EROSION IN ARIZONA. BOLSON REGION 149 
the physiographic agencies in regions of aridity, by some authorities, 
while others have given it an intermediate or even a minor importance. 
The fact is that wind plays a varying réle from major to minor 
depending upon the variety of arid climate. This is investigated 
later and attention is now directed to the method of erosive wind 
attack and the resistant surfaces developed thereto. 
It seems to be commonly believed that wind may attack success- 
fully an unelevated or even a depressed flat surface.t It is true 
that the wind picks up most of the fine material it transports from 
flat lowlands, but this is largely a repicking of material dropped; of 
tourist-passenger particles on a stop-over ticket. The source is 
from certain limited areas that are undergoing erosion on account of 
special exposure and non-protection. 
The larger portion of the surface of the southwestern arid region 
is fortified against the attack of the wind by desert pavements. Take 
as an example of extreme development those north of the Chocolate 
Mountains and west of the Colorado River, protecting an old flood- 
plain deposit of the latter stream. There the surface is covered with 
a perfect mosaic of fitted stones, polished and flattened to the last 
degree of perfection by the wind. The light of the early or late sun 
is thrown back from uncounted faceted mirrors, in a dancing blaze. 
Step on this pavement and you are surprised to find the apparently 
solid rock yielding underfoot. Scratch it with your boot, and you 
find that there is only a single thickness of pebbles, in size up to an 
inch in diameter, and at times less than a quarter of an inch thick, 
covering a deposit of hot, dry dust. A few trips of the wagon in the 
same tracks, will form deep ruts by the breaking of the pave- 
ment. ‘The above description is of the most perfect pavement I have 
seen, but everywhere, except in the sorted sand of the dune regions, 
a strong wind develops resisting pavements. The wind attack upon 
a deposit of dry playa mud, mixed with occasional pebbles (such 
pebbles are common to all deposits on account of the vicissitudes of 
arid deposition) must proceed as follows: First (provided no crust 
deposits interfere), the wind can blow just as much dust as contains 
on the average a single layer of pebbles; second, this layer of pebbles 
t Davis, ‘The Geographical Cycle in an Arid Climate,” Jour. Geol., 1905, pp. 
385, 388, 391. 
