By editor
Chouteau and Cascade Counties
The Highwood Mountains are an "island range," the eroded remnants of an Eocene volcanic complex. The magma that pushed upward here was highly enriched in potassium, forming a rare, dark, heavy igneous rock called shonkinite, composed primarily of augite and orthoclase feldspar with almost no silica.
Visitor Access
The core of the Highwoods is public Forest Service land, open and free for hiking and geology viewing. The roads are mostly gravel and close in the winter. Visitors can hike the drainages and examine the dark, heavy shonkinite outcrops, a testament to the unusual chemistry of the magma that blistered the crust here fifty million years ago.