208 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



Introduction 



That portion of the great oil belt of California lying along 

 the western border of the San Joaquin Valley and the ad- 

 jacent foothills, beginning north of McKittrick, and extend- 

 ing southward through Midway and Sunset, is commonly 

 known as the West Side Oil Fields. 



The geology of this area was first mapped by Dr. Ralph 

 Arnold^ who, while following in the main the divisions 

 worked out at Coalinga, did not differentiate the upper Mio- 

 cene and Pliocene formations, but grouped them together as a 

 unit under the name of the McKittrick Formation. 



During the past four years members of the geological force 

 of the Southern Pacific Company^ have done some careful and 

 detailed mapping in this area and the interpretation of this 

 mapping, particularly in the vicinity of the southern part of 

 the McKittrick Oil Field, has given a clearer knowledge of 

 the deposits than previously existed, and permits the making 

 of a clearer separation, and a closer correlation with the cor- 

 responding formations of the Coalinga region. At the same 

 time a close study of records and logs and of the materials 

 and fossils taken from the oil wells of these West Side Fields 

 enables us to draw some interesting and important conclu- 

 sions regarding the stratigraphy and age of the various oil 

 zones encountered in them. 



This paper is presented with a two-fold object : 



1. Of differentiating the upper Miocene and Pliocene de- 



posits of the West Side Fields, and correlating them 

 with their northern equivalents in the Coalinga 

 Field ; 



2. Of showing the age and correlation of the oil sands 



themselves of the West Side Fields. 



Acknowledgments 



The writer is indebted to many operators and oil men who 

 have kindly given logs, information, fossils, and other ma- 

 terials and data since the Midway-Sunset fields first began 



»Bull. U. S. G. S. 406. 



- Note: This paper was nearly completed three years ago when the author was in 

 the employ of the Southern Pacific Co. 



