Overview

Excerpt: ...UNCONFORMITIES IN THE COLORADO CANYON, ARIZONA. How geological history may be read in unconformities is further illustrated in Figures 207 and 208. The dark crystalline rocks a at the bottom of the canyon are among the most ancient known, and are overlain unconformably by a mass of tilted coarse marine sandstones b, whose total thickness is not seen in the diagram and measures twelve thousand feet perpendicularly to the dip. Both a and b rise to a common level nn and upon them rest the horizontal sea-laid strata c, in which the upper portion of the canyon has been cut. 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, 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. 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...

Full Product Details

9781153701037

Table of Contents

Reviews

Author Information

Tab Content 6

Customer Reviews

Recent Reviews

Countries Available