ENDLICH ON EROSION IN COLORADO. 847 



Torrents caused by violent rain-storms, and by sudden melting of 

 the accumulated masses of snow higher up, have given the first impulse 

 to the formation of this interesting group. Sweeping down the hill- 

 side, over the impregnable masses of trachyte, they have reached this 

 easily eroded deposit of "ash". Eapidly cutting down into the soft 

 material, no resistance has been offered to the eroding action, save by 

 the erratic blocks of basalt scattered along the slope. There the water 

 must separate, thus carving, primarily, sharp, narrow ridges out of the 

 tuffs. Subsequent erosion caused transverse separation of portions of 

 these ridges, and the bowlders that first determined their preservation 

 remained as protectors upon the tops of more or less pyramidal forms. 

 Eain, hail, snow, frost, and wind were the artists that eventually moulded 

 the monuments into their present shape. Ever changing in their detail- 

 features, losing m aterial day after day, they gradually approach that 

 time when the cap can no longer be sustained. Without the protection 

 of this accidentally placed rock, the column rapidly goes toward its final 

 destruction. The constant denudation, the never-ceasing exposure of 

 fresh surface, has precluded the possibility of any vegetation thriving 

 within the area assigned to these monuments. Though utterly devoid 

 of this feature, which constitutes so large a portion of the charm at 

 South Eiver, the exquisite workmanship of the detail-carving and the 

 pure colors exhibited, readily allow one to forget its absence. About 

 two hundred of these monuments are here grouped together, varying 

 in size and in arrangement. Small ones occupy isolated j^ositions, 

 caused not unfrequently by the protection of the basalt after it had 

 abandoned the first column by which it was supported. The largest 

 ones are near the walls of the horseshoe, frequently having one common 

 base, and separating from each other at different points of height. 



PLATEAU CREEK. 



Dr. Peale, in 1874, found some very prominent occurrences belonging 

 to this class.* Plateau Creek flows into the Grand Kiver north of the 

 Great Mesa. About west longitude 108° and north latitude 39o 20', the 

 monuments in question were observed. Tertiary shales compse the bluffs 

 bordering upon the creek. A number of the ridges composed thereof 

 are covered by basalt, which had its origin to the northeast. Erosion has 

 isolated a number of these bluffs, and their edges, fronting the creek, 

 are formed by high, massive monuments. Weathering and fluviatile 

 action has separated portions of the superincumbent basalt, and the 

 fragments form the cap-stones upon the columns. Shales, of light yellow 

 and gray colors, nearly horizontally stratified, are cut into more or less 

 regular cones, and support blocks of black basalt. Dr. Peale says: — 

 "The covering of basalt which once covered it has been partially 

 removed. The remnants left reach from 200 to 250 feet above the general 



* Rep. U. S. Geol. Surv. for 1874, p. 9L 



