The area includes the remains of twelve coal mines and the sites of several long-gone coal mining towns. The Preserve contains over of mine workings. The largest and oldest town, Nortonville, had a peak population of about 1,000. Somersville, Stewartville, West Hartley and Judsonville were located in valleys to the east. The sites of Stewartville and West Hartley are located on private property outside the eastern boundary of the Preserve. The mines were the Empire, Central, Star, Corcoran, Pittsburg, Manhattan, Eureka, Independent, Union, Black Diamond, Mt. Hope, and Cumberland. The coal produced was of a low grade (sub-bituminous or lignite), but for a time in the 19th century, was the only readily accessible and economic source in California. Therefore, it was a very valuable resource and powering the railroads, ships and heavy industry of California. The mineral was often called 'black diamond."