In the Silverton region, ore generally contained only about 5% valuable metals, usually lead, zinc, copper, silver, and gold, and after milling each ton yielded a small amount of metal and a large amount of wet, sand-like tailings. When the Mayflower Mill began operating in 1930, mining companies commonly dumped tailings into the nearest river, but superintendent and part owner Charles A. Chase opposed that practice and in 1935 developed a successful on-land disposal system that impounded wet mill tailings in dam-like tailings ponds. After the ponds dried, the tailings remained high above the Animas River as tall, sandy mounds. As mining became more environmentally conscious, impoundment became a standard disposal technique, and these two mounds are remnants of Chase's pioneering work in the 1930s and 1940s, among the earliest engineered tailings ponds in the Rocky Mountain mining districts and significant in the history of both the American environmental movement and the mining industry.