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The Geological Exploration of Arizona: The Role of State and Federal Surveys and the Geologic Map of Arizona

Article Author(s): 

Steve Reynolds
Jon Spencer
Stephen Richard
Phil Pearthree

The 1924 State Geologic Map

The years 1918 to 1924 represent an important time period in the history of the state geologic map. In 1918, geologist G. M. Butler replaced prominent mining engineer C. F. Willis as director of the Arizona Bureau of Mines. At that time, Butler was also Dean of the College of Mining and Engineering at the University of Arizona. Under Butler's direction, the Bureau began to assume its present role as a state geological survey. The Bureau restructured its priorities and directed most of its efforts toward production of a reconnaissance geologic map of the entire state. In 1919, the Bureau and the U.S. Geological Survey entered into a cooperative agreement to jointly produce the map. The U.S.G.S. assigned its most experienced reconnaissance mapper, N. H. Darton, to coordinate the project. Darton spent a total of 20 months in the field between 1919 and 1922, and, in the process, mapped the geology of approximately a third of the state. Darton was assisted by several Bureau geologists, including Eldred D. Wilson, Carl Lausen, and Olaf P. Jenkins. Lausen and Wilson are credited with mapping nearly all of southwestern Arizona. For northeastern Arizona, Darton relied extensively on the previous geologic mapping of H. E. Gregory.

The field work for the map was finished in 1922 and the map was published in 1924 (Darton and others, 1924). The map is mostly a reconnaissance geologic map, although not labeled as such. Much of the geologic mapping was done from horseback, horse and wagon or Model T. Due to the limited time and access, several mountain ranges in southwestern Arizona were probably mapped in a single day, or less. In areas of most difficult access, the geology may have been sketched from a distance using binoculars. There was simply too much area, too few geologists, and too little time to feasibly map every range in the detail or accuracy desired. For example, a large number of mountain ranges in western Arizona are simply shown on the 1924 map as "granite." More detailed mapping in some of these ranges has revealed the pre- dominance of other rock types, such as limestone, sandstone, and volcanic rocks. However, the geology of other areas was shown almost exactly as we know it today! In all, the 1924 state map is a remarkable accomplishment considering the circumstances under which it was produced. Its chief compiler and contributor, N. H. Darton, will long be remembered as one of the foremost geologists to have worked in Arizona.

Darton capped off his work with an amazing 1925 publication entitled, "A Resume of Arizona Geology." In this report, Darton summarizes the geologic history of Arizona and presents many cogent observations that he made while mapping. Darton wrote the text in 1923 immediately after completing field work on the state map. The resume, in conjunction with state map, provides a useful record of what was known about Arizona geology in 1923-24. It is a remarkable scientific work that remains an important contribution to Arizona geology. To this day, Darton's descriptions and cross-sections constitute the only published information for certain hills and fossil localities in western Arizona. Modern-day geologists still have much to gain by reading this classic summary of Arizona geology.

Geologic Studies Between 1924 and 1969

Publication of the state geologic map in 1924 was an important milestone in the geological exploration of Arizona. The entire state had now been mapped, albeit in a very cursory and over-simplified way. Geologists finally had a map of the whole state that they could study while trying to unravel Arizona's geologic history. The slow and deliberate process of improving and updating the state map could now proceed.

Arizona experienced a surge of geological activity after 1924. The U.S. Geological Survey, Arizona Bureau of Mines and University of Arizona were responsible for most of the geological research published between 1924 and 1969. Geologic studies conducted by the U.S.G.S. during this time period fall into several distinct categories. First, the Survey mapped a number of key quadrangles near important mining districts in southern and central Arizona. Representative examples of this type of study are the publications of James Gilluly (1937) on the Ajo area, C. A. Anderson (1950, 1951), on Jerome and Bagdad, J. R. Cooper (1960) on southeastern Arizona, and N. P. Peterson (1962) on the Globe- Miami area. Important quadrangles were also mapped by M. H. Kreiger, S. C. Creasey, P. T. Hayes, J. R. Cooper, and H. Drewes. These quadrangle studies provided essential information about Arizona's mineral deposits and their geologic setting.

The second type of U.S.G.S. study was concerned with the relationship between geology and water resources. The publications of M. E. Cooley, J. W. Harshbarger, L. C. Halpenny, and C. A. Repenning established much of what we presently know about the layered rocks of northeastern Arizona (Harshbarger and others, 1957). U.S.G.S. geologists L. A. Heindl and D. G. Metzger contributed much information about the relatively recent geologic histories of southern and western Arizona, respectively.

The Arizona Bureau of Mines continued its emphasis on geologic mapping. In 1933, Eldred D. Wilson published a map and discussion of the geology and mineral resources of southern Yuma County. Wilson mapped this hitherto unknown area of southwestern Arizona from 1929-1932. In the process, he discovered a new set of mountains that had been overlooked by previous geologists and explorers. He named this range the Butler Mountains after G. M. Butler, former Director of the Bureau and Dean of the College of Mining and Engineering (Wilson, 1931). Wilson was the first person to describe and map the geology of a large number of mountain ranges in southwestern Arizona. The data from Wilson's 1933 geologic map were incorporated into the 1969 state geologic map.

J. D. Forrester became director of the Bureau in 1956 and supported intensified work on a new state geologic map. Bureau geologists, E. D. Wilson, R. T. Moore, and H. W. Peirce, began mapping selected areas in more detail. As before, the U.S.G.S. agreed to cooperate on the state map project. The University of Arizona continued its tradition of research on the geology and mineral resources of Arizona. Professors A. A. Stoyanow, E. D. McKee, B. S. Butler, J. F. Lance, E. B. Mayo, and their students contributed many important ideas.

An important development in the geological exploration of Arizona was the advent of radiometric techniques for determining the actual ages of rocks in millions of years. The contributions of University of Arizona professor Paul E. Damon cannot be overemphasized in this regard. He and his colleagues determined the ages of hundreds of rock units throughout the state. In many cases, these age determinations drastically modified our perception of the geologic history of Arizona. Later versions of the geologic maps of the state were significantly different from previous maps, simply because of these new age assignments. L. T. Silver of the California Institute of Technology and R. F. Marvin and E. H. McKee of the U.S.G.S. were also important in determining the ages of some Arizona rocks.

Senior Geologist
Arizona Geological Survey


Research Geologist
Arizona Geological Survey

Steve Reynolds

Arizona State University School of Earth and Space Exploration

Steve Richard

Research Geologist
Arizona Geological Survey

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