On July 8, 1806, William Clark traveled down Willards Creek on the southwest side for about 11 miles through a country of high, dry, uneven, stony open plains and very boggy low bottoms, with high mountains whose tops and north sides still held snow. He found great quantities of hyssop, or sagebrush, and shrubs common to the Missouri plains scattered through the valleys and hillsides, judged the road from Travellers Rest Creek to this place an excellent one, and recorded that Shields killed an antelope. At about 6,100 feet above sea level, with Baldy Mountain rising to 10,568 feet to the northeast, the area still retains many of the native plants, animals, and cold July nights known two hundred years ago, and little has changed since the time of Sacajawea's ancestors, long before she guided the Corps of Discovery and Clark here. Over geologic time, however, the landscape has changed greatly: 47 million years ago there were no Pioneer Mountains, this was the edge of the Renova Basin draining eastward, and fossils show a tropical climate. The Pioneer Mountains began rising about four million years ago after crustal extension in the region started about 17 million years ago, thinning the crust and allowing super-hot molten magma of the aesthenosphere to bulge upward. As crustal blocks moved along northwest-trending normal faults, rising blocks built the Pioneer Mountains while adjoining blocks dropped and deepened the valleys. Movement along these faults stretches the rocks and releases energy as earthquakes, whose breaks occur below the surface along a fault and whose magnitude increases with the amount of energy released; though a person might live here a lifetime without feeling one, earthquakes are common on the scale of geologic time.