Peering at the Eureka Slope at the Black Diamond Mines Regional Preserve south of Antioch, California.  The area was rich in production of silica sand & coal (the Mount Diablo coal field being discovered in 1855).  During its heyday, the area had 12 mines that supported thousands of miners and their families from around the world in five town sites. Four million tons of sub-bituminous and lignite coal was extracted from the Mount Diablo coal fields until the quantity, price and quality was overtaken by a larger operation with higher-quality anthracite coal from Washington State. The townsites were abandoned and houses moved when the mines closed around 1906. 

In the 1920s, a mine producing high-quality silica sandstone was started by owner Marvin Greathouse on a hillside above the Somersville townsite. He sold the product to the Hazel-Atlas Glass Company which operated a plant in Oakland manufacturing glass containers. Hazel-Atlas eventually purchased the mine and operated it until about 1945. 

The Eureka Slope, an incline shaft, was the entrance to the Eureka Coal Mine. Between 1860 and 1895, more than 150,000 tons of coal were hoisted to the surface. The slope is 290 feet long and descends at a pitch of 32 degrees. It was fitted with a self-acting, inclined railroad that allowed the weight of a descending empty mine car to partially offset the weight of an ascending loaded car. The slope was extended above grade with a wooden trestle, allowing cars to dump directly into a storage bunker. Coal from the bunker was loaded into rail cars for transportation to a wharf on the San Joaquin River, a distance of about 5.5 miles. Below the ground, the mine workings extend down more than 800 feet, eventually connecting the Independent shaft, located in the parking lot one quarter mile to the north.

From: https://www.kqed.org/science/23430/bay-areas-coal-mining-history-at-the-black-diamond-mines-regional-preserve

https://en.wikipedia.org/wiki/Black_Diamond_Mines_Regional_Preserve

https://www.hmdb.org/m.asp?m=198217