West of the town of Green River, where Interstate 80 and the Union Pacific cross open sagebrush, stacks and long gray buildings stand out on the flats with no town around them. They sit on top of the largest deposit of its kind in the world: beds of trona, a sodium mineral, laid down when this basin held a lake that kept drying up.
A lake that came and went
About fifty million years ago, in the Eocene, a lake geologists call Lake Gosiute filled much of what is now the Green River Basin. Its fine-layered sediments make up the Green River Formation, the same rock that holds the fossil fish of Fossil Butte to the west. Through one long stretch, recorded in a part of the formation called the Wilkins Peak Member, the lake had no outlet and the climate swung dry. Again and again it shrank to a briny, alkaline pan, and as the water evaporated, sodium carbonate minerals crystallized out of it in layers. Forty-two beds of trona, or of trona mixed with salt, lie stacked in that rock between about 600 and 2,000 feet down, some of them ten to twenty feet thick. The Wyoming State Geological Survey puts the resource at about 127 billion tons.
Found while looking for oil
Nobody went looking for it. Sodium brine turned up in wells in the town of Green River in 1896, and a small plant drew soda from it until production stopped after the First World War. Then in 1937 and 1938 Mountain Fuel Supply drilled a wildcat well about eighteen miles west of town, the John Hay well, hunting for oil and gas. It found neither. The driller's log noted salt crystals over hundreds of feet, and the core samples sent off for analysis came back as nearly pure trona, at a depth where the mineral had never been found.
Working out how to mine and refine it took years. In 1947 Westvaco Chemical began mining trona from a bed about 1,500 feet down, and its plant turned it into soda ash, sodium carbonate. Within a few decades Wyoming trona had all but driven out the old way of making soda ash in factories from salt and limestone; the last of those plants in the country closed in the 1970s.
What it becomes
Soda ash is one of the quiet basic chemicals of modern life. After sand it is the main ingredient in glass, where it lowers the melting point, and it goes into detergents, paper, baking soda and a long list of industrial processes. Several companies work the beds today, most in room-and-pillar mines, where crews cut rooms out of the trona and leave pillars standing to hold the roof. Together they produce around nine-tenths of the soda ash made in the United States and a large share of the world's supply.
The lake is long gone, and the plain over it gives no sign of what lies underneath, except for the plants, the rail spurs and the trains of covered hoppers rolling out of the basin.