Some of these have weathered into toppling masses, which required only a heavy wind or slight contractions to send them from their uncertain bases onto the snow or ice beneath.

And the other causes mentioned all had their influences in breaking up the peaks and ridges and depositing great jagged bowlders of rock in the slowly-moving glaciers.
Little by little these masses of rock worked their way down lower into the ice-bed.

Sometime they must reach the bottom, yet, though they rest upon granite, and granite would cleave to granite, the irresistible pressure from above forces the ice and rock masses forward.

Thus the sharp-edged blocks of granite become the _blades_ in the tools that are to help cut out the contours of a world's surface.

In other words the mass of glacial ice is the grooving or smoothing _plane_, and the granite blocks, aided by the ice, become the many and diverse blades in this vast and irresistible tool.