The deposition of the aquifer material dates back 2 to 6 million years to late Miocene to early Pliocene age when the southern Rocky Mountains were still tectonically somewhat active. From the uplands to the west, rivers and streams cut channels in a generally west to east or southeast direction. Erosion of the Rockies provided alluvial and eolian sediment that filled the ancient channels and eventually covered the entire area of the present-day aquifer, forming the water-bearing Ogallala Formation. The depth of the formation varies with the shape of the pre-Ogallala surface, being deepest where it fills ancient valleys and channels. The Ogallala Formation consists mostly of coarse sedimentary rocks in its lower sections, which grade upward into finer-grained lithologies.