Salt Creek Hills was a focus of both prehistoric American activity and historic mining, set in a rich environment that supports numerous species. The area includes the Amargosa/Salt Spring mine in the Salt Spring Hills near a contact zone between Mesozoic granite and Cambrian sedimentary rocks of limestone and limy shales, where recrystallization and alteration produced metamorphic rocks such as hornfels and skarns, and where gold occurred in thin quartz- and carbonate-filled fissure veins adjacent to contacts of granite with limestone, marble, limy shales, and hornfels. Exposed geology includes Quaternary alluvium, Zabriskie Quartzite, red agglomerate, grey tuff-breccia, grey aphanite porphyry, marble, hornfels, banded granofels, and granite. In the southern Salt Spring Hills, where Highway 127 bends, a water gap was cut by the ancestral Mojave River, which flowed into Death Valley from the south; during the Pleistocene, Mojave River silts and clays were temporarily ponded against the hills by an alluvial fan from the Avawatz Mountains to the south, in a lake believed related to the much larger Lake Manly that intermittently occupied Death Valley during Quaternary time, and overflow eventually cut through the barrier while present-day Salt Creek continues to dissect the Pleistocene lake sediments. Mining remains included three Chilean-style arrastres known here in the late 1800s, of which one millstone survives, and a stamp mill originally built in the 1880s to crush ore for precious metals. Amargosa House may be the oldest standing structure in the Mojave Desert; the three-room office-house was originally constructed between 1850 and 1852, and was shown one hundred years later, in 1950, with a modern roof. The stream, wetland, dune, and wash areas around Salt Creek provide food and shelter for as many as nine types of bats, which forage at different heights and times and on different prey, including the pallid bat, which specializes in hard-shelled insects such as scorpions. Most bats in the western U.S. feed on insects and are major predators of night-flying insects, with a single bat able to eat half its body weight in insects in one night, roughly 3000 or more. Historic mines in Salt Creek Hills have become important replacement roosting habitat for bats as natural caves and crevices have been destroyed or disturbed, but bats are highly sensitive to human disturbance, and hibernating bats and nursing mothers are especially vulnerable. Reports from the old mining era tell of violence at the Amargosa/Salt Spring Mine: in October 1864, three miners named Cook, Plate, and Gordon were reportedly attacked by a roving band of Chemehuevis, Cook was killed, the mill was burned, and the other two escaped into the desert and committed suicide about 20 miles away; the Rousseau Party passed through in December 1864 and noted four houses and the burned mill. A new company under superintendent George Rose took over in late 1864, and confused reports describe another attack in either 1864 or 1865 in which eight miners were working the claims near a Paiute camp at Sheep Creek Springs; one miner went 45 miles south to Marl Springs near present Kelso for military help, but the relief party arrived too late, and seven miners who attempted a pre-dawn escape were seen by lookouts and slain.