Introduction.
This landscape is mostly the work of water. The soft sedimentary rocks of the hills and riverbanks are composed of material eroded by water from the young Rocky Mountains over 70 million years ago. Muds and sands were transported by rivers, and deposited in and near an ancient seaway.
The hills and river valley were sculpted from these old sediments, primarily by fresh water streams and rivers. Most of that happened in the last 5 million years. Ice played a role too. Scattered large boulders like those around the main kiosk were dumped here by a melting glacier, probably about 20 thousand years ago.
The late Cretaceous World
During most of the Cretaceous Period, an expedition from the east to the west coast of what is now North America would have required ocean-going boats. Shallow seaways advanced from both the north and south and sometime came together to completely split the continent. The early Rocky Mountains were rising. Some of the mountains were explosive volcanoes like the Cascades in western Oregon and Washington today. Winds and water spread layers of volcanic ash across the whole area. Dinosaurs stomped across the land. What a time!
The rocks you see across the river in the steep river bank and in the hills on both sides of the Missouri valley are part of the geologic record of the Late Cretaceous seaway and coastal plain. Geologists map these rocks as two different formations.
