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Some cliffs may be relatively stable for years following a cycle of erosion, he said.

If while the quartzite remained much as now, the valleys in the sandstone below 1,000 or 1,100 feet were filled, the result would correspond in a general way to the surface which existed in this region when the first distinctly recognizable cycle of erosion was brought to a close.

Thus a new cycle of erosion was begun, and the uplifted peneplain was dissected by the quickened streams which sank their valleys promptly into the slightly resistant sandstone.

The time necessary for the development of such a surface is known as a cycle of erosion, and the resulting surface is a base-level plain, that is, a plain as near sea level as river erosion can bring it.

This cycle of erosion appears to have advanced no farther than to the development of wide flats along the principal streams, such as the Wisconsin and the Baraboo, and narrow ones along the subordinate water courses, when it was interrupted.