Developing the thought in greater detail, let us glance at the illustrations of this chapter and at any photographs which may be at hand, and realization will begin. Let imagination dart back a million years or more to the time when this foreground rim and that far run across the vast chasm are one continuous plain; perhaps it is a pine forest, with the river, no greater than to-day, perhaps not so great, winding through it close to the surface level. As the river cuts downward, the spring floods following the winter snows cave in its banks here and there, forming sharply slanted valleys which enclose promontories between them. Spring succeeds spring, and these side valleys deepen and eat backward while the promontories lengthen and grow. The harder strata resist the disintegration of alternate heat and cold, and, while always receding, hold their form as cliffs; the softer strata between the cliffs crumbles and the waste of spring waters spreads them out in long flattened slopes. The centuries pass. The ruin buries itself deep in the soft sandstone. The side valleys work miles back into the pine forest. Each valley acquires its own system of erosion; into each, from either side, enter smaller valleys which themselves are eating backward into the promontories. The great valley of the Colorado now has broad converging cliff-broken sides. Here and there these indentations meet far in the background behind the promontories, isolating island-like mesas. The rest of the story is simple repetition. Imagine enough thousands of centuries and you will imagine the Grand Canyon. Those myriad temples and castles and barbaric shrines are all that the rains and melting snows have left of noble mesas, some of which, when originally isolated, enclosed, as the marble encloses the future statue, scores of the lesser but mighty structures which compose the wonder city of the depths. These architectural operations of Nature may be seen to-day in midway stages. Find on the map the Powell Plateau in the northwest of the canyon. Once it was continuous with the rim, a noble promontory. It was cut out from the rim perhaps within the existence of the human race. A few hundred thousand years from now it will be one or more Aladdin palaces. Find on the map the great Walhalla Plateau in the east of the canyon. Note that its base is nearly separated from the parental rim; a thousand centuries or so and its isolation will be complete. Not long after that, as geologists reckon length of time, it will divide into two plateaus; it is easy to pick the place of division. The tourist of a million years hence will see, where now it stands, a hundred glowing castles. Let us look again at our photographs, which now we can see with understanding. To realize the spectacle of the canyon, let imagination paint these strata their brilliant colors. It will not be difficult; but here again we must understand.