The Medicine Lake Highlands are a land of volcanic glass flows, pumice deposits, and lava tubes — impressive leftovers from eruptions of the Medicine Lake volcano. Yet, unlike the dramatic profiles of the stratovolcanoes that formed the peaks of Shasta and Lassen, the Medicine Lake volcano is a shield volcano with gently sloping sides. Its magma chamber emptied through side vents, leaving a huge dome on the landscape. On the east, the highlands are made up of 950-year-old lava flows. Highland glass flows include Grasshopper Flat, Little Glass Mountain, Glass Mountain, and Sugar Hill. Above the Modoc Plateau, the highlands described a natural boundary between Modoc and Achomawi lands. The Ilmawi band of Achomawi, who lived along the Pit River, traveled far to obtain the better-chipping obsidian found at Grasshopper Flat. Because both the Modoc and the Achomawi claimed the obsidian quarry at Sugar Hill, there were occasional conflicts. The ancient quarries remain evident today. Obsidian mined here was traded throughout Northern California.

Glass Mountain Round-trip Distance: 2 miles (more or less along informal paths). Location: Prehistoric obsidian quarry. Administration: Doublehead Ranger District, Modoc National Forest. Map: Modoc National Forest, Medicine Lake Highlands.

Trailhead
Exit I-5 at Hwy. 89 about 5 miles north of Dunsmuir, and go 29 miles east to Harris Springs Rd. (FS Rd. 15) near Bartle. Turn left and drive 4 miles to Powder Hill Rd. (FS Rd. 49). Make a right and go 29 miles northeast to Forest Service Rd. 97. Turn right and go 6 miles north to Forest Service Rd. 43N99. Turn left (north) and go to the southern edge of the Glass Mountain Geologic Area. Park along the road (no fee).

On the Trail
A Glass Mountain walk takes in the prehistoric obsidian mines along a glass flow, which, because of its rare geologic formation, has been designated a Special Interest Area in Modoc National Forest. Because dacite and rhyolite obsidian were emitted here simultaneously from the same vent — swirling but not mixing — they created a stronger, higher grade of glass. Quarried by Modoc, Atsugewi, Achomawi, and Wintu, the obsidian was traded to villages over 100 miles away. The Wintu used fire to break off the weathered surface layer of obsidian to expose the high-grade subsurface material. Since the quarrying activity has not been entirely obscured by weathering and vegetation growth, certain places on Glass Mountain remain as though they were only recently altered by Native miners. Pick your route along informal paths of gray dacite. Avoid walking on the slippery and sharp rhyolite glass. Wander as much of the 4,000-acre flow as you like before returning to your car.