The high plateaus of northern Arizona and southern Utah, 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.
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?
THE SEA AND ITS SHORES
We have already seen that the ocean is the goal at which the waste of the land arrives. The mantle of rock waste, creeping down slopes, is washed to the sea by streams, together with the material which the streams have worn from their beds and that dissolved by underground waters. In arid regions the winds sweep waste either into bordering oceans or into more humid regions where rivers take it up and carry it on to the sea. Glaciers deliver the load of their moraines either directly to the sea or leave it for streams to transport to the same goal. All deposits made on the land, such as the flood plains of rivers, the silts of lake beds, dune sands, and sheets of glacial drift, mark but pauses in the process which is to bring all the materials of the land now above sea level to rest upon the ocean bed.
But the sea is also at work along all its shores as an agent of destruction, and we must first take up its work in erosion before we consider how it transports and deposits the waste of the land.
The sea cliff and the rock bench. On many coasts the land fronts the ocean in a line of cliffs. To the edge of the cliffs there lead down valleys and ridges, carved by running water, which, if extended, would meet the water surface some way out from shore. Evidently they are now abruptly cut short at the present shore line because the land has been cut back.