Green River Formation tiger chert

Tiger chert is a banded, brown-and-tan chert from the Green River and Great Divide basins of southwestern Wyoming. It formed in the beds of Eocene Lake Gosiute, where silica cemented the sand, silt and clay of the lake bottom or filled voids left by dissolved shells. The Wyoming State Geological Survey says the alternating dark and light brown or tan bands reflect yearly layers of sediment, and that the stone is homogeneous, ideal for finely flaked tools. Outcrops lie southwest of Rock Springs and at a few other places in the Green River Basin. About 13,000 years ago Clovis knappers made points up to about 8 inches long and very thin from it. Caches of such tools turn up as far as Boulder, Colorado, and the Black Hills, and some were coated with red ocher.

Lake-bottom sediment of Eocene Lake Gosiute (56 to 33.9 million years ago) cemented by silica in the Green River and Great Divide basins. In places silica bound the grains; in others it filled voids left by dissolved shells. Erosion has exposed the layers, which form hard pavements on the basin floors.