The Lone Star Ranger by Zane Grey

Two days later, about the middle of the forenoon, Duane dragged the two horses up the last ascent of an exceedingly rough trail and found himself on top of the Rim Rock, with a beautiful green valley at his feet, the yellow, sluggish Rio Grande shining in the sun, and the great, wild, mountainous barren of Mexico stretching to the south.

Duane had not fallen in with any travelers. He had taken the likeliest-looking trail he had come across. Where it had led him he had not the slightest idea, except that here was the river, and probably the inclosed valley was the retreat of some famous outlaw.