What physical geographical feature in the Old Northwest served as an internal borderland?

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The Ohio River served as an internal borderland in the Old Northwest because it was a natural boundary between free and slave states. It separated the Northwest Territory, where slavery was prohibited by the Northwest Ordinance, from Kentucky, a slave state. The Ohio River also played a significant role in the Underground Railroad, as many escaped slaves crossed it to reach freedom in the North.

What Physical Geographical Feature in the Old Northwest Served as an Internal