In the northern hemisphere the north sides of river valleys are sometimes of more gentle slope than the south sides.  Can you suggest a reason?

The grand canyon of the Colorado river in Arizona.  The Colorado River trenches the high plateau of northern Arizona with a colossal canyon two hundred and eighteen miles long and more than a mile in greatest depth.  The rocks in which the canyon is cut are for the most part flat-lying, massive beds of limestones and sandstones, with some shales, beneath which in places harder crystalline rocks are disclosed.  Where the canyon is deepest its walls have been profoundly dissected.  Lateral ravines have widened into immense amphitheaters, leaving between them long ridges of mountain height, buttressed and rebuttressed with flanking spurs and carved into majestic architectural forms.  From the extremity of one of these promontories it is two miles or more across the gulf to the point of the one opposite, and the heads of the amphitheaters are thirteen miles apart.

The lower portion of the canyon is much narrower (Fig. 54) and its walls of dark crystalline rock sink steeply to the edge of the river, a swift, powerful stream a few hundred feet wide, turbid with reddish silt, by means of which it continually rasps its rocky bed as it hurries on.  The Colorado is still deepening its gorge.  In the Grand Canyon its gradient is seven and one half feet to the mile, but, as in all ungraded rivers, the descent is far from uniform.  Graded reaches in soft rock alternate with steeper declivities in hard rock, forming rapids such as, for example, a stretch of ten miles where the fall averages twenty-one feet to the mile.  Because of these dangerous rapids the few exploring parties who have traversed the Colorado canyon have done so at the hazard of their lives.

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.  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.

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.