The geology of Alaska includes Precambrian igneous and metamorphic rocks formed in offshore terranes and added to the western margin of North America from the Paleozoic through modern times. The region was submerged for much of the Paleozoic and Mesozoic and formed extensive oil and gas reserves due to tectonic activity in the Arctic Ocean. Alaska was largely ice free during the Pleistocene, allowing humans to migrate into the Americas. Geologic History, Stratigraphy & Tectonics Compared with other areas of the North American continent, Alaska formed in the recent geologic past. Until 200 million years ago, western North America terminated at the Rocky Mountains, 120 miles further inland than the current shoreline, until the addition of the Yukon-Tanana Terrane. The Birch Creek Schist is the oldest rock in Alaska's interior and forms the core of the Yukon-Tanana Terrane with muscovite-quartz schist, mica quartzite and graphite schist. This intensely folded and rock unit extends from Fairbanks into the Yukon Territory in Canada and formed between 800 and 600 million years ago in the Proterozoic, due to the metamorphism of shale, sandstone and mudstone along the western margin of the continent. A different schist belt may underlie the Brooks Range and is known from deep boreholes in the vicinity of Prudhoe Bay, reaching greenschist and blueschist on the sequence of metamorphic facies. Precambrian metasedimentary gneiss is found in the Kigluaik Mountains on the Seward Peninsula, while in southeast Alaska, additional Precambrian rocks underlie Prince of Wales Island. Paleozoic (539-251 million years ago) Throughout the Paleozoic, a marine shelf environment dominated the western edge of the proto-North American continent Laurentia, becoming deeper over time and experiencing widespread volcanism over 5000 square kilometers, as well as the deposition of carbonates, mudstone and sandstone. Volcanic rocks remain in the Keevy Peak Formation in the Northern Alaska Range and metamorphosed into the Totatlanika Schist in the interior, as well as in the Alexander terrane in the southeast. Mesozoic (251-66 million years ago) In the Mesozoic, North America separated from the supercontinent Pangea which had formed in the late Paleozoic. Oceanic plates in the Pacific Ocean subducted in trenches offshore, carrying volatiles such as water and carbon dioxide with them, leading to rock melting and volcanic activity. Intense volcanism built up Alaska's mountains until a slowing of eruptions in the late Cretaceous around 80 million years ago. During the Middle Triassic three large terranes—Wrangellia, Alexander and Stikine—remained