About five miles north of New Castle, and a turn out on the west side of U.S. Highway 85. The coal in this region did not derived from a tropical jungle, but is the product of a low, ferm-like vegetation which covered the land in past eons. These plants covered hundred square miles, but only in places like Cambria did the forces of nature form coal of sufficient depths to be worth mining. Coal was not the only product sought by the people who settled in northeast Wyoming in the late 19th century. Beehive ovens were built to convert some of the coal to coke for the use of processing gold at a smelter in Deadwood, South Dakota. Oddly, it was found that by using this Coke the deadwood smelter produced more gold per ton then the first assay of the ore disclosed. Testing of the cool at the mine eventually showed that it contained enough gold to increase its value by $.50 to nearly a dollar per ton, and smelter representatives agreed that a bonus was to be paid for the gold. 

Source: A Guide to Historic Sites by the Wyoming Recreation Commission Page 299.