Where the 49ers found gold was later described as part of the Sierran or Mother Lode Gold Belt, a term used to label a 120-mile long, mile-wide stretch of land between El Dorado and Mariposa counties where rich placers and eventually veins of gold were found.
Geologists put it in the Western Sierra Nevada Metamorphic Belt, a complex geology that includes the Melones, Bear Mountain and Sonora fault zones, among others. It’s usually abbreviated to the Mother Lode, and it is, as geologist John Kramer described, in the southern portion of the Melones Fault Zone.
The gold got there in two ways. One was it was embedded in veins of quartz. Another was through the action of rivers, both ancient and contemporary, that pulverized the quartz, freeing the gold it contained and spreading it in waterways and in alluvial fans. Columbia’s gold was spread about by river flows over a basement marble long before the Sierra Nevada arose.
Ronald H. Limbaugh and Willard P. Fuller, in “Calaveras Gold” (University of Nevada Press, 2004), wrote, “Sierra placer gold comes from quartz veins in bedrock. Disintegration and erosion of the bedrock over geologic time liberate the gold, which then travels downslope under the forces of gravity and running water. Because of its much greater density, gold works down through streambeds toward the bedrock. The bigger the pieces of gold, the slower they move.”
The ancient rivers were much more vigorous at times than any we see in the foothills today, and because the topography was different, they followed mostly different courses than the rivers of today. They coursed the plain that came into existence after the erosion of the ancestral Sierra Nevada. Nineteenth-century miners quickly found gold downstream from where today’s rivers cut through those ancient streambeds and alluvial fans, on the bedrock beneath, and where today’s rivers still pulverize gold-bearing quartz veins.
Eventually, miners discovered they could tunnel into the dry gravel of the ancient streambeds to get down to bedrock in places like San Andreas and Murphys in particular. They also found gold literally on hilltops, put there by erosion, and deposited in valleys or exposed in outcrops on hillsides. But as the pickings got harder, the miners dug deeper. Eventually, too, they followed the gold-bearing quartz veins and by crushing the quartz and processing it they laboriously extracted gold.
This is part 4 in a series of stories about the geology of the Mother Lode. Contact Buzz Eggleston at firstname.lastname@example.org.