QUATERNARY STRATIGRAPHY AND ARCHAEOLOGICAL GEOLOGY ALONG THE MISSOURI RIVER AND GEOMORPHIC CONTEXTS ASSOCIATED WITH THE LEWIS AND CLARK EXPEDITION

The Expedition of Lewis and Clark traveled westward towards the Rocky Mountains in A.D. 1804 and 1805. The journals of several of the expedition members and especially the maps of William Clark document the landscapes of the upper Missouri Basin. Studies focused on the landscapes associated with Fort Mandan, in North Dakota, and around the Great Falls, in Montana, provide an opportunity to compare the records of Lewis and Clark with geomorphic and stratigraphic contexts along the Missouri River. The winter camp of Fort Mandan was situated on a low fluvial terrace on the north side of the Missouri, below the confluence of the Missouri and Knife Rivers. Higher terraces and upland areas contain fluvial and glacial facies of the Pleistocene Coleharbor Group and the late Pleistocene and Holocene Oahe Formation. A low terrace consists of a sequence of three texturally distinct sets of fluvial beds. At one exposure, a 140+ cm thick set of laminated and crossbedded “lower sands” are overlain by four silt beds with a combined thickness of 40 cm. A bone fragment from one of the silt beds has a radiocarbon age of 160 +/-40 yr B.P. (Beta-156172). The silts are overlain by two beds of sandy silt and silty sand with a combined thickness of 40+ cm. In another exposure further downstream, fragments of charcoal from the uppermost silt provided a radiocarbon age of 190 +/-40 yr B.P. (Beta-156171). Thus the terrace reflects episodic aggradiation since approximately the time of the Lewis and Clark Expedition. The Lower Portage camp was located on the south side of the Missouri downstream from the confluence of the Missouri with Belt Creek. The landscape contains Cretaceous bedrock, fluvial gravels, lacustrine deposits, and till. Alluvial fan deposits downstream contain the Mazama tephra. Radiocarbon ages on bone collagen, wood, charcoal/charred material and soil-bulk organics/organic sediments demonstrate slow deposition since about 2,810 +/-50 yr B.P. (eg. Beta-123275). The geomorphic, stratigraphic, and geochronologic evidence appears to indicate little change in the landscape context of the Lower Portage area during the late Holocene.