The geology of Nevada began to form in the Proterozoic at the western margin of North America. Terranes accreted to the continent as a marine environment dominated the area through the Paleozoic and Mesozoic periods. Intense volcanism, the horst and graben landscape of the Basin and Range Province originating from the Farallon Plate, and both glaciers and valley lakes have played important roles in the region throughout the past 66 million years. Geologic history, stratigraphy, and tectonics The oldest rocks in Nevada are in the East Humboldt Range in the northeast, with lead isotope data suggesting an age of 2.5 billion years, at the boundary of the Archean and Proterozoic. Metamorphic and igneous rocks formed 1.7 billion years ago underlie Clark County and the populous areas around Las Vegas. The region was part of the supercontinent Rodinia one billion years ago, situated at the equator. The continent rifted apart between 700 and 600 million years ago. No continental rocks exist in western Nevada from more than 700 million years ago, because the western part of the region was rifted away, becoming part of current day Siberia. Paleozoic (539-251 million years ago) After the breakup of Rodinia, southern and eastern Nevada remained as a passive margin on the western edge of the proto-North American continent of Laurentia. Up to 20,000 feet of sediment accumulated along the new margin as it subsided, building up thick layers of sandstone, siltstone, limestone and dolomite. The Monitor, Egan, Schell Creek and Arrow Canyon ranges in the east are dominated by limestone and dolomite formed during 400 million years of marine conditions. Central Nevada by contrast, preserves shale, siltstone and chert formed under deep water conditions as fine ooze settled into deep water offshore. Continental collisions began to form the supercontinent Pangea leading to subduction along the western margin of Laurentia. As plates subducted, they spurred intense volcanic and tectonic activity, and volcanic island arcs and carbonate platforms accreted against the western shore. Dozens of terranes accumulated over 250 million year and many rock units were thrust on top of others as thrust sheets along thrust faults. Large granite bodies formed from magma. The Antler Orogeny, named by geologists in 1951 after Antler Peak on Battle Mountain, was an early orogeny mountain building event in the creation of the Basin and Range Province. The Antler highland, including the Roberts Mountains, formed as fractured deep ocean sediments