I was raised on the Snake River Plain that makes up the bulk of Southern Idaho.  The Snake River "snakes" through the plain rising in Western Wyoming and entering Idaho near Jackson Hole before flowing South and then West across the Magic Valley before turning Northwestward to the Oregon border.  Much of the canyon near where I was raised was carved out by the river and the Great Bonneville Deluge. The sides are perpendicular and it is difficult to enter from the top.  Further downriver the entrance is easier and provided a great wintering and encamping site for Native Tribes for centuries.