Geology of Placer County An Overview

In Spanish, Sierra Nevada translates to “snow covered mountain range.”

These mountains were created over millions of years as tectonic plates collided and one dove under the other, lifting the region up out of the sea. Volcanoes formed and erupted, and glaciers expanded and retreated over time.

In Placer County, these processes created a region that is extremely rugged and resource rich.

The elevation rises sharply from 164 feet to a height of 9,044 feet, stretching from the Sacramento Valley all the way to Lake Tahoe. It is over one hundred miles long but with an average width of just twenty-five or thirty miles.

The northern and southern boundaries of Placer County are the Bear River and the Middle Fork of the American River. The North Fork of the American River runs through the county, as well.

North Fork American River

Gold miners from around the world began flocking to the area in 1849 and collecting the “easy pickings” of gold dispersed in the region’s ravines and waterways.

But, buried deep within the many layers of these snow covered mountains are even richer gravel deposits, where vast quantities of gold has been found.

Through millennia of water-caused erosion, this gold was extracted from its source and carried through these ancient waterways. As the local terrain continued to transform over the centuries, these were locked in place and covered with sediment from the continued erosion of the mountains.

These ancient, gold-bearing “auriferous” river channels were aggressively pursued by gold seekers. By the early 1850s, drift and hydraulic mining operations targeted their valuable gravel beds.

Entire towns sprang up around these different mining operations, oftentimes becoming ghost towns when the mine closed up or the entrance to the tunnel was relocated.

Sunny South

Other towns, like Rocklin and Penryn, trace their beginnings to another valuable resource in Placer County.

Granite makes up the backbone of the Sierra Nevada as well as many of the monuments, curbs, and buildings from Auburn to San Francisco.

The success of this industry is owed to the high quality (iron free) granite of the area, as well as the decision of the Central Pacific Railroad to build their line of the transcontinental railroad through the length of the county.

The Griffith Quarry in Penryn is on the National Register of Historic Places and visitors can explore the museum to learn more about this important enterprise and take a hike around the quarry.

Clay, discovered at Lincoln is still mined today and used for making terra cotta and clay products at the Gladding McBean Factory.

Gladding McBean

Many other valuable minerals have been mined and quarried in Placer County. Silver, marble, copper, chromite, coal, and lime, among many others, are part of the geologic makeup and manufacturing legacy of the area.

For more information, check out the 10th Annual Report of the State Mineralogist (1890) which was written by Assistant Surveyor, J. B. Hobson on the topic of Placer County.

J. B. Hobson

This very informative report also included a set of beautifully illustrated geologic maps. This transcription was created by the Foresthill Divide Historical Society.

Hobson knew Placer County well, having gained notoriety and riches as a hydraulic miner. At one point, Hobson claimed a federal land patent underneath the entire town of Iowa Hill in order to secure access to the mineral wealth he presumed to be there.