 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.