It’s one of those overlooked facets of Nature. Much of the Great Plains sits on the Ogallala Aquifer. This is an underlying terrain, to depths of hundreds of meters, filled in by sediment from the Rockies (https://en.m.wikipedia.org/wiki/Ogallala_Aquifer#General_cha...).

The greater part of the LA Basin is a floodplain which consists of hundreds of feet of sediment from the San Gabriel mountains.