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Map of an Exploring Expedition to the Rocky Mountains . . . and to Oregon and North California . . . , hand-colored map by Brevet Capt. J.C. Frémont, 1845

John C. Frémont led five scientific expeditions into the West between 1842 and 1854; three of them were for the Army Corps of Engineers. This map, based on two expeditions, accompanied a report to the U.S. Senate. It accurately depicted large areas of the West, including locations of American Indian nations, but left blank the places unexplored by Frémont’s expeditions.

Geography and Map Division, Library of Congress

Map of an Exploring Expedition to the Rocky Mountains . . . and to Oregon and North California . . . , hand-colored map by Brevet Capt. J.C. Frémont, 1845

Mapping Western Territories

Before the Civil War, members of Congress grappled with the anticipated benefits of expansion into western territories. To inform legislative decisions that would affect the territories and their inhabitants, Congress relied on maps and reports of expeditions, such as those led by explorer John C. Frémont, a member of the Army Corps of Topographical Engineers. The subsequent expansion, however, ignited a passionate debate over the expansion of slavery, pitting the proslavery South against the antislavery North.