C. E. DUTTON. Tertiary History of the Grand Cañon District, with Atlas, Mon. II, U. S. Geol. Surv., 1882, pp. 264, pls. 42, maps 23.
G. SWEINFURTH. Map Sheets of the Eastern Egyptian Desert. Berlin, 1901-1902, 8 sheets.
W. D. JOHNSON. The High Plains and their Utilization, 21st Ann. Rept. U. S. Geol. Surv., Pt. iv, 1901, pp. 601-741.
=The weathering processes under control of the fracture system.=—In an earlier chapter it was learned that the rocks which compose the earth’s surface shell are intersected by a system of joint fractures which in little-disturbed areas divide the surface beds into nearly square perpendicular prisms (Fig. 36, p. 55), more or less modified by additional diagonal joints, and often also by more disorderly fractures. Throughout large areas these fractures may maintain nearly constant directions, though either one or more of the master series may be locally absent. This distinctive architecture of the surface shell of the lithosphere has exercised its influence upon the various weathering processes, as it has also upon the activities of running water and of other less common transporting agencies at the surface.
Within high latitudes, where frost action is the dominant weathering process, the water, by insinuating itself along the joints and through repeated freezings, has broken down the rock in the immediate neighborhood of these fractures, and so has impressed upon the surface an image of the underlying pattern of structure lines (plate 10 A).
In much lower latitudes and in regions of insufficient rainfall, the same structures are impressed upon the relief, but by other weathering processes. In the case of the less coherent deposits in these provinces, the initial forms of their erosional surface have sometimes been determined by the dash of rain from the sudden cloudburst. Thus the “bad lands” may have their initial gullies directed and spaced in conformity with the underlying joint structures (Fig. 238).
FIG. 238.—Rain sculpturing under control by joints. Coast of southern California (after a photograph by Fairbanks).]
In such portions of the temperate regions as are favored by a humid climate, the mat of vegetation holds down a layer of soil, and mat and soil in coöperation are effective in preventing any such large measure of frostwork as is characteristic of the subpolar regions or of high levels in the arid lands. In humid regions the rocks become a prey especially to the processes of solution and accompanying chemical decomposition, and these processes, although guided by the course of the percolating ground water along the fracture planes, do not afford such striking examples of the control of surface relief.