EOLIAN GEOLOGY 
175 
fact in question is nowhere especially noted. It should be one of 
the first points to arrest the attention. 
While the gradient of the sand-plains is quite uniform from one 
end to the other that of the arroyo bed is at first greater than that 
of the plain, and then less. Some distance, often several miles 
up the canyon from its mouth, the planes of the sand-plain and 
of the stream bed intersect. At the cross line where mountain 
gives way to plain the surface of the sand-plain is highest above 
the arroyo, frequently as much as 50 to 100 feet. Here the 
arroyo traverses the sand plain in a walled gorge, for the sand 
which with some clay constitutes the structure of the sand-plain 
is somewhat indurated. Beyond the mouth of the canyon the 
planes of arroyo and sand-plain finally come together again. 
Abrupt replacement of the normal pebble and boulder bed of 
desert range arroyos by-fine sands a few miles above the mouths 
of their canyons seems perfectly inexplicable on the hypothesis of 
any known water-action. Moreover boulder-trains of the arroyos 
often extend several miles from the canyon mouths out over 
surfaces of the adjoining intermontane plain. This fact alone 
points to a different origin of the arroyo trains and the sand-belts. 
Microscopical examination of the sands composing the sand-plains 
indicates very different lithologic characters from the sands found 
in the arroyo farther up-stream beyond the limits of the sand-plain 
— a fact suggesting a difference of source. 
The actual process of formation of the sand-plain at the mouths 
of arroyos debouching from desert ranges is a phenomenon that 
is readily observed. During gales on the desert the prodigious 
amount of drifting of the sands from off the intermontane basins 
is a common sight. Old gorges of the arroyo through the sand- 
plain are not infrequently filled to overflowing by drifting sands. 
Until the next flow of storm-waters from the mountains the sand- 
plain remains as untrenched and as smooth as any earth plain 
can be. The spreading out of storm-waters over such a sand-plain 
lying at the mouth of an Arizona canyon is geographically described 
by W J McGee but by him given a totally different interpretation. 
The sands of the sand-plain and that of the arroyo come from 
opposite directions. The one is mainly an accumulation of mate¬ 
rials drifting in off the intermontane plain; the other is derived 
from the heights of the mountains. The first is wind-driven, the 
