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.