Boulder Cave formed over millions of years from ongoing volcanic action and erosion. Periodic lava flows deposited rock over soft, loose rock and soil. The lava cooled, forming hard layers of basalt and trapping the softer layers of loose sediment between. Wind and water then scoured the softer layers out. Today the cave is home to a dwindling population of Pacific western big-eared bats (a.k.a. Townsend bats), a listed sensitive species in both Washington and Oregon. Only fifty remain of thousands that inhabited the cave in the 1920s and '30s, hence the cave's winter closure to ensure a safe hibernation period. Do bring a reliable light source for exploring the cave.

The Boulder Cave Trail was built by the Civilian Conservation Corps (CCC) in 1935 and was improved in 1987 by the Youth Conservation Corps (YCC), with the voluntary help of a local stone mason. The gravel trail follows the rim of a small gorge that is part of a deep ravine. The path climbs gradually away from the parking area, piercing a pine and fir forest that also supports some big, old maples and alders.

As the wide trail nears the cave mouth, it winds down to the slit in the rock that serves as the cave entrance. Hikers MUST have a reliable light source to explore the cave--a bright headlamp is recommended, though a powerful hand-held flashlight would be adequate. A backup light should also be carried just in case your primary light goes dead. You'll also want a jacket to slip on, even in the heat of August. The cave is damp and cool, with temperatures typically hovering in the 50s.

Explore the shallow cave (about 400 feet deep) at your leisure before heading back out the way you came.