Cultural Geology Guide—Wyoming’s Trona Resource and Opal Wyoming

Cultural Geology Guide—Wyoming’s Trona Resource and Opal Wyoming

Today we conclude our tour of the Great Divide and Green River Basins with a look at Wyoming’s trona resource and the town of Opal. As you will recall from the first Cultural Geology Guide post, this all relates to the ancient Lake Gosiute.

“Lake Gosiute disappeared millions of years ago but we can see evidence of how it shaped the cultural landscape since the early days of humans habituation at the end of the last Ice Age, more than 15,000 years ago,” says Julie Francis, WYDOT archeologist for the cultural geology guide.

Wyoming’s Trona Resource

Cultural Geology Guide—Wyoming's Trona Resource and Opal Wyoming
An ancient lake called Lake Gosiute today contains the largest deposits of trona in the world. Trona is mined and refined to create glass, detergents, baking soda, and other products.

SWEETWATER COUNTY — Lake Gosiute was a large (up to 15,000 square miles) Eocene lake that occurred about 56 to 34 million years ago in the Green River Basin. At that time, the basin was quite large. It extended across much of southern Wyoming, including the Great Divide and Washakie basins and into northwestern Colorado, northeastern Utah, and southeastern Idaho. This area is referred to as the Greater Green River Basin. A similar lake to the south, Lake Uinta, was sometimes connected to Lake Gosiute and had a drainage basin that covered most of western Colorado, eastern Utah, and the corners of Arizona and New Mexico. The lifespan of Lake Gosiute was between 4 to 8 million years. The size of both lakes fluctuated widely in reflection of wet and dry cycles. During arid times, the lakes became extremely saline, resulting in the deposition of sodium salts that included trona.

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Two-thirds of the world’s supply of soda ash comes from this area. There are 25 major trona beds that occur within the Wilkins Peak Member of the Upper Eocene Age Green River Formation. The trona deposit as a whole covers about 1,000 square miles within the Green River Basin, mostly in Sweetwater County, Wyoming.

Like trona, many of the Earth’s minerals are used for industrial purposes, but consumers may not realize they also benefit from these minerals in a number of their everyday products. Trona is used in many of the products we use every day such as glass bottles. Because of the ancient Lake Gosiute, Wyoming supplies this natural resource to places across the globe.

Opal, Wyoming

LINCOLN COUNTY — Opal, Wyoming is located in the southern part of Lincoln County. It represents one of Wyoming’s early railroad centers for cattle and sheep ranchers. James Davison opened the first store in Opal, operating from a tent. In 1890, Hugh McKay and his partner G.M. Miles of Big Piney bought and enlarged the business. The business became very prosperous, according to the Lincoln County Historical Society. It included everything from dry goods to farm equipment.

In 1892, Buckhalter and Cotton merged with the business and the Opal Mercantile Company was established with James W. Chrisman and the Petrie brothers as its officers. These partners also operated a branch establishment at Granger, the junction of the Union Pacific and Oregon Short Line railroads. The establishment became the largest stocked store in the county. The mercantile received up to 10 railcars of merchandise at a time. The building later served as the Opal Mercantile and was used for a variety of the town’s purposes, including the Post Office and stage station.

How Opal Got its Name?

One might suspect that with the town’s name there had to be opal found in the area. The first settler to Opal was Mr. Charles Robinson who came to this area in 1877. In 1881, two visitors arrived at the Robinson Ranch. One was a promoter for the Oregon Short Line Railroad Company, the other an editor from Omaha. As Mr. Robinson was giving them a tour of the ranch, the promoter suddenly stopped, picked up a peculiar stone, and began to examine it. Mr. Robinson said, “It’s an Opal.” Other specimens found later on in the area were also considered to be opal, and thus the area was referred to as Opal. A short while later, a little settlement grew up there and the town was named Opal.

But did they really find opal? Agate and some other calcified rocks can be found on the gravel terraces surrounding the valley and sitting on top of the Bridger Formation, says Wayne Sutherland, WSGS gemstones geologist. “We have a lot of different gravels some from higher up of the Paleozoic era and some of the gravels that are locally derived, ” he says. “Some of these gravels have some opalescence.” These gravel terraces were most likely created during the Late Pliocene (5.3 million years old) and in fact, says Sutherland, may contain some opal. “Ostracod shells found have been opalized, or replaced by opal so there is opal in the area,” Sutherland says. “But we have not seen any large size opal.” Mr. Robinson likely found a piece of agate that had been opalized, and so goes history with the name of this little town.

The Town of Opal sits on Quaternary gravel below the Laney Member of the Green River Formation and below an upland of the Bridger Formation.