The Lewis and Clark Expedition passed here by sail in May 1805, taking advantage of a southeast breeze on the Missouri River. The surrounding landscape preserves a much older history: during the Cretaceous Period, shallow seas advanced from the north and south and sometimes split North America, while the early Rocky Mountains rose and some volcanoes spread ash across the region. The present hills and river valley were carved by erosion of Cretaceous sedimentary rocks, mostly within the last 5 million years, and the Missouri River itself has shifted across its floodplain since 1805 from the opposite side of the valley. In the last 2 million years, glaciers advanced from the north, at least once extending past this location and leaving scattered large boulders behind. Salts deposited in ancient marine sediments still dissolve into streams and groundwater, creating water that Lewis found unfit for use and painfully purgative. Exposed surfaces of the Bearpaw Shale contain large clear gypsum crystals that Lewis and Clark's journals misidentified as quartz. Geologists map the Late Cretaceous rocks here as the Bearpaw Shale, formed from offshore mud in the seaway more than 70 million years ago, and the Judith River Formation, laid down about 75 million years ago in estuaries, deltas, swamps, lagoons, and river floodplains and now famous for dinosaur fossils.