The American frontier comprises the geography, history, folklore, and cultural expression of life in the forward wave of American expansion that began with English colonial settlements in the early 17th century and ended with the admission of the last mainland territories as states in 1912. A "frontier" is a zone of contact at the edge of a line of settlement. The leading theorist Frederick Jackson Turner went deeper, arguing that the frontier was the defining process of American civilization: "The frontier," he asserted, "promoted the formation of a composite nationality for the American people. " He theorized it was a process of development: "This perennial rebirth, this fluidity of American life, this expansion westward. . . furnish[es] the forces dominating American character. " Turner's ideas since 1893 have inspired generations of historians (and critics) to explore multiple individual American frontiers, but the popular folk frontier concentrates on the conquest and settlement of Native American lands west of the Mississippi River, in what is now the Midwest, Texas, the Great Plains, the Rocky Mountains, the Southwest, and the West Coast.