Page:Popular Science Monthly Volume 54.djvu/73

Rh contains one of these fractures (due to weathering) and is not thick, some crevice is sure to open a path to the enemy, which is soon widened to a highway for the frost and rain, and a cascade in shower-time pours down, picking up sand as it goes to help in the attack. The weathering becomes more rapid, the arch opens up, and in time a natural bridge (Fig. 2) spans the air where once there was but solid stone. The process continuing, the bridge will disappear, a vacancy will take its place, and far off in the river bottom, or still farther in the sea, will rest the disintegrated material that once made part of the continuous cliff. Where the cliff is too thick to be perforated (Fig. 3), the arch breaks back into a deep cavern whose roof falls and falls till the blue sky takes its place. Thus has a natural bridge, like a flower, its birth, its growth, perfection, and decay. Wind erosion also plays a part, but the chief work is due to water.

Besides bridges there are numberless other forms. Who has not seen Castle or Pulpit Rocks, or Devil's Slides, or Palisades, etc.?



But it is in the West, perhaps, that the most remarkable rain carvings and wind carvings occur, and especially in that part called the Southwest, that "land as old as time is old," that strange, weird land of red rocks, of tall, long cliff lines like mountain ranges split asunder to span the desert in their nakedness; that land of labyrinthine canons, where the bloom of morning lingers to kiss the gloom of