Thus, a glacier is constantly breaking off blocks and making angular surfaces, and then grinding off the angles both of the fragments and the bed, and thus forming rounded bowlders and _moutonnees_ surfaces.

Its erosion is a constant process of alternate _rough hewing and planing_.

If the rock be full of fissures, and the glacier deep and heavy, the rough hewing so predominates that the plane has only time to touch the corners a little before the rock is again broken and new angles formed.

This is the case high up on the _canyon walls_, at the head of Cascade Lake and Emerald Bay, but also in the _canyon beds wherever the slate is approached_.

If, on the other hand, the rock is very hard and solid, and the glacier be not very deep and heavy, the planing will predominate over the rough hewing, and a smooth, gentle billowy surface is the result.