Complex sentence: 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.
Split into simpler sentences: 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.