178 



DYNAMICAL GEOLOGY. 



158. 



Drop-made columns. D. '87. 



sion on soft earth or mud by denudation, Avhicli is circular or elliptical, 

 according as the wind blows or not. These impressions, if they escape 

 obliteration by succeeding drops and are soon covered by a layer of sediment, 

 become " fossil rain-marks," and many surfaces so marked exist in the older 

 rocks, bearing evidence as to former rains, and also as to the above-water 

 level of the surface rained on. It may have been a mud-flat exposed between 

 high and low tides. When the drops strike a gravel bed, stones in the gravel 

 will protect the material directly beneath, while erosion around may cut 

 away the material, and leave standing slender columns, each capped with a 

 stone, as monumental evidence of the work done. 



A miniature specimen of this work was observed by the author in 1887, 

 alongside of the path leading down into Kilauea. It had been produced by 



drops falling from shrubbery, wet with the heavy 

 mist of the night, to a bed of earth, three or four 

 feet below. A portion of the scene is represented, 

 natural size, in Fig. 158. 



Columns of 10 to 30 feet are often made out of 

 beds of gravel, glacial drift, and the like. Fig. 159 

 represents a case near Antelope Park, on a small trib- 

 utary of the Rio Grande, where a bed of tufa, over 

 500 feet thick, contains large stones. The waters of 

 the rains descending along the surface of a vertical wall first made, beneath 

 the stones, bas-reliefs of columns, and then the free columns ; and, in the 

 end, an area three miles long and half a mile wide 

 was thickly covered with the columns, many 60 to 80 

 feet high, and some 400 feet (Endlich, 1875) . 



The power of water-strokes is well illustrated by 

 the effects in gold-washings from a jet under a head 

 of pressure derived from the water in an elevated 

 reservoir, as in California hydraulic mining. The 

 beds of compact auriferous gravel gradually return to 

 their original condition of loose earth and stones, 

 although struck only by a mass of pure water. 



At Niagara, the spray made by the waterfall, 

 carried forcibly into an open chamber behind the 

 fall, causes the wear of the shales (James Hall). 



2. The excavation of valleys ; Denudation. — Ero- 

 sion, excavation and denudation, or land-sculpture, are 

 parts of one process. The simplest illustrations of 

 the subject are afforded by the great, gently sloping, 

 volcanic mountains, made up chiefly of stratified 

 streams of basaltic lavas. In them, the slopes are 



but 5° to 10°, and conditions determining direction of drainage are in general 

 reduced to two, the first and the last of those mentioned on page 177. The 

 facts here presented are from the author's observations of 1839-1841, pub- 

 lished in his Exploring Expedition Report, 1849. 



159. 



Rain-made columns x '04. 

 Endlich, '75. 



J*>i 



