 The Missouri and many of its tributaries cross the Great Plains , flowing over or cutting into the <unk> Group and older mid @-@ Cenozoic <unk> rocks . The lowest major Cenozoic unit , the White River Formation , was deposited between roughly 35 and 29 million years ago and consists of <unk> , sandstone , limestone , and conglomerate . Channel <unk> and finer @-@ <unk> <unk> deposits of the <unk> Arikaree Group were deposited between 29 and 19 million years ago . The Miocene @-@ age <unk> and the slightly younger <unk> @-@ age <unk> 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 <unk> . 
