Spanish Diggings stones

Spanish Diggings is a huge field of ancient quarry pits in eastern Wyoming, in the Hartville Uplift. Settlers once blamed Spanish gold-seekers, but scientists showed that Native people dug the pits for toolstone. Two kinds of rock were taken. Fine-grained orthoquartzite from the Morrison and Cloverly formations comes in purple, gold, gray, tan and red. Chert nodules from the older Guernsey and Hartville limestones are often butterscotch with black dendrites. The Wyoming Geological Survey says some of the chert is poor, though good patches exist. People quarried here from Paleo-Indian times, roughly 12,000 years ago, into the historic period, and artifacts from it have turned up across the Northwestern Plains. Large ceremonial bifaces in Hopewell burials near La Crosse, Wisconsin, may come from here.

Orthoquartzites from the Upper Jurassic Morrison and Lower Cretaceous Cloverly formations, on the central-western slope of the Hartville Uplift (outcrops cover about 15 square miles). Chert nodules come from the Pennsylvanian-Mississippian Hartville Formation and Mississippian-Devonian Guernsey Formation along the flanks of the uplift. The uplift is a structural arch raised in the Laramide mountain-building.