Prospecting revealed that Seams Nos. 3, 1, 7, and 13, the latter also known as the Van Dyke Seam, were of sufficient thickness to justify the development of mines. Preliminary railroad surveys were made up the long canyon from Thayer Junction on the main line.
In the meantime, locations for mines had been selected and a town site laid out. On October 23, 1903, the drift into "C" Mine was started, and on February 24, 1906, the slope was begun. "A" and "B" Mines were opened April 30, 1906, and "D" Mine on June 30th of the same year. "A" Mine was opened on No. 7 Seam, and later included Seams Nos. 1 and 3.
The work was under comparatively light cover, and haulage was carried on by mules, with a few electric locomotives. The haulage from Nos. 1 and 3 Seams was conducted by an endless rope haulage on an outside plane to the tipple, a distance of 3,500 feet. The coal was all undercut with electric mining machines, and drilled with electric drills.
"E" Mine was opened during 1910, and a modern steel screening plant was constructed that year. A shaft had been sunk for a distance of 100 feet from ground level, with a 200 HP electric hoist with full automatic control erected on the steel headframe that mounted the shaft. Self-dumping cages were used for hoisting purposes. By 1937, "E" 's reserves had been depleted and the mine closed down.
The first shaking conveyor installed in the Rock Springs district was put into operation in "C" Mine during 1927. About 1910 a gas producer plant was installed at "D" Mine, the gas being used to operate an electric generator, but the quantity of electric power developed was so small and the expense so great, that the plant was abandoned in 1922.
The Superior mines were originally opened by the Superior Coal Company, organized December 28, 1905, with a capital stock of 10,000 shares, with a par value of $100 each, headquarters located in Cheyenne, Wyoming. The company merged with the Union Pacific Coal Company on May 1, 1916.
In 1934, when it became evident that the coal reserves contiguous to the present Superior mines were nearing exhaustion, active prospecting was started in the vicinity of the mines. By mid 1936, reliable evidence indicating reserves of 40,000,000 tons justified the opening of a new mine of large capacity, with further substantial reserves partially prospected.
Put on a producing basis on New Year's Day 1939, the D.O. Clark Mine, was modern in every respect. Named for the man who did so much pioneer coal mining work in Wyoming, Utah, Colorado and Washington, it was designed for production of 7,500 tons per day of two shifts.
Premier Mine and Copenhagen Mine were opened and managed by private coal companies not directly associated with the UP Coal Company.