Ogallala Aquifer | What Happens When the Water Runs Out, National Geographic Magazine: Rocks Formed During Uplift

Sediment that formed the Ogallala aquifer sloughed off from the Rocky Mountains, creating gravel that is mined for construction materials. Rocky Mountain uplift and the natural weathering allowed the material to scuff off the slopes. Then materials transported by huge streams became the channels in the aquifer. The Rocky Mountains are compositionally different fhaving more granite than from those in the south.  Sixteen acres of the gravel are stored near Slaton, Texas.