Approximately "fifty miles long and fifteen miles wide," Big Hole Prairie "is hemmed in by high mountains on every side except the southeast where Wisdom River passes out from it." Plate XLIX.
"This valley for the most part is wide and open." There are "spurs separating it from the Hell-Gate Valley on the south," and "separate on the north the various tributaries flowing into the Blackfoot River." Plate LXII.