Green River Basin

The Greater Green River Basin is vast. Historically, it is revered as the heart of fur trapping county, in the era of the Mountain Men. It also served as a major crossing for migrant travelers headed West. It extends from the Overthrust Belt on the west to the Rawlins Uplift on the east. It is bookended by the Uinta Mountains to the south and the Wind River Mountains to the north. The Rock Springs Uplift lies in the center of this basin.

Meet your tour guides, Wayne Sutherland, geologist with the Wyoming State Geological Survey and Julie Francis, archeologist for the Wyoming Department of Transportation.

Tour guides
Tour guides, Julie Francis and Wayne Sutherland.

"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.

The Great Divide and Green River Basins

By Julie Francis

Modern-day travelers on Interstate-80, west of Rawlins, cross the Continental Divide twice. Between the southern Wind River Mountains and Atlantic Rim southwest of Rawlins, the divide splits, forming the Great Divide Basin along the crest of the continent. It is also called the Red Desert Basin for the red soil derived from Eocene formations that cover the basin’s floor. It, along with the neighboring Green River Basin to the west were covered by Lake Gosiute during the Eocene Epoch, 56 to 33.9 million years ago.

Wyoming's Great Divide Basin - WY Cultural Geology
Great Divide Basin
Great Divide Basin

Though it disappeared millions of years ago, Lake Gosiute has shaped the cultural landscape of the Great Divide and Green River basins since the earliest days of human occupation at the end of the last Ice Age (15,000 years ago). As the lake dried and was buried by younger sediments, lake-bottom muck slowly became cemented into rock. In some portions of the ancient lake bed, high concentrations of silica bonded the grains of sand, silt and clay. In other places, silica filled the voids left when the hard shells of many small sea creatures dissolved. This resulted in the formation of a variety of hard, glass-like rocks often termed “cherts.” These layers of rock were exposed by subsequent erosion and today form hard pavements on the basin floors. Because of their glass-like quality, many of these cherts are well-suited for the manufacture of stone tools and were extensively used by native peoples.

“Tiger chert” is a distinctive variety of these materials. Named for the alternating bands of dark and lighter browns or tans, the banding reflects the yearly deposition of sediment into the bottom of Lake Gosiute. Outcrops of the material occur southwest of Rock Springs and at a few other places in the Green River Basin. The homogeneous nature of the material made it ideal for manufacturing into finely flaked tools, and it occurs in archaeological sites of all ages.

About 13,000 years ago, Clovis flint knappers, with unparalleled skill and craftsmanship, used this material to fashion Clovis points, large (up to 8” long) and exceptionally thin (ca ¼ to ½”) bifaces and other exotic tools. These, along with similar tools made of different raw materials such as obsidian, were stored in caches found as far away as Boulder, Colorado and the Black Hills of Wyoming. Some of the artifacts have been coated with red ocher, suggesting some type of ritual or ceremonial use.

Killpecker Sand Dunes
Killpecker Sand Dunes. [Credit: Chamois Andersen, WSGS, 2015.]

With no major topographic features to serve as shelter, high winds often blow across the open, flat floors of the Great Divide and Green River basins. One result of these winds has been the formation of sand dunes and vast sand sheets. The Killpecker Sand Dunes, in the Red Desert portion of the Green River Basin, is the second largest active sand dune field in the world. It is nearly 50 miles long and up to 10 miles wide. The largest active sand dune is in the Great Nefud Desert of Saudia Arabia, called Jafura and covers an area of more than 550 square miles. The Killpecker Sand Dunes and other dunes in Wyoming have played a major role in the cultural landscape. Native peoples used active dunes to trap game, as well as for gathering a variety of nutritious grasses and plants, which colonize the sandy surfaces. By at least 5,000 years ago, people began digging semi-subterranean lodges into the dunes, providing a home base for families and perhaps the basis of small communities.


Bamforth, and Douglas, B., 2014, Clovis caches and Clovis knowledge of the North American landscape: the Mahaffy Cache, Colorado, in Clovis caches: Recent discoveries and new research, edited by Bruce B. Huckell and J. David Kilby: Albuquerque, University of New Mexico Press, p. 39-60.

Frison, G.C., Bradley B., 1999, The Fenn cache: Clovis weapons and tools: One Horse Land and Cattle Company, Santa Fe, New Mexico.

Kornfeld, M. and, Frison, G.C., and Larson, M.L., 2010, Prehistoric hunters and gathers of the High Plains and Rockies: Walnut Creek, Left Coast Press, Inc., 598 p.

Miller, J. C., 2010, Lithic resources, in Prehistoric hunters and gathers of the High Plains and Rockies, by Kornfeld, M., Frison, G.C., and Larson, M.L.: Walnut Creek Left Coast Press, Inc., p. 533-598.

Oregon Trail

By Julie Francis

"It was likely a foolish business, this going to Oregon, but it was good to think about, like thinking about getting out of old ways and free of old places. Like his pa had said once, telling about coming down the Ohio in a flatboat, there wasn't any place as pretty as the one that lay ahead." ~ Epigraph from A.B. Gutherie's novel The Way West

The Oregon Trail - WY Cultural Geology

The Oregon-California trail was a 2,170-mile route from Missouri to Oregon and California that enabled the migrating of the early pioneers to the western United States.

During eight decades in the 1800s the Oregon Trail served as a natural corridor as the United States moved from the eastern half of the continent toward the West Coast. The Oregon Trail ran approximately 2,000 miles west from Missouri toward the Rocky Mountains to the Willamette Valley. A trail to California branched off in southern Idaho. The Mormon Trail paralleled much of the Oregon Trail, connecting Council Bluffs to Salt Lake City.

It began as an unconnected series of trails used by Native Americans. Fur traders expanded the route to transport pelts to trading posts and rendezvous. In the 1830s, missionaries followed the still faint trail along the Platte River and the Snake River to establish church connections in the Northwest.

Large-scale migration started in 1843, when a wagon train of more than 800 people with 120 wagons and 5,000 cattle made the five-month journey. In 1847, Mormons escaping persecution headed toward Salt Lake, and the discovery of gold in California in 1848 sent a wave of fortune seekers west. Military posts, trading posts, shortcuts, and spur roads sprang off the Oregon Trail over the next three decades. The Central Pacific Railroad connected California in 1869, and the Oregon Shortline finished a railroad from Portland, Oregon to the Union Pacific railroad in Wyoming in 1884.

The trail became a route for eastward cattle drives, but by the 20th century, the Oregon Trail was considered part of a historic past, and the image of covered wagons and heroic pioneers had become an American icon.

The challenge of crossing many rivers and the Continental Divide created other severe tests for the emigrants. Summer temperatures, miles of shadeless trail and choking dust compounded to make life decidedly unenjoyable. Though confrontations with Indians were rare, the fear of attack was a constant worry. It was the only feasible way for settlers to get across the mountains.

The glory years of the Oregon Trail finally ended in 1869, when the Transcontinental Railroad was completed. Actual wagon ruts from the Oregon Trail still exist today in many parts of the American West; and many groups are working hard to preserve this national historic treasure.

In 1978, Congress designated it the Oregon National Historic Trail.

Oregon Trail
Trail ruts on the Oregon Trail, near La Barge, Wyo. [Credit: Chamois Andersen, WSGS, 2015]

Pioneer Routes Out West

By Julie Francis

American Progress
American Progress, an 1872 painting by John Gast.

Upon cresting the Continental Divide at South Pass, westbound emigrants on the Oregon, California and Mormon Pioneer trails faced an arduous journey across the Green River Basin. The high desert landscape, as well as the mountain ranges and hogbacks of the overthrust belt, presented a variety of challenges to travelers during the mid-1800s. Geological landforms often played an important role in determining specific routes.

“Manifest Destiny” was the belief during the 19th century that American settlers were to expand and move across the continent, from the Atlantic to the Pacific Coast.

From South Pass, the primary route headed southwest toward what is now the town of Granger and onwards to Fort Bridger. This route provided much needed fresh water, grass for livestock, and the opportunity to rest and replenish supplies at Fort Bridger. From Fort Bridger, Mormons continued southwest across the Wasatch Range and faced perilous descents of the steep canyons on the western slope to the Salt Lake Valley. Most Oregon and California-bound travelers took a sharp turn north at Fort Bridger to Muddy Creek. Cumberland Gap, cut by Muddy Creek through the Hogback, served as access to the overthrust belt and the Bear River Divide. Emigrants followed several variants established along different creeks to reach the divide, often traveling at elevations significantly higher than South Pass. The routes converged at a low pass and descended Bridger Creek to the Bear River and then onto the Snake River plain in Idaho.

The Fort Bridger route to Oregon and California was far from direct, leading to the opening of several significant cutoffs. These generally followed a more westerly course across the Green River Basin and joined the main trail north of present-day Kemmerer. These cutoffs shortened travel time by as much as three days, but emigrants had to cross vast waterless stretches between South Pass and the Green River and were unable to go to Fort Bridger to resupply. The Sublette Cutoff, established in the 1840s, left the main route at “Parting of the Ways” near South Pass and headed west over the high, dry benches of the Green River Basin. After crossing the Green River south of present-day LaBarge, emigrants inscribed and painted their names on the soft Wasatch Formation sandstones at Names Hill, alongside Native American images. The Slate Creek cutoff took a more southerly route, and the Lander Cutoff headed west across the upper Green River Basin, crossing the river north of present day Big Piney. This road was built by the U.S. Army in 1857 as part of a network of wagon roads across the West.

Travelers on all routes had to cross the Green River to continue their journey. At low water, the river could be forded. During high water, ferries operated at key crossings. Ferry owners charged high fees, and the crossings were dangerous, often resulting in loss of wagons, livestock, belongings and lives. Eventually, the ferries were replaced by bridges on what became the modern state highway system. Even today, there are only three highway crossings of the Green River, all in the general vicinity of the historic ferry crossings for the pioneer routes going west.


Wyoming State Historic Preservation Office, 2014, The way West: A historical context of the Oregon, California, Mormon Pioneer, and Pony Express National Historic Trails in Wyoming, at

Bureau of Land Management, 2015, Basic facts about the Oregon Trail Oregon/Washington, at, accessed July 9, 2015.

Kornfeld, M. and, Frison, G.C., and Larson, M.L., 2010, Prehistoric hunters and gathers of the High Plains and Rockies: Walnut Creek, Left Coast Press, Inc., 598 p.

U.S. Forest Service, 2015, Making the trek, at, accessed July 9, 2015.

Opal, Wyoming

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.

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.

How Opal got its name?

Welcome to Opal, Wyoming
Welcome to Opal, Wyoming. [Credit: Chamois Andersen, WSGS, 2015]

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.

Wyoming's Trona Resource

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.

Wyoming's Trona Resources - WY Cultural Geology

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.

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 countries across the globe.

Trona products
Trona is used in a variety of every day products, including tooth paste, bottles and baking soda.

Trona Trail Historic Mine Byway, Sweetwater County

Trona Trail
The Trona Trail Historic Mine Byway, interpretive signs at the Green River Visitor Center.

The Trona Trail Byway can begin or end at Granger Stage Station, or at a pullout on U.S. 530. Visitors can access the trail from east- or westbound I-80 or northbound U.S. 530. The Green River Visitor Center is a great place to stop and see trona and industry equipment on display.

White Mountain Petroglyphs - Green River Basin Rock Art

By Julie Francis

White Mountain Petroglyphs
Steamboat Mountain is the highest point in Wyoming's Red Desert. It is the site of numerous petroglyphs, dating back 11,000 years. [Credit: Dan Frazer]
White Mt. Petroglyphs - WY Cultural Geology

The vestiges of Eocene Lake Gosiute exposed on the modern land surface often consist of soft sandstones and mudstones. Easily sculpted by both water and wind, slightly harder beds of rock have sometimes been exposed by downcutting of the Green River and other streams or along the sides of buttes capped by harder rocks more resistant to erosion. The rocks of these bluffs and walls are often quite massive, with few bedding planes, fractures or seams, and they made for an ideal canvas on which native peoples and emigrants engraved, chiseled, abraded and painted a variety of images. Unfortunately, because the rocks are so soft, the glyphs do not stand up well to the ravages of time, and most of the Native American rock art of the Green River basin is likely less than 1,000 years old.

Hints that native people of the Green River Basin made rock art for thousands of years have been found on the flanks of Steamboat Mountain. There, a small overhang contains the images of humans and mountain sheep, made by a method called pecking, and almost completely coated by layers of patina (or rock varnish). Both the varnish and petroglyphs have survived because the overhang is protected from the wind, which has “sandblasted” nearby rock surfaces. Accelerator mass spectrometry (AMS) radiocarbon dating of minute pieces of organic material embedded in the varnish suggest that these images could have been made as long as 11,000 years ago. The radiocarbon age certainly does not provide an exact date of manufacture but clearly indicates a substantial antiquity of these images.

White Mountain Petroglyphs

The incised glyphs of the White Mountain Petroglyph site stand in stark contrast to these older images. Hundreds of renderings of bison, bear, elk and other animals, along with some human figures, animal tracks, plants and geometric designs have been incised into the soft sandstone north of Rock Springs. Many of the animals pictured at this site are considered to be spiritually powerful creatures, whose power and wisdom was sought by humans, and these images indicate ceremonial and spiritual use, probably beginning in the Late Prehistoric Period about 1,000 to 1,500 years ago. Human hand and finger grooves, made simply by repeated gouging of the rock over the millennia, provide testament to the power of the place as well as to the fragility of the soft rock. In addition to these images, White Mountain contains many historic period images - horse-mounted warriors, some holding bows, others holding rifles and clubs, protected by shields and often engaged in battle. This imagery, commonly termed Biographic style art, reflects the major disruption, conflict and changes experienced by native peoples - many of whom were ancestors of the modern Shoshone and Ute – after the entry of Euro-American emigrants into the region.

Archeologist Julie Francis
Archeologist Julie Francis discusses the petroglyphs found at White Mountain. [Credit: Chamois Andersen, WSGS, 2015]
White Mountain Petroglyphs
Petroglyph sites were often used as sacred places where people came to communicate with the supernatural.  Some sites may have been places where medicine men or spiritual leaders went to meditate to receive visions or guidance to lead or heal their people. Some of the symbols may have had special significance for hunting such as with this petroglyph of a bison. [Credit: Chamois Andersen]


Francis, J. E., and L.L., Loendorf, 2002, Ancient visions: Petroglyphs and pictographs of the Bighorn and Wind River Country, Wyoming and Montana: Salt Lake City, University of Utah Press.

Keyser, and D., James, and G.R., Poetschat, 2005, Warrior art of Wyoming’s Green River Basin: Biographic petroglyphs along the Seedskadee: Oregon Archaeological Society Publication No.15.

Liu, Tanzhao, R.I., Dorn, 1996, Understanding the spatial variability of environmental change in drylands with rock varnish microlaminations: Annals of the Association of American Geographers 86(2), p.187-212.