It may fall to the very bottom most of the day, or only for an hour or two, depending on the trend of the canyon with reference to the sun's course.

At the bottom of the Kanab where it joins the Grand, the sunlight 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.