The hills south and west are part of the Rawlins Uplift, a 40 mile long, north-trending fold in the earth's crust that rises a thousand feet above the surrounding plains. Granite rocks ranging in age from 2.6 billion years to 10 million years are exposed along its crest and slopes. Much of the distant reddish hills are Flathead Sandstone from the Cambrian, 520 million years old, and the grey strata above them are Madison Limestone from the Mississippian Period, 360 million years old; both were deposited in shallow seas that once covered this area. The Flathead Sandstone contains hematite, a red iron mineral mined here from 1870 to the early 1900s at outcrops a mile to the south. Some of this hematite became a durable barn red paint pigment known as Rawlins Red, chosen in 1893 for the newly constructed Brooklyn Bridge. Hundreds of tons of ore were extracted annually between 1879 and 1886, and paint made from it was used across the country on many kinds of buildings and structures, until rising extraction, shipping, and processing costs, along with advances in paint manufacture, led to the mines' permanent closure.