Sims Corner Eskers and Kames National Natural Landmark of Douglas County, Washington, and nearby McNeil Canyon Haystack Rocks and Boulder Park natural landmarks contain excellent examples of Pleistocene glacial landforms.

During the middle to late Miocene epoch, one of the largest flood basalts ever to appear on the earth's surface engulfed about 163,700 km2 (63,000 mile2) of the Pacific Northwest, forming a large igneous province with an estimated volume of 174,300 km3.

[2][3][4][5][6] Two million years ago the Pleistocene epoch began and ice age glaciers invaded the area.

Grooves in the exposed granite bedrock are still visible in the area from the movement of glaciers and numerous glacial erratics in the elevated to the northwest of the coulee.

Flowing across the current Grand Coulee and Dry Falls regions, the ice age Columbia then entered the Quincy Basin and joined Crab Creek, following Crab Creek’s course southward past the Frenchman Hills and turning west to run along the north face of the Saddle Mountains and rejoin the previous and modern course of the Columbia River just above the main water gap in the Saddle Mountains, Sentinel Gap.