Sanderson saw the outlines of animal and rider as they appeared for an instant, partly screened from him by the trees and undergrowth on the arroyo edge.

Then horse and rider vanished, going northward, away from the arroyo, silently, swiftly.
Schooled to caution by his long experience in a section of country where violence and sudden death were not even noteworthy incidents of life, and where a man's safety depended entirely upon his own vigilance and wisdom, Sanderson got up carefully, making no noise, slipped around the thicket of alder, crouched behind a convenient rock, huge and jagged, and waited.
Perhaps the incident was closed.

The rider might be innocent of any evil intentions; he might by this time be riding straight away from the arroyo.

That was for Sanderson to determine.
The rider of the horse--a black one--had seemed to be riding stealthily, leaning forward over the black horse's mane as though desirous of concealing his movements as much as possible.

From whom?
It had seemed that he feared Sanderson would see him; that he had misjudged his distance from the gully--thinking he was far enough away to escape observation, and yet not quite certain, crouching in the saddle to be on the safe side in case he was nearer than he had thought.
Sanderson waited--for only a few minutes actually, but the time seemed longer.