The canyon has been shaped by several agencies. Its depth is due to the river which has sawed its way far toward the base of a lofty rising plateau. Acting alone this would have produced a slitlike gorge little wider than the breadth of the stream. The impressive width of the canyon and the magnificent architectural masses which fill it are owing to two causes. Running water has gulched the walls and weathering has everywhere attacked and driven them back. The horizontal harder beds stand out in long lines of vertical cliffs, often hundreds of feet in height, at whose feet talus slopes conceal the outcrop of the weaker strata (Fig. 15). As the upper cliffs have been sapped and driven back by the weather, broad platforms are left at their bases and the sides of the canyon descend to the river by gigantic steps. Far up and down the canyon the eye traces these horizontal layers, like the flutings of an elaborate molding, distinguishing each by its contour as well as by its color and thickness. [Illustration: Fig. 55. Diagrams illustrating Conditions which produce Falls or Rapids _A_, vertical succession of harder and softer rocks; _B_, horizontal succession of the same. In _A_ the stream _ab_ in sinking its bed through a mass of strata of different degrees of hardness has discovered the weak layer _s_ beneath the hard layer _h_. It rapidly cuts its way in _s_, while in _A_ its work is delayed. Thus the profile _afb´_ is soon reached, with falls at _f_. In _B_ the initial profile is shown by dotted line.] The Grand Canyon of the Colorado is often and rightly cited as an example of the stupendous erosion which may be accomplished by a river. And yet the Colorado is a young stream and its work is no more than well begun. It has not yet wholly reached grade, and the great task of the river and its tributaries--the task of leveling the lofty plateau to a low plain and of transporting it grain by grain to the sea--still lies almost entirely in the future. [Illustration: Fig. 56. Longitudinal Section of Yellowstone River at Lower Fall, _F_, and Upper Fall, _F´_, Yellowstone National Park _la_, lava deeply decayed through action of thermal waters; _m_ and _m´_, masses of decayed lavas to whose hardness the falls are due. Which fall will be worn away the sooner? How far upstream will each fall migrate? Draw profile of the river when one fall has disappeared] [Illustration: Fig. 57. Diagram illustrating Migration of a Fall due to a Hard Layer _H_, in the Midst of Soft Layers _S_ and _S_, all dipping upstream