.

Every storm that falls upon them wastes them, cutting gaps, disintegrating boulders, and carrying away their decaying material into new formations, until at length they are no longer recognizable by any save students, who trace their transitional forms down from the fresh moraines still in process of formation, through those that are more and more ancient, and more and more obscured by vegetation and all kinds of post-glacial weathering.
Had the ice-sheet that once covered all the range been melted simultaneously from the foot-hills to the summits, the flanks would, of course, have been left almost bare of soil, and these noble forests would be wanting.