gorges only wide enough at the base to accommodate the stream. The
steepness of the valley slopes depends on the relative rates at
which the bed is cut down by the stream and the sides are worn
back by the weather. In resistant rock a swift, well-laden stream
may saw out a gorge whose sides are nearly or even quite vertical,
but as a rule young valleys whose streams have not yet reached
grade are V-shaped; their sides flare at the top because here the
rocks have longest been opened up to the action of the weather.
Some of the deepest canyons may be found where a rising land mass,
either mountain range or plateau, has long maintained by its
continued uplift the rivers of the region above grade.

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.