Overview
Twin Sisters is not a trail for casual hikers. The 4,700-foot elevation gain crammed into 4.2 miles of climbing is among the most demanding one-day efforts in Washington State, and the dunite geology of the summit creates conditions unlike any other peak in the region. The rocks are orange-tinged, slippery when wet, and loose in places near the ridge. But for hikers who want a true summit experience with minimal traffic and views that span from the Cascades to the saltwater lowlands, Twin Sisters delivers.
The massif sits south of Mt. Baker in Mt. Baker-Snoqualmie National Forest and is actually two distinct summits: North Twin (the higher at 6,589 feet) and South Twin (6,454 feet). Day hikers typically aim for the ridge between them or for North Twin depending on conditions. Both require the same approach trail.
The dunite geology deserves some attention. Dunite is an ultramafic rock formed in the Earth's upper mantle, and it almost never appears at the surface. The Twin Sisters exposure is one of the largest in the world. The rock is heavily fractured near the summit and weathers to yellow-orange talus fields that give the peak its distinctive appearance from miles away.
The Route
Miles 0 to 2.0: Skookum Creek Forest
The trail begins near Skookum Creek at approximately 1,900 feet and immediately starts climbing. There is no gentle warm-up here. The first two miles gain roughly 2,000 feet through a dense Pacific Northwest forest of Douglas fir, western red cedar, and alder. The trail is well-worn but steep, with some sections using roots and rocks as natural steps.
Seasonal stream crossings occur in this section; these are easy by July but can be significant earlier in the season. The forest is beautiful in its own right, offering some of the most lush old-growth hiking in the area.
Miles 2.0 to 3.5: Subalpine and Upper Forest
The forest thins at around 4,000 feet as the trail enters the silver fir and mountain hemlock zone. The climbing continues at an unrelenting grade. Views begin appearing to the west through breaks in the canopy, showing the Nooksack drainage and the Cascade foothills. The trail gains approximately 2,000 more feet in this section.
The transition to open terrain comes suddenly at around 5,500 feet, where the trail emerges from the last trees onto the open dunite slopes. The orange-yellow rock is immediately visible here, and the landscape feels abruptly different from the Pacific Northwest forests below.
Miles 3.5 to 4.2: Summit Ridge Scramble
The final 0.7 miles crosses open dunite talus and scree, following cairns to the summit ridge between North and South Twin. The route involves Class 2 scrambling: using hands for balance on steeper sections, stepping carefully on loose rock. No technical gear is needed, but solid footwear is essential.
From the ridge, Mt. Baker fills the northern sky. The Cascade Range spreads in both directions. On the clearest days, Puget Sound and the Olympic Peninsula are visible to the west. The views justify the climb.
When to Visit
July through September is the realistic window. Snow covers the upper dunite slopes well into July in average years. July attempts may require traction devices and snow travel experience. August offers the most reliable snow-free conditions. September is quieter and often clearer, though early snow is possible at any time after the first of the month.
This trail dries out earlier than some nearby peaks because the open dunite slopes above 5,500 feet lose snow quickly once summer arrives. The lower forest section can be muddy through June.
What to Bring
- Many hikers carry 3 liters of water on this demanding climb. Water from lower streams should be treated. Above treeline, water is essentially absent. Individual hydration needs vary significantly with effort level.
- High-ankle hiking boots with rigid soles. The dunite rock is uneven, loose, and hard on ankles.
- Trekking poles are very useful on the steep descent.
- Traction devices for early season travel.
- Food for a full high-effort day.
- Wind and rain protection. The summit ridge is exposed.
- An offline map or GPS. Cell service is absent throughout.
Practical Details
A Northwest Forest Pass is required at the trailhead (as of 2026). No restrooms at the trailhead. Follow Leave No Trace principles for waste management; see the Leave No Trace guide for details on wilderness waste practices.
Dogs are allowed but must be leashed. The rocky summit terrain is hard on dog paws; consider whether your dog's paws are suited for the dunite surface. Group limits apply within any wilderness zones along the route.
For multi-day trips, review bear canister requirements. For information on free passes for veterans, see veteran benefits for national forests.
Getting There
From Highway 542 near Glacier, turn south on Mosquito Lake Road and connect to Skookum Creek Road (Forest Road 12). The trailhead is approximately 20 miles from Glacier via this route. FR 12 is rough and high-clearance vehicles are strongly recommended.
From Bellingham, the drive is approximately 65-70 miles total. Check current forest road status through the Mt. Baker Ranger District before attempting this approach. Road conditions can change significantly with weather. Download offline maps and GPS coordinates before leaving cell coverage. Check current conditions for trail snow reports specific to the Twin Sisters area.