Mapping in the Sonoma Range, 1939

In the summer of 1939, I boarded a bus for Nevada, where I had accepted a position as junior geologist in the metals branch of the United States Geological Survey (USGS) under D.F. Hewett.

My first assignment was a new project under the direction of Henry Ferguson. Our job was to map the geology of the Sonoma Range one-degree quadrangle. A quadrangle is a rectangular map area; a one-degree quadrangle runs one degree latitude and one degree longitude. In Nevada, that covers about 70 miles from north to south and 50 miles from east to west. We were to make the map, assemble a geologic framework of the region, and study the mineral deposits. I now had a full-time job with the USGS at a salary of US$2,000 per year, plus field expenses. Not bad for those days!

When I stepped off the bus on July 4 in Winnemucca, a blast of hot, dry air hit me. Henry Ferguson then introduced himself, saying “Hello, I’m Fergie.” He was short, about five feet, four inches, and his unassuming, almost shy personality immediately put me at ease. I soon learned that his self-effacing manner hid a great intellect.

Siemon (Si) Muller, professor of geology (paleontology) at Stanford University, was with Fergie. I sensed I was in the right place to learn a lot of field geology.

We went to Bob’s Cafe in Winnemucca for a quick lunch, and Fergie exclaimed, “We want to go out to the East Range to look at the Triassic rocks there.” Si, a specialist in Triassic rocks and faunas, had previously worked in the Sonoma Range south of Winnemucca, but he had never had a chance to look at the East Range. On that first, brief trip, we found what Si was looking for: Triassic formations identical to those he had studied in the Sonoma Range. I got a serious blister on my heel during the hike but didn’t complain. I found out right away that we were going to tackle the geology of the Sonoma Range quadrangle head on! The rocks and structures we eventually were to map would change the face of Nevada geology forever.

Initially, my fieldwork responsibilities were to study and map the igneous rocks, both volcanic and intrusive. I also studied mineral deposits, principally gold- and silver-bearing veins. When time permitted I helped Fergie map the Paleozoic rocks that proved to be critical elements of the regional geologic framework.

Fergie was first and foremost a field geologist. For a time, he had been in charge of the USGS’s metals branch in Washington state. Fergie worked primarily in the Great Basin and authored many important papers on its mining districts. He was largely responsible for setting up the geologic framework of Nevada as we know it today.

My association with Fergie permitted me to see how his agile mind worked and how he solved complex structural problems in the field. Initially, I wondered about the relevance of some of his far-reaching predictions, but looking back I see that he taught me a great deal about mineral deposits. Fergie insisted that the best way to solve a geologic problem was to map it, and the solution would miraculously appear.

The first inkling that the Sonoma Range quadrangle might hold the key to the regional geology came in Willow Creek in the East Range.

Entering Willow Creek from the west, we first encountered the Inskip Formation, which contained a coral of Mississippian age (about 330 million years old) and which rested unconformably on the Leach Formation, a chert, quartzite, shale and volcanic unit. The Leach had been thrust over a unit of limestone and shale.

Initially, Muller thought the limestone and shale might be Triassic in age, thereby dating the thrust as post Triassic. But work in the 1980s by Donald Whitebread revealed that they were early Paleozoic, thus permitting us to date the thrust as pre-Inskip and therefore Mississippian or older. This proved to be a critical element in the regional geology.

In Willow Creek we visited Wallace Calder, a dentist from Winnemucca who had mined handsome nuggets, some weighing as much as 20 ounces, from a placer deposit. Calder said the nuggets came from a fault zone in Willow Creek and showed us the fault in the Wadley mine that he owned.

We later determined that it was a thrust fault that brought oceanic-facies rocks (rocks from deep water) over limestone of the continental shelf. We did not know whether these events were merely of local significance or whether they signaled a larger regional pattern, but we recognized two important facts: that deep-water oceanic rocks had been thrust over a continental margin in shallow water, and that gold occurred in and below the thrust zone.

These were only the first steps in assembling a geologic framework in which gold was to be found. Following my old professor Hoover Mackin’s advice, taken from Through the Looking Glass, we were believing in impossible things before breakfast. We now had two of them.

When in the field, Fergie and I generally worked the same area. It didn’t take me long to become totally hooked on the rocks of the region. Certainly they were new to me, but they gave tantalizing glimpses of spectacular formations folded into complex structural forms. They reminded me of photographs and drawings of the Alps I had seen in textbooks. Like Lewis Carroll’s Alice, my curiosity was aroused, and before I knew it, I had fallen down the rabbit hole, never once considering how in the world I was to get out again.

— Ralph Roberts worked with the United States Geological Survey for 44 years, in the western U.S., Central America, and the Middle East. In 1981, he was a founding partner of exploration consulting firm Victor E. Kral Associates. This is the first in a series of excerpts from his recent book, A Passion for Gold: An Autobiography, published by the University of Nevada Press and used here with its permission.