The top exposed layer of
Columbia River Basalt Group basalt in the Saddle Mountains is the Saddle Mountain Basalt, which ranges from in thickness and is interspersed by sedimentary layers of the Ellensburg Formation. The Saddle Mountain Basalt is composed of the Umatilla Member flows, the Wilbur Creek Member flows, the Asotin Member flows (13 million years ago), the Weissenfels Ridge Member flows, the Esquatzel Member flows, the Elephant Mountain Member flows (10.5 million years ago), the Bujford Member flows, the Ice Harbor Member flows (8.5 million years ago) and the Lower Monumental Member flows (6 million years ago). The
Cordilleran Glacier diverted the ancient route of the
Columbia River 15,000 years ago, backing up water to create
Lake Spokane. As the Okanogan lobe grew the Columbia was rerouted into the
Grand Coulee. Flowing across the current Grand Coulee-
Dry Falls region, the ice-age Columbia then entered the
Quincy Basin near
Quincy, Washington and joined
Crab Creek at Moses Lake, following Crab Creek's course southward past the
Frenchman Hills and turning west to run along the north face of the Saddle Mountains, there to rejoin the previous and modern course of the Columbia River just above the main water gap in the Saddle Mountains,
Sentinel Gap. It eroded the north face of the Saddle Mountains, creating the conditions for the
Corfu Slide. == Geography ==