BOLSONS OF NORTHERN MEXICO 73 



"At first sight the Sonoran district appears to be one of half-buried moun- 

 tains, with broad alluvial plains rising far up their flanks, and so strong is 

 this impression on one fresh from humid lands that he finds it difficult to trust 

 his senses when he perceives that much of the valley-plain area is not allu- 

 vium, but planed rock similar to or identical with that constituting the moun- 

 tains. To the student of geomorphy this is the striking characteristic of the 

 Sonoran region — the mountains rise from the plains, but both mountain and 

 plain (in large part) are out of the same rocks. The valley interiors and the 

 lowlands are, indeed, built of the torrent-laid debris, yet most of the valley 

 area carries but a veneer of alluvium so thin that it may be shifted by a single 

 great storm. Classed by surface, one-fifth of the area of the Sonoran district, 

 outside of the Sierra and its foothills, is mountain, four-fifths plain ; but of 

 the plain something like one-half, or two-fifths of the entire area, is planed 

 rock, leaving only a like fraction of thick alluvium. This relation seems 

 hardly credible. During the first expedition it was noted with surprise that 

 the horseshoes beat on the planed granite and schist or other hard rocks in 

 traversing the plains 3 or 5 miles from the mountains rising sharply from the 

 same plains without intervening foothills ; it was only after observing this 

 phenomenon on both sides of different ranges and all around several buttes 

 that the relation was generalized, and then the generalization seemed so far 

 inconsistent with facts in other districts that it was stated only with caution 

 even in conversation." 



The author just quoted ascribes to sheetflood erosion the origin of the 

 rock-floors of the Sonoran plains. The activity of wind as an accom- 

 panying planation agent is not suggested. The main facts, however, are 

 the singular character, the shallowness, and the great extent of the rock- 

 floors of the northern Mexican intermont plains. > 



DEBRIS OF THE B0L80N PLAINS 



The soft mantle covering the intermont plains of the arid region is 

 usually described as made up of gravels. The plains deposits are, how- 

 ever, in large part fine clays or loams. There are some sands; but on 

 the whole there is comparatively little real gravel and coarse rock ma- 

 terial. The latter appears mainly in the paths of the torrential drainage- 

 ways debouching from the mountains and on the piedmont slopes, but 

 seldom extends very far out into the plains area proper. 



There are two or three aspects of the plains debris that perhaps tend to 

 give a false impression regarding its composition. The arroyos leading 

 from the mountains have high gradients ; their waters, whenever there 

 are any, are invariably torrential in character; the fans produced at the 

 mouths of the canyons are mainly made up of coarse waste. To the 

 traveler who is accustomed only to the geologic phenomena of the humid 

 climate the natural inference is that the coarse materials from the high 

 peripheral belt are carried down to level up the low plain. 



