Prospecting showed that Seams Nos. 3, 1, 7, and 13, also called the Van Dyke Seam, were thick enough to develop, and after preliminary railroad surveys up the long canyon from Thayer Junction, mine locations were chosen and a town site was laid out. On October 23, 1903, work began on the drift into “C” Mine, and on February 24, 1906, the slope was started; “A” and “B” Mines opened on April 30, 1906, and “D” Mine on June 30 of that year. “A” Mine worked No. 7 Seam and later included Seams Nos. 1 and 3. Mining took place under comparatively light cover, with haulage by mules and a few electric locomotives, while coal from Nos. 1 and 3 Seams was moved by endless rope haulage on an outside plane to the tipple three thousand five hundred feet away. The coal was undercut with electric mining machines and drilled with electric drills. “E” Mine opened during 1910, when a modern steel screening plant was also built, and a shaft one hundred feet deep was equipped with a two hundred HP electric hoist with full automatic control on a steel headframe, using self-dumping cages for hoisting. By 1937, “E” Mine's reserves were exhausted and it closed. In 1927, the first shaking conveyor in the Rock Springs district began operating in “C” Mine. Around 1910, a gas producer plant was installed at “D” Mine to run an electric generator, but because the power produced was too small and the expense too great, it was abandoned in 1922. The Superior mines were first opened by the Superior Coal Company, organized on December 28, 1905, in Cheyenne, Wyoming, with ten thousand shares of capital stock valued at one hundred dollars each, and the company merged with the Union Pacific Coal Company on May 1, 1916. In 1934, as nearby coal reserves neared exhaustion, active prospecting resumed, and by mid-1936 evidence of forty million tons of reserves justified opening a large new mine. Brought into production on New Year’s Day 1939, the modern D.O. Clark Mine was named for a pioneer coal mining figure in Wyoming, Utah, Colorado, and Washington and was designed to produce seven thousand five hundred tons per day in two shifts. Premier Mine and Copenhagen Mine were opened and managed by private coal companies not directly associated with the UP Coal Company.