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.

[7] As the Okanogan lobe melted, the northern half of the Waterville Plateau, including the upper portions of Moses Coulee, were littered with clear evidence of its passing.

This glacial till, composed of clay, silt, sand, gravel, cobblestones, and erratic boulders, covers most of the upper Moses Coulee.