ht in November
remains in the bottom just two hours, but outside in the main gorge the
time is very much longer.

The walls of a great canyon, and usually a small one, are terraced;
seldom are they wholly vertical for their entire height, though
occasionally they may approach this condition on one side or the other,
and more rarely on both sides at once, depending on the geological
formations of the locality. Owing to the immense height of the walls
of such canyons as those on the Colorado, the cliffs frequently appear
perpendicular when they are far from it, just as a mountain peak often
seems to tower over one's head when in reality it may be a considerable
distance off. In the nature of the formation and development of canyons,
they could not long retain continuous vertical walls. What Powell calls
the "recession of cliffs" comes into play. The erosive and corrasive
power of water being the chief land sculptors, it is evident that there
will be a continual wearing down of the faces of the bounding cliffs.
The softer beds will be cut away faster than the harder, and where these
underlie the harder the latter will be undermined and fall. Every canyon
is always widening at its top and sides, through the action of rain,
frost, and wind, as well as deepening through the action of its
flowing stream. EROSION is this power which carves away the cliffs,
and CORRASION the one which saws at the bottom, the latter term, in
geological nomenclature, meaning the cutting power of running water.*
This cutting power varies according to the declivity and the amount of
sediment carried in suspension. It is plain that a stream having great
declivity will be able to carry more sediment than one having little,
and in a barren country would always be highly charged with sand, which
would cut and scour the bed of the channel like a grindstone. As Dutton
says, a river cuts, however, only its own width, the rest of a canyon
being the "work of the forces of erosion, the wind, frost, and rain."
That is why we have canyons. The powers of erosion are far slower than
those of corrasion, especially in an arid region, because they are
intermittent. Where rocks take a polish, as in Marble Canyon, the
scouring and polishing work of corrasion is seen in the shining bright
surface as far as the water rises. This all belongs to the romance of
the Water-gods, those marvellous land sculptors.

* The introduction of this subject may seem unnecessary