134-2 Meriwether Lewis, William Clark, and the Corps of (Geologic) Discovery: Geoheritage Sites along Missouri National Recreational River, Nebraska–South Dakota, U.S.A.

Session: Crossing Boundaries: Histories of Geoheritage

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Following the Louisiana Purchase in 1803, President Thomas Jefferson commissioned the Corps of Discovery to explore the expanded American western frontier—including its geologic resources. To gain access to the west, the Lewis and Clark Expedition navigated the Missouri River, including what is now the Missouri National Recreational River (MNRR), which offered the most direct and practical means of transportation. Throughout the journey, Lewis and Clark mapped the locations of several geologic commodities, including metallic ores, salt, coal, grinding stone, and freshwater springs. Although these maps are not considered geologic in the modern sense, they are accurate enough to locate these resources today. Multiple geologic features along MNRR factored into the westward journey and offer glimpses into the expedition’s early encounters from late August through early September of 1804.

On the southern banks of the Missouri River, the Corps of Discovery encountered the intense heat and haze of the Ionia Volcano, a river bluff of the Cretaceous Carlile Shale that was a historical source of misconception regarding igneous activity. North of MNRR, members of the expedition hiked to Spirit Mound, a significant spiritual site among the Indigenous Peoples of the Great Plains. The mound is a product of the last ice age and is a scenic drumlin that rises above the prairies of South Dakota. On 28–31 August 1804, the Corps of Discovery camped below Calumet Bluff, a rocky bluff along the southern riverbank consisting of the Cretaceous Niobrara Formation. The bluff was the setting of the expedition’s first council with leaders of the Yankton Sioux Tribe and symbolized one of Lewis and Clark’s early diplomatic accomplishments. Not far from Calumet Bluff, the expedition discovered the fossilized remains of a marine reptile approximately 14 m (45 ft) long; today, multiple plesiosaurs and mosasaurs have been discovered within MNRR. In the upstream portion of MNRR, the Corps of Discovery visited Old Baldy, a conical knob of the Cretaceous Pierre Shale. The knob was an important cultural feature noted by Lewis and Clark but is also where the expedition first captured and scientifically documented the prairie dog (genus Cynomys). Due to their scientific, cultural, historic, and scenic values, both Spirit Mound and Old Baldy are prominent geoheritage sites listed in the National Register of Historic Places.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6979

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