Note that the crystalline rocks a have been crumpled and crushed. Comparing their structure with that of folded mountains, what do you infer as to their relief after their deformation? To which surface were they first worn down, mm´ or nm? Describe and account for the surface mm´. How does it differ from the surface of the crystalline rocks seen in the Torridonian Mountains ([Fig. 206]), and why? This surface mm´ is one of the oldest land surfaces of which any vestige remains. It is a bit of fossil geography buried from view since the earliest geological ages and recently brought to light by the erosion of the canyon.

Fig. 207. Diagram of Wall of the Colorado
Canyon, Arizona, showing Unconformities 		Fig. 208. View of the North Wall of the Grand Canyon of the Colorado River, Arizona, showing the Unconformities illustrated in [Figure 207]

How did the surface mm´ come to receive its cover of sandstones b? From the thickness and coarseness of these sediments draw inferences as to the land mass from which they were derived. Was it rising or subsiding? high or low? Were its streams slow or swift? Was the amount of erosion small or great?

Note the strong dip of these sandstones b. Was the surface mm´ tilted as now when the sandstones were deposited upon it? When was it tilted? Draw a diagram showing the attitude of the rocks after this tilting occurred, and their height relative to sea level.

The surface nn´ is remarkably even, although diversified by some low hills which rise into the bedded rocks of c, and it may be traced for long distances up and down the canyon. Were the layers of b and the surface mm´ always thus cut short by nn´ as now? What has made the surface nn´ so even? How does it come to cross the hard crystalline rocks a and the weaker sandstones b at the same impartial level? How did the sediments of c come to be laid upon it? Give now the entire history recorded in the section, and in addition that involved in the production of the platform P, shown in [Figure 130], and that of the cutting of the canyon. How does the time involved in the cutting of the canyon compare with that required for the production of the surfaces mm´, nn´, and P?

Fig. 209. Unconformity between the Cambrian and Pre-Cambrian Rocks, Wisconsin

a, pre-Cambrian rocks, igneous and metamorphic, greatly deformed; a´, zone of decomposed pre-Cambrian rocks and residual clays on which rest the Cambrian sandstones b. What unconformity do you find here? What two peneplains do you discover? Which is the older? Which was the more complete? To what stage has the present erosion cycle advanced? Suggest a reason why the valleys in the Cambrian are wider than those in the pre- Cambrian. When did the decay of the pre-Cambrian rocks of zone a´ take place, and under what circumstances? Give the entire history recorded in this section, stating the successive cycles of erosion in their order and the causes which brought each cycle to a close