ACADEMY OF SCIENCES] POWELL'S SURVEY 99 



LAND SCULPTURE AND CLIMATE 



The detailed features to be discussed under "Sculpture and climate" were the sharp crests 

 of certain summits as contrasted with the rounded crests of other summits of similar structure 

 in the Henry Mountains; and the contrast was shown to depend initially on altitude; for 

 altitude increases rainfall, rainfall increases vegetation, and vegetation increases the retention 

 of weathered detritus. Hence the lower crests, where rainfall is small and vegetation is lack- 

 ing, are sharp; while the higher crests, on which rainfall is somewhat more plentiful and 

 vegetation is encouraged, are soil cloaked and round. This was one of the topics which was 

 well thought out in the field; for it was noted on October 27, 1876, under the heading, "Moun- 

 tain sculpture & climate": 



The mts which lift the largest masses highest get most moisture. This stimulates vegetation & thereby 

 aids disintegration (weathering) & hinders transportation. The result is a smoothness of slope & contour as 

 compared with lower, smaller mts. In general, the other conditions of erosion being the same, aridity tends to 

 ruggedness, to canons, to pinnacles. 



The report next takes up a discussion of bad-land forms carved in homogenous soft rocks, 

 where the law of divides leads to the expectation that the minutely subdivided crests shoidd 

 have angular cross profiles; but it was noted that as a matter of fact the profiles just over the 

 crests are round. 



Evidently some factor has been overlooked in the analysis, — a factor which in the main is less important 

 than the flow of water, but which asserts its existence at those points where the flow of water is exceedingly 

 small, and is there supreme (123). 



It is curious to read here that "some factor has been overlooked," in view of an explanatory 

 entry in a field notebook on the same date with the extract last quoted. The entry concerns 

 "any process which penetrates from the surface of a solid," and opens with the statement: 

 "I think I have solved the problem of the rounding of the crests of the bad-land ridges"; 

 then after a page of general considerations it reads.: " Where the surface is convex there is 

 greater penetration than where it is plane; and where it is concave, these is less. ... It is 

 under this action that angular blocks of crystalline rocks (& others) become round by simple 

 weathering, without attrition." A significant application of this principle follows: 



Now what is the action which penetrates in the case of bad-lands? It is weathering. It is the action of 

 water in dissolving, in decomposing, in expanding (either with or without frost) the shale & thus disintegrating 

 it and preparing it for transportation. Thus in badlands, transportation and corrasion are tending to produce 

 angular forms & weathering opposes their production. The action of weathering is most potent where the 

 deviations from a plane is greatest [that is, along the convex crest of a divide]. Angularity is greatest where 

 erosion is most rapid [that is, along the sharply incised channel of a rain-water rill]. 



This explanation would seem sufficient to most students of land forms; but perhaps Gil- 

 bert's failure explicitly to recognize "soil creep" as an effect of weathering — a process which 

 Lesley had recognized and explained 20 years before — left hhn dissatisfied with it. In any 

 case he certainly showed great conscientiousness if, after getting so "warm," he still felt that 

 the rounding on the crests of bad-land divides needed further examination. He returned to 

 this subject 30 years later in an article on " The convexity of hilltops," 2 and explained it fully on 

 the basis of observations made on the uplands of the Sierra Nevada. He might have at the 

 same time adduced examples of extremely sharp crests in the plant-covered ridges of certain 

 volcanic islands in the tropical seas, like Oahu and Tahiti, where the rainfall is so heavy that its 

 action is far in excess of soil creep; as a result the slopes on the two sides of a ridge increase in 

 steepness upward until they meet in an extraordinarily acute edge, and thus warrant the "law 

 of divides" fully. But Gilbert's experience even in the wettest parts of the Henry Mountains 

 and the high plateaus was with only a moderate rainfall; so moderate that instead of producing 

 acute-edged ridges like those of the tropical islands just mentioned, it favored the production 

 of rounded crests and summits in the manner stated in the first paragraph of this section. 



' Journ. Geol., xvii, 1909, 344-350. 



