Located northwest of Craig in Moffat County, the Sand Wash Basin is an area of Bridger Formation rock outcrops that prehistoric peoples mined extensively as a source for stones to make tools with. Bridger Formation chert is typically light to dark brown, though some of the chert in the basin is referred to as “tiger chert” because of its distinct alternating light and dark brown banding. Tiger cherts have been found across Colorado and in both Utah and Wyoming. The distinct patterning of tiger chert has allowed archaeologists to trace the movement of prehistoric people in and out of northwestern Colorado.

Geology

The Sand Wash Basin is the southern portion of the Green River Basin system in Wyoming, which is an Eocene-aged lake system that drained south into the Piceance Basin of Colorado and the Uinta Basin of Utah. Deposits in the Sand Wash Basin are sedimentary and contain many fossils, including well-preserved vertebrate, invertebrate, and plant fossils. Because of high silica content in the region’s geology, the Sand Wash Basin contains layers of chert bedrock, chert nodules, petrified wood, and fossilized stromatolites that lend themselves to striped banding.

There are different varieties of chert tool-stone quarries in the Sand Wash Basin. The basin’s center contains bedrock layers of chert. The basin’s perimeter contains more nodules of petrified wood and stromatolites that are available as eroded gravel deposits and isolated clusters. Whether quarried from layers of chert bedrock or collected from erosional deposits, all the stone material in the basin was usable for tool blanks and is typically identified as Bridger Formation chert.

Archaeology

In 1976 Richard Stucky did an archaeological survey of the Sand Wash Basin. Stucky noted that the basin’s Bridger Formation cherts had a long history of use and can be associated with the bison-hunting Paleo-Indian and Clovis populations of 13,000 years ago as well as later Archaic, Formative, and historical groups, including the Shoshone and Ute. In some places in the basin one can still see the large quantities of stone that were quarried and tested by prehistoric inhabitants of the area. One additional piece of evidence for a long period of use of the basin’s cherts comes from the Mahaffy cache site in Boulder, with its impressive tiger chert artifacts. Stucky suggested that prehistoric families camped around the periphery of the Sand Wash Basin while mining resources in the middle of it, which was supported by a subsequent archaeological study in 2010. Stucky’s work resulted in a collection of pristine materials now housed at the Denver Museum of Nature and Science, including a fourteen-centimeter-long cold-worked copper knife found at the Cathedral Butte site, which is similar to knives found in Oklahoma and the Great Lakes region.

Bridger Formation chert artifacts have been found in archaeological sites in neighboring states, such as the John Gale Cache in Wyoming and Fremont villages in western Colorado and Utah, and archaeologists are studying their chemical structure to connect these artifacts to specific quarries in the Sand Wash Basin. Interestingly, heating the Bridger Formation chert in a fire alters its structure and makes it easier to shape into a tool. While this can create sharper tools, it can also crack the chert and make it more brittle. The heat-treating of cherts has been shown to occur more often around the periphery of the Sand Wash Basin than in the heart of the Sand Wash Basin, though it is unknown how heat-treating alters the ability of scientists to source the Bridger Formation chert to specific quarries.

Bridger Formation cherts from the Sand Wash Basin help archaeologists understand the way prehistoric families lived and moved through Colorado over the last 13,000 years. Beyond that strong archaeological value, tiger chert artifacts can be beautiful examples of prehistoric craftsmanship. Thus, in addition to being utilitarian tools that now serve as markers of trade and antiquity, they were likely admired and appreciated for their striking visual characteristics as much in the past as they are today.