Contributions to the geography of the United States, 1923-1924 / Marius R. Campbell, geologist in charge.

Abstract

are not numerous, for the blocks of the pedestal rocks weather slowly, as shown by their smooth brown and incrusted surfaces. Those blocks which are so placed that the bedding planes are vertical have no such smooth crust. Their surfaces are rough and are bright in color. Pebbles and fragments of the matrix are easily detached with the fingers. The blocks shed numerous durable fragments, which, lying at the bases of the blocks, protect the underlying shale from the drip. Doubtless also the rough surfaces retard the movement of water and more of it penetrates into the rock. Light rains probably cause no drip from such blocks. For these reasons rocks with vertical bedding planes stand on conical hillocks, whose slope is determined by the ability of entirely local rain wash to erode the shale under the protecting film of gravel. That the type of surface characteristic of the Shinarump blocks enables the film of water to extend to the under side of the block for 3 feet before falling probably explains the fact that only large blocks much more than 6 feet in diameter stand on pedestals. The block about 6 feet in diameter shown on the left in Plate II, B, has so small a pedestal that its fall to the left seems imminent. The pedestal, if formed whoUy by the action of the drip curtain, should be as large as the annular ring of the drip furrow. However, most of the pedestals are slimmer, like those of the rocks shown in Plates I, B, and II, A. These pedestals and the pedestals of many other rocks have been eroded more than 3 feet back of the drip curtain. They have almost vertical sides, and there is no talus or slope of loose material between the pedestal and the drip furrow. These features may be explained by the action of the film of water that extends inside the drip curtain. In the rain of July 14, 1921, the film merely moistened the top *of the pedestal, but in great storms water may completely cover it. Even a small amount of water is effective in the erosion of the Moenkopi formation, for it is sandy and porous, has well-defined vertical joints, and is impregnated with gypsum and other soluble salts. A small amount of water applied at the top and soaking into the pedestal is sufficient to open the joints and bring to the surface the soluble salts, as shown in Plate IV, A and B. The shale not only crumbles and ''slacks" at the surface but sloughs off along the vertical joints. The vertical sides of the pedestal may therefore be explained by the work of the film of water due to ordinary storms, but it is not certain that even great rains would produce enough water for the film to cover the entire pedestal and run off at the foot in large enough quantities to carry away the talus. The direct erosive action of wind-driven rain,^the splash from the drip furrow, and the lateral migration of "rivulets sKinahan, G. H., Valleys and their relation to fissures, fractures, and faults, p. 83, London, 1875. Attributes part of erosion of pedestal rocks of Devonshire to wind-driven rain. 6 Barrel!, Josepb, The piedmont terraces o...

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Archaeological and Historical Investigations at the Site of the New Bexar County Justice Center in Downtown San Antonio, Texas

“…The soil beneath the house was identical to that between the house and the kitchen. Artifacts found in that area (Table 2,c) were not remarkable, with the exception of a cache of 11 copper printing plates found beneath the front porch that bore the imprints of illustrations from Dr. Campbell's (1925) book, Bats, Mosquitoes and Dol/ars. One interesting thing about these plates is the fact that several of the illustrations did not appear in the book, perhaps victims of a zealous editor during the last stages of production of the book.…”