EROSION IN ARIZONA BOLSON REGION 137 
Structure of the playa muds. (a) Chemical deposits. (b) Surface 
markings. (c) Animal and plant remains. Erosion of the mud 
sheet. 
CLIMATE AS AFFECTING THE EROSION OF THE BAJADA 
Discussion of (1) the remains of an older bajada near Tucson, Arizona, and 
(2) the incision of the younger bajada by the larger gullies; showing 
that while climate theoretically might be competent, the facts of the 
case suggest that other conditions were the immediate cause of this 
erosion, and finally that climatic changes are better read in the detail of 
structure and composition of the deposits, rather than in topography. 
INTRODUCTION 
Before the publication of Barrell’s studies on climate and deposi- 
tion,’ I wrote: 
It does not seem probable, therefore, that theoretical analysis of the complex 
relations that obtain between climate and deposition will accomplish what it has 
in the case of deposition by running water and by glacial action. The problem 
will be solved by detailed studies in each region. In each case the disturbing 
factors must be evaluated, and the intensity of each process gauged.? 
The study of Barrell’s article did not seriously affect the above 
opinion, but it did suggest and encourage the following contribution, 
on account of the evident value of his analysis in directing attention 
to the newly developing study of climatic effect on geological processes, 
in showing its possibilities, and in suggesting some of the criteria 
available. 
In this contribution attention will be confined to a certain definite 
portion of the arid southwest, which is here called the bolson region. 
A criticism by Keyes of Hill’s definition of the term “bolson’’3 and 
its use by Tight, Lee, and others gives point to an investigation of 
the history of the use of the word, and a discussion of its proper 
meaning. 
The data to which this treatment owes its development have been 
collected during a residence in the arid southwest since 1901. The 
‘Joseph Barrell, “The Relation between Climate and Terrestrial Deposition,”’ 
Jour. Geol., 1908, pp. 159-90, 255-95, 303-84. 
2C. F. Tolman, “The Geology of the Vicinity of Tumamoc Hills,” Publication 
113, The Carnegie Institution of Washington (in press). 
3 Hill, Topographic Atlas of the United States, U. S. G. S.; Keyes, “‘Bolson Plains 
and the Condition of Their Existence,” Am. Geol., Vol. XXXIV, pp. 160-64; “Bolson 
Plains,” Am. Jour. Sci., Vol. XV, pp. 207 ff.; ‘Rock Floor of the Intermont Plains 
of the Arid Region,” Bull. Geol. Soc. Am., Vol. XIX, pp. 63-92; Tight, ‘“Bolson Plains 
of the Southwest,”’ Am. Geol., Vol. XXXVI, pp. 271-84. 
