His eyes took in the hollow, the edge of the lake, and the approach to the timber, and his nose gauged the wind as accurately as the pointing of a compass.

The reason he remained quiet was that he was almost on the danger-line.

In other words, the mountains and the sudden dip had formed a "split wind" in the hollow, and had Thor appeared fifty yards above where he now crouched, the keen-scented caribou would have got full wind of him.
With his little ears cocked forward and a new gleam of understanding in his eyes, Muskwa now looked upon his first lesson in game-stalking.

Crouched so low that he seemed to be travelling on his belly, Thor moved slowly and noiselessly toward the creek, the huge ruff just forward of his shoulders standing out like the stiffened spine of a dog's back.

Muskwa followed.