There exist some greyish walls; but most are red except in the granite gorges of the Grand Canyon, where, for a thousand feet, they are black.

Below the junction, forty miles, they came to the mouth of the San Juan! Yet Cataract Canyon and Narrow together, the first canyons of the Colorado proper, are fifty miles long and the San Juan comes in at least seventy-five miles below their end.

The walls of the San Juan he describes as being as high as those of the Colorado, which he has just been talking about, that is, five thousand feet, yet for these seventy-five miles he would have actually been passing between walls of about one thousand feet.

He says he could not escape here because the waters of the San Juan were so violent they filled its canyon from bank to bank.

In reality, he could have made his way out of the canyon (Glen Canyon) in a great many places in the long distance between the foot of Narrow Canyon and the San Juan.
There is nothing difficult about it.