Geologists say Mount Diablo’s geologic history is complicated. To put it simply, the collision of oceanic plates and continental plates and other conditions resulted in the formation of many different types of rock.
Diabase quarries began operation at Mount Zion and Eagle Peak in 1947, with the crushed, gravel-like rock used as a construction aggregate to provide a stable foundation for roads and buildings. It can also be a low-cost extender when added to more expensive ingredients like asphalt and cement to make concrete.
The success of Coal Hill
In earlier days, miners brought out coal from the hills northeast of Clayton. The area we know today as the Black Diamond Mines Regional Preserve was once the largest coal mining operation in California. In the 1860s, legislators even proposed changing the name of Mount Diablo to Coal Hill.
From the 1850s to the early 1900s, mule-drawn carts brought 4 million tons of “soft coal” to the surface. Railroads transported it to landings on the San Joaquin River. Then barges carried the coal to San Francisco, Sacramento and Stockton to be used in homes and factories and to power trains and boats.
Miners often enjoyed spending their wages and free time in Clayton. Several saloons offered beer from the brewery on Main Street, or the miners could catch a play or musical event at the Social Hall. The discovery of a higher grade of coal in the Northwest and the increasing use of oil as an energy source marked the end of the coal era.
The sands of time
From the 1920s until 1949, firms mined silica-rich sand in the same area as the former coal mines. The Hazel-Atlas Mine sent sand to Oakland for use in the manufacture of glass jars and bottles, and the Colombia Steel Works Foundry in Pittsburg used sand from the Roberts Sand Co. Mining of sand ended because of competition from imported Belgian glass sand and the closing of the steel foundry.
The Cowell Cement Co. quarried limestone at Lime Ridge from 1908 to 1946. Dust generated by this enterprise settled on the leaves of crops, raising the ire of many Clayton farmers until the company erected a large smokestack to contain the fallout.
People also mined gold, silver and copper but in relatively small amounts. A cinnabar mine just east of Clayton was still operating at a profit in 1940. Cinnabar is an ore from which mercury or quicksilver is derived, which in turn is used to separate gold and silver from the ore that contains it.
The complex nature of Mount Diablo geology has yielded many types of mining riches and presents us with the challenge of balancing the needs of our increasing population and the preservation of the beauty of our mountain and its surrounding hills.
Debbie Eistetter is a board member of the Clayton Historical Society. For more information or to become a member, visit claytonhistory.org The Clayton Museum, at 6101Main St., is open 2-4 p.m. Wednesdays and Sundays. Admission is free.