The walls of a great canyon, and usually a small one, are terraced; seldom are they wholly vertical for their entire height, though occasionally they may approach this condition on one side or the other, and more rarely on both sides at once, depending on the geological formations of the locality. Owing to the immense height of the walls of such canyons as those on the Colorado, the cliffs frequently appear perpendicular when they are far from it, just as a mountain peak often seems to tower over one’s head when in reality it may be a considerable distance off. In the nature of the formation and development of canyons, they could not long retain continuous vertical walls. What Powell calls the “recession of cliffs” comes into play. The erosive and corrasive power of water being the chief land sculptors, it is evident that there will be a continual wearing down of the faces of the bounding cliffs. The softer beds will be cut away faster than the harder, and where these underlie the harder the latter will be undermined and fall. Every canyon is always widening at its top and sides, through the action of rain, frost, and wind, as well as deepening through the action of its flowing stream. _Erosion_ is this power which carves away the cliffs, and _corrasion_ the one which saws at the bottom, the latter term, in geological nomenclature, meaning the cutting power of running water.[3] This cutting power varies according to the declivity and the amount of sediment carried in suspension. It is plain that a stream having great declivity will be able to carry more sediment than one having little, and in a barren country would always be highly charged with sand, which would cut and scour the bed of the channel like a grindstone. As Dutton says, a river cuts, however, only its own width, the rest of a canyon being the “work of the forces of erosion, the wind, frost, and rain.” That is why we have canyons. The powers of erosion are far slower than those of corrasion, especially in an arid region, because they are intermittent. Where rocks take a polish, as in Marble Canyon, the scouring and polishing work of corrasion is seen in the shining bright surface as far as the water rises. This all belongs to the romance of the Water-gods, those marvellous land sculptors. [3] The introduction of this subject may seem unnecessary to the general reader, but no just comprehension of this river can be reached without some knowledge of the forces creating its chasms. The “Hole in the Wall,” near Ft. Defiance, Arizona. The “Hole in the Wall,” near Ft. Defiance, Arizona. This kind of sandstone has the peculiarity of weathering in this way, sometimes producing larger arches, alcoves, etc. Photograph by BEN WITTICK.