 The Missouri and many of its tributaries cross the Great Plains , flowing over or cutting into the Ogallala Group and older mid @-@ Cenozoic sedimentary rocks . The lowest major Cenozoic unit , the White River Formation , was deposited between roughly 35 and 29 million years ago and consists of claystone , sandstone , limestone , and conglomerate . Channel sandstones and finer @-@ grained overbank deposits of the fluvial Arikaree Group were deposited between 29 and 19 million years ago . The Miocene @-@ age Ogallala and the slightly younger Pliocene @-@ age Broadwater Formation deposited atop the Arikaree Group , and are formed from material eroded off of the Rocky Mountains during a time of increased generation of topographic relief ; these formations stretch from the Rocky Mountains nearly to the Iowa border and give the Great Plains much of their gentle but persistent eastward tilt , and also constitute a major aquifer . 
