EROSION IN ARIZONA. .BOLSON REGION 157 
Arroyo trains.—The larger streams often run, for a portion of 
their course, at least, below the surface of the bajada, and neverthe- 
less their work may be on the whole aggradational. Down such 
channels the streams bring large boulders, forming extensive radial 
deposits of rock trains.' 
Sand pockets—kIn the larger stream bottoms, especially in the 
cross drainage when developed at the foot of the bajada, deposits of 
perfectly sorted sand occur in “whirl pockets”? sometimes composed 
of clean mica, forming then bad quicksand deposits. 
Changes in layers.—The shifting character of the aggrading 
streams records itself in the rapid wedging-out of the layers. This is 
especially well shown in Tucson well sections where the boulder 
layers in adjacent wells do not occur in the same place in the column. 
t' Tolman, “Notes on Desert Processes and Desert Deposits,” Journal of the 
Proceedings of the Annual Convention of the Arizona Miners Association, 1905-6, p. 15: 
‘“The distribution of coarse material in the outwash desert deposits is in some ways 
remarkable. ‘The larger stuff is, of course, somewhere in the vicinity of the mountains. 
It is also distributed to great distances down the temporary torrent channels. ‘To 
appreciate what is accomplished in these waterways at a distance from the mountains, 
it is necessary for one to have seen some such a display as I once witnessed on the desert 
plain between Altar and Puertocito on the road to Santa Ana, Sonora, Mexico. I was 
driving with no thought of rain, when suddenly I came to one of the numerous washes 
that cross the road, and found it filled to the brim with a foaming roaring flood, which 
was as impassable as the Niagara. Looking towards the mountains perhaps ten or 
fifteen miles distant, I noticed for the first time the black speck of the cloudburst, 
which happened to be at the headwaters of this arroyo and no other. In a few hours 
I was able to cross, and on my return found some boulders to approach three feet in 
diameter. And this was ten to fifteen miles distant from the mountains! On the west 
slope of the Santa Rita mountains (east of Tucson) there is a ridge of rocks that was 
piled there by a torrent which later took a different course. These large boulders 
have remained invincible against the subsequent attacks of the water. This deposit 
has been mistaken for the moraine of an old glacier... . . The rate at which the 
mountains are torn down and the outwash deposits are built up, is rapid indeed when 
compared with the slower processes of ordinary erosion. As an example of this rate 
I shall mention the fact that the bottom layer of the Tucson outwash deposit, south 
of the Santa Catalina mountains, contains fragments of porphyries and lavas which 
do not appear in place on the south side of the range. The conclusion is evident that 
this layer represents a portion of the mountains entirely washed away... . . Some- 
times great boulders are found at some distance from the mountains. I recall one 
case where, between a great boulder and the parent mass from which it had been 
detached, there was a short ridge or knoll fifty feet high. J was asked how the boulder 
could have traveled up over the hill to its present position without the aid of ice. I 
answered ‘the surrounding country has been washed away and the hill left as a rem- 
nant, since the boulder rolled down from above.’”’ 
