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The Elements of Geology eBook

This eBook from the Gutenberg Project consists of approximately 328 pages of information about The Elements of Geology.

Wind deposits both of dust and of sand may be expected to contain the remains of land shells, bits of wood, and bones of land animals, testifying to the fact that they were accumulated in open air and not in the sea or in bodies of fresh water.


Sand-laden currents of air abrade and smooth and polish exposed rock surfaces, acting in much the same way as does the jet of steam fed with sharp sand, which is used in the manufacture of ground glass.  Indeed, in a single storm at Cape Cod a plate glass of a lighthouse was so ground by flying sand that its transparency was destroyed and its removal made necessary.

Telegraph poles and wires whetted by wind-blown sands are destroyed within a few years.  In rocks of unequal resistance the harder parts are left in relief, while the softer are etched away.  Thus in the pass of San Bernardino, Cal., through which strong winds stream from the west, crystals of garnet are left projecting on delicate rock fingers from the softer rock in which they were imbedded.

Wind-carved pebbles are characteristically planed, the facets meeting along a summit ridge or at a point like that of a pyramid.  We may suppose that these facets were ground by prevalent winds from certain directions, or that from time to time the stone was undermined and rolled over as the sand beneath it was blown away on the windward side, thus exposing fresh surfaces to the driving sand.  Such wind-carved pebbles are sometimes found in ancient rocks and may be accepted as evidence that the sands of which the rocks are composed were blown about by the wind.

Deflation.  In the denudation of an arid region, wind erosion is comparatively ineffective as compared with deflation (Latin, de, from; flare, to blow),—­a term by which is meant the constant removal of waste by the wind, leaving the rocks bare to the continuous attack of the weather.  In moist climates denudation is continually impeded by the mantle of waste and its cover of vegetation, and the land surface can be lowered no faster than the waste is removed by running water.  Deep residual soils come to protect all regions of moderate slope, concealing from view the rock structure, and the various forms of the land are due more to the agencies of erosion and transportation than to differences in the resistance of the underlying rocks.

But in arid regions the mantle is rapidly removed, even from well-nigh level plains and plateaus, by the sweep of the wind and the wash of occasional rains.  The geological structure of these regions of naked rock can be read as far as the eye can see, and it is to this structure that the forms of the land are there largely due.  In a land mass of horizontal strata, for example, any softer surface rocks wear down to some underlying, resistant stratum, and this for a while forms the surface of a level plateau (Fig. 129).  The edges of the capping layer, together with those of any softer layers beneath it, wear back in steep cliffs, dissected by the valleys of wet-weather streams and often swept bare to the base by the wind.  As they are little protected by talus, which commonly is removed about as fast as formed, these escarpments and the walls of the valleys retreat indefinitely, exposing some hard stratum beneath which forms the floor of a widening terrace.

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