Limestone, near Ruby Valley, East Humboldt Mountains, Nevada

Most of O'Sullivan's effort was devoted to recording geological subjects-features and formations of special interest to the survey's geologists. The Fortieth Parallel terrain was a vast open-air geological laboratory, with abundant evidence of the region's complicated and often violent geological past. In the rocks, the team read a story of ancient lake beds, periodic volcanic and glacial activity, forces of weathering and erosion and processes of mountain building involving the uplift, faulting and folding of rock strata. Here, O'Sullivan photographed the cliffs of the Ruby Mountains whose canyons and valleys had been formed by glaciers.