At the time of impact, the Alamo crater was about 120 miles (200 km) west of its current location, said Charlie Sandberg, an emeritus geologist with the U.S. Geological Survey in Denver, Colorado, and one of the scientists who first identified the crater. Since the Devonian Period, western North America has crumpled inward because of collisions with other tectonic plates, building up the region's spectacular mountain ranges. The tectonic forces faulted and tilted the impact crater rocks, partially burying the crater. Volcanic eruptions also concealed some Alamo impact rocks. Finally, some rocks are also inaccessible, because they cross Area 51 and the Nevada Test Site.