Square Hills, 1200 Miles above St. Louis

“An hundred miles above [Belle Vue], I passed a curious feature, called the ‘Square Hills’ . . . On ascending them I found them to be two or three hundred feet high, and rising on their sides at an angle of 45 degrees; and on their tops, in some places, for half a mile in length, perfectly level, with a green turf, and corresponding exactly with the tabular hills spoken of above the Mandans.” (Catlin, Letters and Notes, vol. 2, no. 32, 1841; reprint 1973)