Norris Road: Heating, Cooling and Clearing Obsidian Cliff

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Obsidian Cliff, a steep wall of black volcanic glass beside the road between Norris and Mammoth Hot Springs, is the place this picture shows, and the title describes the work done there: heating the rock, cooling it, and clearing away the broken pieces. Before power equipment, crews often built fires against stubborn rock and then doused it with water, so the sudden temperature change would crack it apart. Early Yellowstone roads were cut largely by hand through rugged, geothermally active ground, which made every mile slow to build. The cliff's glassy face gives the scene its unusual sheen.

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