There were convulsions in those days when the sea washed close to the bases of the mountains. Through the crevasses and fissures in the rocks the water crept into the fires of the earth, and explosions—volcanic eruptions—were the result. Wandering over these stony tracks you might fancy that all strata and all geological ages were blown into discord by those explosions. For here are many kinds of splintered and twisted rocks—rocks aqueous and igneous, gritstones, conglomerates, shales, slates, syenite, basalt. And everywhere the white coatings of carbonate of lime that look as though they were run hot from a puddling furnace; and the dust of sulphur, copper, and iron blown upon granite as though oxidized by fire.
Glaciers.
Land slips.
The evidence for glaciers is not so convincing. There is no apparent sign of an ice age. Occasionally one sees scratches upon mountain walls that are suspicious, or heaps of sand and gravel that look as though pushed into the small valleys by some huge force. And again there are places on the Mojave where windrows of heavy bowlders are piled on either side of mountain water-courses, looking as though ice may have caused their peculiar placing. But there is no certainty about any of these. Land slips may have made the windrows as easily as ice slips; and water can heap mounds of sand and gravel as readily as glaciers. One cannot trace the geological ages with such facility. Things sometimes “just happen,” in spite of scientific theories.
Movement of stones.
The talus.
Besides, the movement of the stones into the valleys is going on continuously, irrespective of glaciers. They are first broken from the peaks by erosion, and then they fall into what is called a talus—a great slope of stone blocks beginning half way down the mountain and often reaching to the base or foot. Many of them, of course, are rolled over steep declivities into the canyons and thence carried down by flood waters; but the talus is the more uniform method for bowlders reaching the plain.
Stages of the talus.
In the first stage of the talus the blocks are ragged-edged and as large as a barrel. Nothing whatever grows upon the slope. It is as bare as the side of a volcanic crater. And just as difficult to walk over. The talus is added to at the top by the falling rock of the face-wall, and it is losing at the bottom by the under blocks grinding away to stone and gravel. The flattening out at the bottom, the breaking up of the blocks, and the push-out of the mountain foot upon the plain is the second stage of the talus. In almost all the large valleys of the desert the depressed talus extends, sometimes miles in length, out from the foot of the mountain range. When it finally slips down into the valley and becomes a flat floor it has entered upon its third and last stage. It is then the ordinary valley-bed covered with its cactus and cut by its arroyos. Yet this valley-floor instead of being just one thing is really many things—or rather made up of many different materials and showing many different surfaces.
Desert-floors.