GEOLOGY OF THE LOWER GILA REGION, ARIZONA. 



By Olype p. Ross. 



INTRODUCTION. 



In 19] 7 and the early part of 1918 the writer 

 made an investigation of desert watering 

 places and routes of travel in a part of south- 

 western Arizona. The results of this work 

 are to be published in two water-supply papers 

 of the United States Geological Survey — a 

 preliminary report giving information in regard 

 to roads and watering places and a final report 

 which is to include also much miscellaneous 

 information on the geology, geography, and 

 hydrology of the region. In the present report 

 the geologic information obtained in the 

 course of the work is summarized. As the 

 geologic investigation was necessarily of a 

 reconnaissance character, the information ob- 

 tained is incomplete, but much of it is new and 

 it is hoped will prove of value. 



The area covered lies in the central part of 

 Yuma Coimty and the western part of Mari- 

 copa Coimty, Ariz. In Maricopa CoTmty it 

 includes an irregularly triangular I'egion with 

 Phoenix at its eastern vertex, bounded on the 

 north and northeast by the road from Phoenix 

 through Wickenburg to Wend en and on the 

 south by the valleys of Salt and Gila rivers 

 and extendmg westward to the coimt}- boimd- 

 ary. In Yuma County the area is bounded on 

 the north by the road between Wenden and 

 Parker through Cunningham Pass and on the 

 south by the valley of Gila River and extends 

 entirely across the county to the western 

 boundary, Colorado River. 



The commercial development of such a region 

 as the one here described is intimately related 

 to the geology. The hope of finding mineral 

 deposits usually furnishes one of the initial 

 incentives for pioneering in such regions. 

 When promising deposits are found, as they 

 have been here, towns spring into existence 

 and the settlement of the country commences. 



In the early days in southwestern Ai'izona fur 

 trapping vied with prospecting as an occupa- 

 tion for the adventurous frontiersmen. When 

 the country became a little better known and 

 more settled, cattle raising and farming were 

 introduced. Both of these industries, particu- 

 larly farming, depend for their success on a 

 supply of water. The available surface water 

 here soon proved insufficient, and the settlers 

 began to utilize the ground water by means of 

 wells. The distribution, cjuantity, and quality 

 of the ground water in a region ai-e directly 

 dependent on the geology and physiography of 

 the region. 



ROCK FORMATIONS. 



At first glance most of the mountains in this 

 section of the country present a very similar 

 appearance. Examination soon shows, how- 

 ever, that they are composed of rocks of a num- 

 ber of very diverse types. There are great 

 masses of ancient metamorphic rocks, of 

 granites and granitic gneisses, and of lavas and 

 tuffs belonging to at least two distinct periods, 

 together with subordinate amounts of sedi- 

 ments associated with the older lavas and 

 tuffs, and sand and gravel filling the valleys 

 between the ranges. More detailed work will 

 undoubtedly result in still further subdivision 

 of the rocks. The metamorphic rocks certainly 

 represent two and probably more than two 

 periods. The granitic rocks belong to at least 

 two periods of intrusion. 



BASAL COMPLEX. 



Definition. — Highly metamorphosed sedi- 

 mentary rocks with associated granitoid 

 gneisses and other rocks of igneous origin 

 make up the whole or a large part of many 

 of the mountain ranges in this region. These 

 rocks will he referred to collectively as the 

 basal complex. They may be divided into 



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