The high plateaus of northern Arizona and southern Utah ([Fig. 130]), north of the Grand Canyon of the Colorado River, are composed of stratified rocks more than ten thousand feet thick and of very gentle inclination northward. From the broad plat form in which the canyon has been cut rises a series of gigantic stairs, which are often more than one thousand feet high and a score or more of miles in breadth. The retreating escarpments, the cliffs of the mesas and buttes which they have left behind as outliers, and the walls of the ravines are carved into noble architectural forms— into cathedrals, pyramids, amphitheaters, towers, arches, and colonnades—by the processes of weathering aided by deflation. It is thus by the help of the action of the wind that great plateaus in arid regions are dissected and at last are smoothed away to waterless plains, either composed of naked rock, or strewed with residual gravels, or covered with drifting residual sand.
Fig. 130. North-South Section, Eighty-Five Miles Long, across the Plateau North of the Grand Canyon of the Colorado River, Arizona, showing Retreating Escarpments
O, outliers; V, canyon of the Colorado; A-H, rock systems from the Archean to the Tertiary; P, platform of the plateau from which the once overlying rocks have been stripped; dotted lines indicate probable former extension of the strata. How thick is the mass of strata which has been removed from over the platform? Has this work been accomplished while the Colorado River has been cutting its present canyon?
The specific gravity of air is 1⁄823 that of water. How does this fact affect the weight of the material which each can carry at the same velocity?
If the rainfall should lessen in your own state to from five to ten inches a year, what changes would take place in the vegetation of the country? in the soil? in the streams? in the erosion of valleys? in the agencies chiefly at work in denuding the land?
In what way can a wind-carved pebble be distinguished from a river-worn pebble? from a glaciated pebble?