for hydrocarbons. Plate rotation produced the Brooks Range with a plate collision visible at the surface in the Kobuk River. Paleozoic and Mesozoic rocks separated from the descending slab, leaving mineral deposits in the Brook Range related to older rock units. Cenozoic (66 million years ago-present) The Talkeetna Mountains north of Anchorage preserve granite formed from igneous activity during the Cenozoic accretion of the Chugach terrane. During the Eocene, the Prince William terrane was joined to Alaska. The Eocene brought a change in the motion of the Pacific Plate, marked by a bend in the Emperor Seamount–Hawaiian Islands linear chain of volcanoes. The small Kula plate, which was being overridden by the Eurasian and North American plates in the Mesozoic, became "jammed" in place, creating the Bering Sea shelf. The St. George and Navarin extensional basins formed in the Bering Strait and filled with sediments. For much of the Cenozoic, Alaska experienced very cold conditions, preserved by fossils in the 5000 foot thick terrestrial sediments of the Chickaloon Formation in the Cook Inlet. By 10 to 13 million years ago, mountain glaciers had formed. A steady supply of moisture from the Pacific Ocean supported coastal glaciers beginning five million years ago, recorded by the glacial mudstone of the Yakataga Formation along the Gulf of Alaska. During the Pleistocene, the interior of Alaska remained ice free, except for the Yukon-Tanana Upland west of Fairbanks, which experienced six phases of glaciation (the western Alaska Range experienced the same number, while the Brooks Range experienced four). North of the Brooks Range, a polar desert developed with windblown sand dunes. Mammoth steppe allowed humans to cross into North America from Siberia. A return to cold conditions 3000 years ago in the Holocene brought major glacial advances in Glacier Bay and Icy Strait, reaching new maximums by 1750 before going into retreat. Currently, Alaska is experiencing an ongoing terrane collision, with the uplift of the Saint Elias Mountains by the Yakutat Block, volcanism and deep granite formation. References Alaska