Fig. 209.—Hollow hewn blocks in a wall in the Wadi Guerraui (after Walther).
The brown desert varnish is one of the most characteristic marks of an arid country. It is found in all deserts under much the same conditions, and is especially apt to be present in sandstone. When scratched, the surface of the rock becomes either cherry-red, indicating anhydrous ferric oxide, or it is yellowish, due to the hydrated iron oxide which we know as iron rust. Thus it is seen that the sands of deserts, in contrast to those yielded by other processes within humid regions, have a characteristic red color, and this may vary from brownish red upon the one hand to a rich carmine upon the other.
The mechanical breakdown of the desert rocks.—The chemical changes of decomposition within desert rocks are, as we have seen, largely due to the action of concentrated solutions of salts at high temperatures. That there is a certain mechanical rending of these rocks, due to the “freezing” of salts within the capillary fissures, has been already mentioned. A further strain effect arises in rocks like granite, which are a mixture of different minerals. Heated to a high temperature during the day and cooled through a considerable range at night, the different minerals alternately expand and contract at different rates and by different relative amounts, so that strains are set up, tending to tear them apart. The effect of these strains is thus a surface crumbling of rocks.
But rock is, as already pointed out, a relatively poor conductor of heat, and hence it is a relatively thin skin only which passes through the daily round of wide temperature range. This outer shell when heated is expanded, and so tends to peel off, or exfoliate, like the outer skin of an onion. The process is therefore described as exfoliation. In all rocks of homogeneous texture the continued action of this process results in convexly spherical surfaces, the material scaled off in the process remaining as a slope or talus which surrounds the projecting knob ([Fig. 210]). Naked, these projecting domes rise above the rim of débris at their bases. Not a particle of dust adheres to the fresh rock surface—no dirt interferes with its glaring whiteness. Yet close at hand lie masses of débris into which wells may be carried to depths of more than six hundred feet without encountering either solid rock or ground water. The bare walls of granite sometimes mount upwards for thousands of feet into the air, as steep and as inaccessible as the squared towers of the Tyrolean Dolomites.
Fig. 210.—Smooth granite domes shaped by exfoliation and surrounded by a rim of talus. Gebel Karsala, Nubian Desert (after Walther).
Rock is such a poor conductor of heat that special strains are set up at the margin of sunlight and shade. This localization of the disintegration on the margin of the shaded portions of rock masses is known as shadow weathering (see [Fig. 215], [p. 206]).
There is, however, still another mechanical disintegrating process characteristic of the desert regions, which is likewise dependent upon the sudden changes of temperature. Rains, though they may not occur for a year or more, come as sudden downpours of great volume and violence. Rock masses, which are highly heated beneath the desert sun, if suddenly dashed with water, may be rent apart by the differential strains set up near the surface. That rocks may be easily rent as a result of sudden chilling is well known to our Northern farmers, who are accustomed to rid themselves of objectionable bowlders by first building a fire about them and then dashing water upon their surface. Thus split into fragments, even the larger bowlders may be handled and so removed from the farming land. The natural process of rock rending by the occasional cloudburst may be described as diffission. Blocks as much as twenty-five feet in diameter have been observed in the desert of western Texas, soon after being broken into several fragments at the time of a downpour of rain ([Fig. 211]).
