EROSION IN ARIZONA BOLSON REGION I4I 
of the region has been worked out, and not justified, considering the 
great priority of the Spanish usage. 
The geologist is not the only one that has a right to be heard in 
this matter, for the problems of the desert are being attacked enthu- 
siastically and successfully by the botanist, and it is only by the 
combined action of the two that the broad problems of desert history 
and climate will be solved. There is a need for a word by which the 
individual self-centered drainage systems of the desert can be desig- 
nated, and we should return to the original and broader meaning 
of the word bolson. I therefore suggest that the word be used to 
cover the watershed of a centripetal drainage system, including all 
the area within the limits of the divides. 
The bolson may depart somewhat from a perfect topographical 
basin, for evaporation on a slope may prevent the development of a 
through drainage, and foster the centripetal variety. Those bolsons 
whose surface water i times of flood reaches some river thoroughfare, 
some lower bolson, or the ocean direct, and consequently the playa 
portion described below is poorly developed or lacking, may be 
called semi-bolsons. In Arizona, therefore, there is every gradation 
between bolson, semi-bolson, and ordinary river drainage, the latter 
becoming more prominent as the Colorado River is approached. 
TOPOGRAPHY OF THE BOLSON 
The well-developed bolson presents three distinct topographic 
features. (1) The Upper Rock Surface.t The slope of this surface 
is a function of rock structure and composition, and erosive attack. 
The top surface may be a mesa, developed either on account of 
protective action of some hard layer, or an older erosional flat, which 
on the top of the higher mountains is protected on account of the 
forest growth. The latter are so discontinuous that they have as 
yet defied any general correlation, and the discovery thereby of a 
faulted peneplain. (2) The bajada. Extending down from the 
rock surfaces are the flanking detrital slopes,3 built up by terrestrial 
t Tolman, /oc. cit., gives photographs and diagram illustrating bolson topography. 
2 Example, top of Catalina Mts., north of Tucson, Arizona. 
3 Blake, ‘‘The Flanking Detrital Slopes of the Mountains of the Southwest Portion 
of the United States,’ Science, new series, Vol. XXV, p. 974, states conclusions at 
variance with those presented here. Probably the best descriptions of all the different 
features of bolsons are found in Carnegie Publication 26, ‘Explorations in Turkestan.” 
