110 WJMCGEE — SHEETFLOOD EROSION. 



torrent may, indeed, divide on the fan surface into a number of streamlets 

 divaricating over the contiguous part of the plain, but these are subject 

 to the same law as the main stream and are continually divided and 

 subdivided as they move down the diminishing incline; and thus the 

 ultimate effect is to distribute the water widely and plane the fan into 

 gentle curves, blending with the adjacent baselevel and sharply dis- 

 cordant with the mountain slope. As the process continues, the fan of 

 one episode, particularly if produced by an exceeding torrent, may re- 

 sist the powers of succeeding freshets, and thus initiate a butte in the 

 line of the barranca, like Fresnal hill; and in some cases the salients 

 between adjacent barrancas are cut through by the divergent floods in 

 such maimer as to form outlying buttes or cusps in line with the aretes 

 ribbing the sierra, like some of the isolated buttes rising from the eastern 

 apron of Sierra Seri, and the granite picacho with its half-mile pediment 

 in the Seriland section near Puerta Infierno. The immediate conse- 

 quence of this retrogression is the cumulative sharpening of the inflec- 

 tion in profile marking the base of the mountain; and it is noteworthy 

 that the smaller mountain remnants on the flatter plains are (other 

 things equal) steeper and more rugged than the greater mountain masses, 

 as illustrated by Sierra de Tonuco and the little buttes in the north- 

 eastern part of Seriland in comparison with the high Sierra. 



In brief, it may be affirmed, with so much confidence as the conditions 

 of observation during two expeditions warrant, that the general effect of 

 sheetflooding in the Sonoran district is to carve baselevel plains, lightly 

 veneered by the carving material, about the medial altitudes ; that these 

 plains tend ever to retrogress into the mountains, and thereby steepen 

 their slopes and render them exceptionally rugged ; and that the anoma- 

 lous topography of the region is not susceptible of explanation by other 

 agencies. 



It would be of interest to consider the natural history of sheetflooding 

 in the region, and to obtain thereby a distinctive geomorphic record 

 which might be compared with the stratigraphic record in reading the 

 geologic history of the southwestern portion of the continent ; but the 

 time for this study has not yet come. 



Sheetflood Work in other Districts. 



While the requisite conditions for sheetflooding are especially favor- 

 able in the Sonoran district, and while the effects of the process are pro- 

 portionately conspicuous, it is not to be supposed that the process is 

 confined to the district — indeed it may be suggested that the process will 

 be found a main or minor one in various districts, particularly those 



