174 
EOLIAN GEOLOGY 
rected are (1) the mergence of the sand-plain with the surface of 
the bajada belt, (2) the angular divergence of the grade-plain of 
the arroyo and that of the sand-plain, (3) the abrupt replacement 
of the very coarse bed materials of the mountain torrent by fine 
materials or sands, and (4) the deep gullying of the sand-plain 
whenever storm-waters fill the arroyo channels. 
Extension of the general intermontane plain’s surface far up 
into the narrow arroyo valleys of the desert ranges is a phenom¬ 
enon which is usually explained on the assumption that these sand- 
plains are the direct product of the soil-ladened waters of the 
mountain brooks depositing their loads at the foot of the high¬ 
lands partly because of lower gradient, partly owing to the spread¬ 
ing out of the stream waters over an unrestricted plains surface, 
and partly on account of absorption of the waters by loose valley- 
fill. These are the natural inferences derived from experience 
in a moist climate. Under conditions of dry climate other factors 
enter into consideration. 
In the mergence of the arroyo sand-plain with the general sur¬ 
face of the neighboring plain the smooth expanse of earth has 
much the same appearance as that of the sea reaching into all 
indentations of a rugged coast. In fact the sand-plain is perfectly 
continuous with the general plain. Whatever formed the one 
manifestly played a leading role in the case of the other. There 
are a number of features which militate against the assumption 
that the sand-plain is a stream phenomenon. There are other 
features which go to show conclusively that the sand-plain is a 
strictly desert phenomenon. It seems to have no counterpart in a 
moist country. It appears to constitute one of the best illustra¬ 
tions known of anti-gravitational gradation. It is one of the 
necessary consequences of wind erosion. It is produced because 
in the drifting of sands over an intermontane plain, all of the 
driven material of a particular gale is not ground to dust, floated 
away and exported from the district. 
The difference in angularity between the gradient of an arroyo 
and the surface of a sand-plain through which the storm-waters 
pass and deeply gully is so great as to be readily perceptible to 
the eye. It seems strange that the discordance of the two should 
so long go unnoticed. The terracing of the bajada belt near the 
desert piedmonts has been in late years widely discussed. Yet the 
