102 WJMCGEE — SHEETFLOOD EROSION. 



lined depressions to depths of several inches. A highly significant effect 

 was found on examining the track of one of the more violent rushes 

 within the flood : At the upper end this was a gully reaching two feet 

 in depth and one or two yards in width, newly gouged in the gravelly 

 and sandy silt of the plain ; at the lower end it was an elongated delta 

 or fan, composed chiefly of sand but containing occasional pebbles 

 (which were not borne by the sheetflood, and must accordingly have 

 been washed out of the gully). A score of other gullies and deltas 

 were seen, some well developed, some nearly obliterated, and it seemed 

 clear that the well developed examples were only those produced toward 

 the end of the freshet, the earlier examples having been masked or oblit- 

 erated by the later flooding. These distinctive marks of the sheetfLood- 

 ing were distributed as extensively as the flotsam heaps, and in like 

 manner were found on the plains below neighboring arroyos 



On traversing a characteristic torrential apron stretching southward 

 from the southern end of Baboquivari range toward the great arroyo 

 known as Rio Seco, in northern Sonora, the track of a still more exten- 

 sive sheetflood was crossed, its traces (apparently some months old) being 

 the characteristic accumulation of flotsam or drift lodged against shrub- 

 bery and elevations, and the equally characteristic gullies terminating in 

 fans. The route lay almost directly across the slope, three to five miles 

 from the base of the mountain ; and the sheetflood-plain was so smooth 

 that, with a little eare in avoiding the occasional gullies, the wagon 

 passed over it at a rapid trot, save in crossing a single sand-lined arroyo 

 eight or ten feet deep and twenty yards across, the torrent marks indi- 

 cating that the entire plain had been flooded for a width of nearly or 

 quite ten miles to a depth exceeding a foot and not exceeding a yard, 

 save in the central arroyo and one on either side (down which the drain- 

 age from the barrancas had evidently flowed streamwise after the force 

 of the torrent was spent). This plain is a typical torrential apron of the 

 Sonoran district ; its slope, five miles from the mountain base, is perhaps 

 150 feet to the mile, increasing to 200 feet near the mountains and per- 

 haps 300 in the reentrants between projecting spurs, while toward Rio 

 Seco the inclination diminishes to 100 feet to the mile or less where the 

 surface passes by a low crenulate escarpment into the broad wash of the 

 psen do-river, itself sloping probably 40 or 50 feet to the mile. The de- 

 posits of the plain as revealed in gullies and the marginal and central 

 arroyos are gravelly and even bouldery sands near the mountain, grad- 

 ing into silty sands toward Rio Seco. These deposits have the customary 

 air of great depth, }^et, as shown in the walls of the barranca (the main 

 head of Rio Seco) passing the frontier post of Sasabe and the lesser bar- 

 ranca near the Indian village of Poso Verde, they are usually but a yard 



