In the Wind River area, the Cascades sit atop Cenozoic era lava flows and volcaniclastic deposits, which have a calc-alkaline composition. Divided into the Ohanapecosh Formation, Eagle Creek Formation, Yakima Basalt, and Quaternary basalt flows, they show variable thickness and topographies. The Cascade Arc formed during the Oligocene epoch, and by the late Miocene epoch there was a low but broad gap between the Washington segment of the arc and Cascade volcanoes in Oregon. Activity picked up during the Quaternary period in Washington, as andesitic stratovolcanoes and small, olivine basalt cinder cones and shield volcanoes erupted. Before the Pleistocene, uplift and erosion was widespread, which was followed by eruptions from the northeastern part of the Wind River area during the early Pleistocene.