ROCK-CUT SURFACES IN THE DESERT RANGES 447 
that the original remnant of the mountain left by the gravel en- 
croachment was probably narrower even than at present. 
It should be stated that the gravels here referred to are the 
borders of very extensive desert deposits, stretching for miles to 
to the south and west and are, or have been, the encroaching 
tongues of the great sheets; the tentacles, so to speak, by which 
gravel accumulation incorporates a mountain mass. 
The scene for a moment may now well be shifted to ‘‘the broad 
expanse of plain and mountain in southwestern Arizona and west- 
ern Sonora (Mexico), stretching from the Sierra Madre to the Gulf 
of California and lying between Gila and Yaqui rivers.’ 
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 areais not alluvium, but 
planed rock similar to or identical with that constituting the mountains. 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 carved out of the same rocks. The valley interiors and the 
lower lowlands are, indeed, built of torrent-laid débris, 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 mountainous, 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. 
Thus is clearly set forth by Dr. W. J. McGee the product of a 
long cycle of erosion. 
A short digression is necessary here in order to define the term 
sheet flood, the force of which agent is of importance in the present 
discussion. In the words of W. J. McGee, 
Under certain conditions, sand-laden water flowing over an erodable plain 
tends at first to divide into parallel streams like those of pure water on an 
indestructible surface, yet, since the streams formed in this way at once begin 
to scour and overload themselves and thus check their own flow, this tendency 
is soon counteracted and the water is distributed again; so that the ultimate 
tendency is toward movement in a more or less uniform film or sheet. 
McGee, W. J., ‘“‘Sheet Flood Erosion,” Bull. Geol. Soc. Amer., VIII, February 
13, 1897, pp. 87-112. 
