104 W J MCGEE — SHEETFLOOD EROSION. 



The 1895 expedition skirted the western and southern bases of Sierra 

 de Tonuco, just outside the area mapped as Seriland ; and along the 

 southwestern margin, near Rancho de Tonuco, the steep mountain side 

 passed within a fraction of a mile into fairly smooth plain, sloping per- 

 haps 150 feet to the mile. Numerous water-cut gullies and fans of ex- 

 ceptional size w r ere found. Most of these were apparently some years 

 old ; and the deltas and fans contained quantities of angular and sub- 

 angular boulders, sometimes reaching a foot in diameter. The deeper 

 scorings here revealed a sheet of fragmental debris, evidently drift from 

 the neighboring sierra, rarely reaching five feet in thickness, composed 

 of little-worn gravel with occasional boulders embedded in a matrix of 

 loam or silt ; this rested on a sharply-eroded surface of granite or black 

 marble not at all decomposed. The mantle was variable in thickness, 

 averaging probably less than a yard over considerable areas, and fre- 

 quently disappearing, leaving the rock to form the surface. About the 

 rancho the alluvium thickened locally, and the ground water circulating 

 on the subjacent rock surface was tapped by a well 40 or 50 feet deep; 

 but even here, in the line of natural flowage from the deepest barranca 

 of the sierra toward Encinas desert, where the slope was least, the usual 

 gullies terminating in low fans abounded, some having been evidently 

 produced by the latest storm. On the steeper slopes adjacent a number 

 of low circular or elliptical mounds, apparently made up chiefly of angu- 

 lar boulders, were noted ; and comparison of the evidently older with 

 the manifestly young indicated that these were remnants of ancient 

 gully-fans modified, rounded, and relatively raised above the mean sur- 

 face by subsequent erosion, though in a few cases the boulder mounds 

 were half buried by finer silt deposited about their flanks. 



The 1894 expedition traversed Baboquivari or Moreno valley (west of 

 Baboquivari range), crossing the course of " Fresnal creek " as shown on 

 the excellent official map of Pima county, Arizona ; and though the line 

 of the •' creek " was not traceable (the wash having been, obliterated in 

 consequence of local failure of rains during recent seasons), a consider- 

 able flotsam -marked area was found in the eastern part of the valley, 

 below the Papago Indian villages collectively known as Fresnal. Here, 

 too, in the shadow of the high and remarkably rugged granite range and 

 its dominant peak, boulder mounds similar to those adjacent to Sierra 

 de Tonuco were observed, and in two or three instances low buttes, to- 

 gether with the more considerable eminence known as Fresnal hill, were 

 noted as apparently the product of continued circumdenudation, initiated 

 about accumulations produced in the manner inferred in the Tonuco 

 region. In this valley, too, it was observed that sometimes gullies of 

 considerable dimensions are produced well out on the plain ; in one in- 



