Several hundred thousand square miles of the Great Plains have fewer than 6 persons per square mile-the population density standard Frederick Jackson Turner used in his Frontier Thesis to declare the American Frontier "closed" in 1893.
Several hundred thousand square miles of the Great Plains have fewer than 6 persons per square mile. This was the population density standard of settlement which historian Frederick Jackson Turner used in his "Frontier Thesis" to declare the American Frontier "closed" in 1893.