In Wyoming, the lack of rail service across large areas did not prevent small coal mines from opening, because in much of rural Wyoming wood was scarce and too valuable to burn on the plains and basins, while coal provided heat and cooking fuel for remote homesteads during nine months of cold weather and year-round food preparation. Wagon mines, served by wagons instead of railroads, were common wherever coal was available, using seams visible at the surface that did not require elaborate mining methods. Cora Ellis, who grew up on a ranch south of Medicine Bow, recalled that most coal from the small local mines was sold to ranchers and used only for local consumption, and in some cases neighbors bartered cream, butter, eggs, or meat for a load of coal. As national attention turned again to coal reserves, interest shifted westward to the American West, especially the northern plains of Wyoming, Montana, and North Dakota, where billions of tons of sub-bituminous coal were found, geologically younger than the bituminous and anthracite coals of the Midwest and East. The largest unbroken concentration of coal in the United States is the Wyodak bed in Campbell County, Wyoming, which reaches the surface along a 120-mile line, ranges from 25 feet to 150 feet thick, contains an estimated 15 billion tons of strip mineable coal to a depth of 200 feet, and may contain 100 billion tons to a depth of 2000 feet. By the 1960s, as coal demand rose again, the environmental effects of strip mining were becoming a growing issue because increasingly large and modern equipment could devastate large tracts of land in a short time.