Where granite is exposed, rapids, or cataracts, form. Along the Nile, there are 10 cataracts in total. The First Cataract, 7 miles in length, is just to the south of Aswan today, and it logically formed the southern border of Egypt. A person traveling along the river by boat would have to leave the river at this point and carry the boat across the desert, since it was not passable on the rapids.
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