ll; but when we have descended six thousand feet of altitude to its
edge we find it a rushing turbulent torrent of muddy water. Its average
width is three hundred feet; its average depth thirty feet. It is
industriously digging the Grand Canyon still deeper, and perhaps as
rapidly as it ever dug since it entered the granite.

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 pe