What I've found so far is that the sand hills region is composed of Quaternary-age " Eutric " regosols, bounded by much sediment and / or uplift of Eocene to Mesozoic ( think Niobrara Sea ) age, located at the edge of the Denver Basin, with loess deposits extending to its east and southeast ( yes, I have citations for all this info ), reaching all the way to the Mississippi River and beyond, while the loess sediment also reaches down to the mouth of the Mississippi River-perhaps this makes sense, since the Platte River runs through the southern sandhills.
