SKETCH OF THE GEOLOGY AND SOILS OF THE CAHUILLA BASIN. 



By E. E. Free. 



INTRODUCTION. 



The first geological examination of the region covered by this volume was made by 

 the late Professor Blake as a part of his work for the Pacific Railway Survey and is de- 

 scribed in the reports of that survey, ' and more fully in the present volume. Following 

 Professor Blake's examination, the region received only incidental notice 2 until 1909, 

 when Mcndenhall 3 published a geologic sketch of the desert in connection with a discussion 

 of water resources. More recently Harder 4 has published some observations incidental 

 to a study of the region just to the north. 



The conclusions of the present paper are based mainly upon personal study of the 

 region, though much use has been made of the data of Blake and Mendenhall. On various 

 trips to the region the writer has had the advantage of accompanying the late Dr. W. J. 

 McGee, Dr. J. M. Bell, and Professor J. C. Jones, as well as others associated with prepa- 

 ration of this volume, though not concerned with the geology or soils. Acknowledgments 

 are due to all of these gentlemen for many valuable suggestions. The soil studies were 

 made while the writer was connected with the Bureau of Soils of the United States Depart- 

 ment of Agriculture, and were carried out with the permission and advice of Professor 

 Milton Whitney and Dr. F. K. Cameron, of that Bureau. The writer is indebted to these 

 gentlemen both for facilitating the course of the work and for permitting the present use 

 of the data obtained. During these soil studies the writer was accompanied by Mr. L. D. 

 Elliott, to whom thanks are due for most efficient assistance. 



DESCRIPTIVE GEOLOGY. 



The general topography of the Cahuilla Basin and its bordering mountain ranges is 

 outlined in Mr. Sykes's paper in this volume and need not be reviewed. In its major 

 features it differs little from the topography characteristic of nearly all desert basins of 

 North America — rugged bordering mountains half-buried in long smooth-sloped aprons of 

 mountain waste, which merge finally in a central salt flat, in this case covered by the Salton 

 Sea. The main exception in the Cahuilla Basin is the openness of the southern end toward 

 the Gulf of California. Here the mountain rim is lacking and the basin is separated from 

 the Gulf only by the alluvial ridge of the Colorado Delta described by Mr. Sykes. Perhaps 

 the depression as a whole is better described as a "trough" than a "basin." 



The geological structure of this trough is not well known. The bordering mountain 

 ranges have not been studied in detail and little is known of their structure, except that 

 it is not simple. The tilted block structure, so frequent in the "basin ranges" to the north, 

 is not discernible here. There is some reason to believe that the axis of the trough is the 

 locus of a major fault-line continuous with the great Cajon fault to the west, and running 

 substantially northwest by southeast. Whether or not this be the case, there has un- 

 doubtedly been much faulting in all the bordering mountains and the trough in general 

 is certainly structural, whatever may have been its origin in detail. 



1 Pacific Railway Reports, Sen. Ex. Doc. No. 78, 33d Congress, 2d Session, vol. 5, part 2, 1856. 



2 See especially, Bailey, Saline Deposits of California, Cal. State Mining Bureau, Bulletin 24, 1902. 



3 U. S. Geological Survey, Water Supply Paper 225, 1009. 



4 U. S. Geological Survey, Bulletin 503, 1912. 



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