the floor of the bridge, which is about above the stream, was examined by fanning and showed evidence of once having formed the bed of the stream. erosion had nearly obliterated this old bed, but had not been sufficient to destroy the sides of the valley. at one time, the river must have flowed at this higher elevation to a place several kilometers down stream from the present tunnel and then passed over a cliff. this cliff gradually receded upstream forming the present box canyon. as illustrated by the sketches, seepage began at a point several hundred meters above the falls and, owing to the horizontal bedding of the rock, it first took a path at right angles to the river course, then descended from bed to bed to the bottom of the falls. the porous, soft nature of the rock favored the rapid enlargement of the underground channel, and eventually the entire stream followed this course.