(Oreamnos americanus), as well as one of the highest densities of yellow-bellied marmot (Marmota flaviventris) in the region. Above tree-line, pikas (Ochotona princeps) are common. Below tree line, elk (Cervus canadensis) and mule deer (Odocoileus hemionus) are common. Among birds, the white-tailed ptarmigan (Lagopus leucurus) are present on the mountain, but so well camouflaged that they are difficult to see even when almost underfoot. Brown-capped rosy finches (Leucosticte australis), pipits and rock wrens (Salpinctes obsoletus) are also seen near the summit. Geology Mount Evans was carved from the rock of the Mount Evans Batholith, formed by an intrusion of magma into the earth's crust about 1.4 billion years ago (in the Mesoproterozoic Era of the Precambrian Eon). Much of the rock is granodiorite, a close relative of granite, modified by later intrusions of quartz and pegmatite. The body of this batholith has been deeply cut by glacial cirques and canyons. Each of the nearby lakes, Summit Lake, the Chicago Lakes, Lincoln Lake and Abyss Lake are tarns located in cirques or glacial canyons surrounding Mount Evans. Echo Lake was dammed by a lateral moraine of the glacier that formed Chicago Canyon. Prior to glaciation, Mount Evans, Long's Peak and several other summits were monadnocks in an upland Peneplain. Glaciation has not entirely destroyed the ancient Flattop Peneplain, named for Flattop Mountain in Grand County. The peaks of these mountains are all remnant features of this peneplain. Scientific research The easy access to the summit provided by the Mount Evans Highway has made it a popular location for scientific research. Arthur H. Compton conducted pioneering research on cosmic rays on the mountain in 1931, shortly after the road to the summit was completed. The University of Denver built a pair of A-frame buildings on the summit to house cosmic-ray researchers. By the 1950s, Mount Evans, the Aiguille du Midi, the Pic du Midi and the Jungfrau were considered the premier locations for high-altitude physics experiments. The first accurate measurement of the lifetime of the muon (originally called the mesotron) by Bruno Rossi in 1939, used sites at Mount Evans, Echo Lake, Denver and Chicago. This experiment verified the reality of time dilation, one of the key predictions of Einstein's theory of relativity. In the summer of 1948, MIT, Cornell, Princeton, NYU and the universities of Michigan and Chicago and Denver conducted an intensive experimental program on the mountain and at Echo