High Rock Lake, for which the wilderness was named, was created about 11,800 years ago after a large rockslide closed the outlet (Box Canyon) to High Rock and Little High Rock Canyons.

One special geological feature in Fly Canyon is the potholes.

The potholes were carved by whirlpool action of sand and gravel in the stream.

Another unique feature is the Fly Slide where emigrants lowered their wagons with ropes into Fly Canyon portion of the Applegate-Lassen Trail.

Elevations in the wilderness range from 4,000 to 7,660 ft (1,220 to 2,330 m) with scattered vegetation of saltbrush and sagebrush.