70 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
ture. It is therefore a function of both rock composition and structure 
and of climate. Strange to say, erosional attack is given strongest topo- 
graphical expression in regions of very scanty rainfall, where the torrent 
of every dozen years takes everything before it (plate 25). Slope C is 
the most striking and typical of the topographical features of the semi- 
arid regions, and not only the larger ranges surrounding the bolson, but 
also each little individual hill appears, as it were, mounted on a sloping 
pedestal (plate 25). These slopes have often been described.’ Herrick? 
has aptly named the extensive slopes of low gradient in New Mexico 
“clinoplains,’’ and Ogalvie® has called the talus apron of the individual 
hills “conoplains.”’ 
While these names have much to commend them, there seems to be a 
need for a simple term for this extensively developed topographic feature, 
a name that will go with such apt and commonly accepted terms as mesa, 
playa, bolson, etc. I therefore suggest the term slope for the inclined 
plain, and the word wash to cover the various forms of outwash deposited 
on the slope. 
It would not be necessary to emphasize the subaerial nature of the wash- 
deposits of the slopes, were it not for the fact that erroneous interpre- 
tation has recently been given them. They are without question the 
depositional phase of the erosional activities above. Running water 
plays a varying role, having no part in the formation of the symmetrical 
slopes under certain arid conditions, temperature change and gravity 
alone being involved. Except for the steepening of the angle close to the 
base of the mountains, these plains keep their even gradient sometimes 
for miles (plate 25). 
The undrained flat in the center is the playa. With increased pre- 
cipitation or decreased evaporation it becomes a lake. Its material is 
almost entirely wind-blown, deposition being most active during periods 
of water occupancy, when the dust from the mountains and slopes is 
caught by the water-sheet. 
In bolson topography there are all gradations, from those showing 
marked development of these features to those in which some of the 
features can scarcely be recognized, on account of milder climatic con- 
ditions, partial drainage by some master stream that has conquered 
the desert conditions, or the lack of the tectonic features necessary to 
initiate the cycle. 
It needs but a cursory investigation to suggest that the past climate 
can be read, provided that the deposits laid down in the center of the 
bolson under the varying conditions of no water (desert flat), playa, and 
expanding and contracting water-sheet can be separated from the sur- 

“Blake, W. P.: Some salient features in the geology of Arizona, etc., Am. Geol., 
XXVII, 167. 
* Herrick, C. L.: The clinoplains of the Rio Grande, Am. Geol., xxxrv, 376-381. 
*Ogalvie, L. H.: The high altitude conoplain, Am. Geol. xxxvI, 27-34. 
