Geology Guide to Hidden Lakes Trail, North Cascades, Washington

By Dave Tucker  October 5, 2015

The trail up to the Hidden Lakes Lookout is among the premier hikes in the northern Cascades. The restored fire lookout at the end of the trail is perched on a narrow fin of rock with great vistas into the glaciated heart of North Cascades National Park: Hidden Lake nestles just below in a granitic cirque; the ice-mantled peaks of the Eldorado-Forbidden-Johannesburg group dominate to the east. And much more beyond. Geology along the trail is beautiful and varied- metamorphic schist of the basement, 75-million-year-old intruding granitic rock rich in crystals, younger dark andesitic dikes that invaded the pale granodiorite, and overlying  volcanic ash deposits sandwiched between buried soil layers. Go for the view, sure, and the exercise [plenty] but keep your eye on the rock, too. The trail is 9 miles round trip, and gains 2900′ elevation. A great full day outing. It is very popular. If you want to sleep over in the lookout, it is first-come, first-served and you’ll need an early start on the weekend, or maybe on pretty much any other summer day. If you don’t get to the lookout early enough to claim the bed, it is a long way to the next suitable sleeping spot; unless you have excellent negotiation skills. Jagged boulders surround the building, and camp spots at the lake are a ways below you. So you might end up with a bivouac among the rocks.

Getting there:

Click to enlarge map and any other images.

Go to Marblemount on Highway 20. At the east end of ‘downtown’ turn up the Cascade River Road. Drive 10.2 miles and go left onto USFS Rd #1540 up Sibley Creek (signed “Hidden Lake Road”. Drive 4.5 miles to the small trailhead (3680′). Park smart and leave room for people to turn around. The road is moderately steep, can be pretty rough, and is prone to washouts. It was in pretty good shape when I drove it in June 2015 [Subaru Forester] and there were some much less rugged cars at the lot. The North Cascades National Park website has a detailed trail description– but no geology. The Washington Trails Association (have you joined yet?) has a trail description and a history of the lookout; again, no geo. Guess that is my job.

Geologic maps: The area lies on the margins of two USGS geologic maps. Most of it is on the Geologic Map of the Mount Baker 30- x 60-Minute Quadrangle. The lookout at the end of the trail is on the Geologic Map of the Sauk River 30- x 60-Minute Quadrangle. These maps are made for the geologist. They come with a technical pamphlet that describes the many rock units- don’t be afraid to try them out. Your proficiency in geology may be tested, but it is useful information. The Mount Baker quadrangle would suffice for all the rocks on this trail, except that the description of the Hidden Lake stock (the granodiorite that underlies the peaks) is described in much more detail in the Sauk River pamphlet, page 16.The trail is shown on the Eldorado Peak and Sonny Boy Lakes 7.5 minute topographic maps.

Hiker guide to the geology of the Hidden Lakes Trail

Foliation has aligned minerals in the schist to give the rock a layered look.

The first part of the trail ascends through forest and then ‘weeds’ (nice flowers in season though). Not much geo other than views to the north of rocky crags rising along the forested ridge crest. These consist of silvery green, mica-rich Napeequa schist with a distinct slabby character. You’ll find a few fallen slabs along the trail. The slabs are very well foliated- meaning that the nearly invisible minerals have been oriented parallel to each other to give the rock a sheeted appearance.This is not original sedimentary bedding, but the result of pressure caused by burial during accretion of these rocks to the margin of North America. Biotite mica is the most prevalent non-microscopic mineral (use your ever-handy hand lens, right?).

The pale pencil-shapes are stretched pebbles.

The good stuff begins after you finish the switchback ascent through the bushy meadow and cross the drainage, heading south on a long traverse at about 5500′. First off, just a bit beyond the drainage, watch for a large slab of gray schist on your right with elongated pale domains. This rock is around 220-250 million years old and was once a seafloor sandstone with pebbles. The pebbles were strrrrrrrrrrrrrrrrrrrrretched way out during directed pressure from the force of the collision of these rocks in the subduction zone along the ancient North American coast. Now they look like slender white pencil leads.

Rather suddenly you notice that all the rock along the

The contact between the schist (left) and the granodiorite is in this saddle above the trail.

trail is now pale granodiorite. This is a coarsely crystalline granitic intrusive rock some 73-75 million years old, called the Hidden Lake stock. A stock is a small body of intrusive rock, with an exposed area of only a few square km; larger bodies are called plutons, or batholiths if really big. This rock intrudes the Napeequa schist to the north. The contact is not exposed on the trail, but is pretty obvious on the skyline to the east- once you notice the change in rocks, look up to the ridge crest and note a prominent saddle some 600′ above you. Rocks on the south (to the right) of the saddle are the whitish Hidden Lake stock. Across the saddle is darker schist.

Granodiorite. Dark mineral crystals are biotite. The white crystals are mostly plagioclase feldspar

The rest of the trail is in the granodiorite. The prominent dark minerals are biotite mica; there is also white and clear plagioclase feldspar, and smoky quartz. There is also some white, squarish potassium feldspar.

In a few places the trail cuts reveal layers of volcanic pale ash between dark soil layers. A particularly good one is at about N48° 30.470′  W121° 12.182’I don’t know which ash came from which volcano, except for the thickest, very fine-grained orangish layer at the bottom of the sequence. This is ‘Layer O’, erupted from during the cataclysmic collapse at Crater Lake caldera in southern Oregon, and 7200 years old. It is found virtually everywhere and is pretty distinctive. All the other ash layers are younger. The 6800 year old Mount Baker BA ash is 3 mm thick to the east of this place at Cascade Pass, and it is possible that one of the lowermost is BA. The ca. 5000-year-old Dusty Peak ash from Glacier Peak is likely present as are two young St. Helens layers: Y 4,000 and 3,000 years ago; Layer W is very young, erupted in 1480. However there appear to be more than 4 thin layers above Layer O, so other Glacier Peak, Mount Rainier or MSH ash layers are probably represented. How would we know? Well, samples of each layer would need to be collected for geochemical ‘fingerprinting’ in the hopes that tiny glass fragments in the ash would match known chemistry of ash layers. Bracketing ages from soil layers wold also be very useful. Volcanic ash layers in the northern Cascades are not well-studied, and there may well be unknown Glacier Peak layers especially.

Mazama ash, ‘Layer O’ is the orange unit at the bottom of this section of volcanic ash.

The best of the dike exposures is just west of the saddle between the two Hidden Lakes Peaks

The trail gains a crest and turns south toward the snaggle-toothed summit of the lower Hidden Lake Peak. You may catch a glimpse of the lookout up there, but don’t stop looking at the rock along the trail. Begin to notice gray dikes intruding the granodiorite. To my knowledge, these have not been dated or studied for composition. They are obviously younger than the Hidden Lake stock, but that’s about all I can tell you. Some of these dikes are only a few centimeters wide; others are a couple meters. They mostly trend north-south, and the trail runs along one or more of the dikes as it follows along the east side of a low north-south ridge. This ridge ends in the basin below the summit (about 6400′)

View east from the saddle. Hidden Lake below in its granodiorite-lined cirque. Peaks: near left: north Hidden Lakes Pk; the dark tower of Eldorado is just behind. Above the lake is the dark pyramid of Forbidden Peak; to the right, the Boston-Sahale group above the deep hole of the upper Cascade River.

Hand samples from a dark andesite (?) dike and granodiorite of the salt-and-pepper Hidden Lakes stock.

Now head east to the 6560′ saddle between the two peaks, about 0.1 mile north of the lookout. If there is snow the trail will be buried. Hopefully there will be tracks in the snow and hopefully those folks will know where they were going. It is only a couple hundred yards to the saddle. Once there, turn right and hike up an exposed trail traversing the steep east face through the granodiorite to the summit. Along the way pass some interesting features. In some places the granodiorite is pocked with shallow pits a few inches across, but I don’t know why the rock weathers in this manner. Look sharp in a few other places to bits of other rock enclosed in the granodiorite. These must be blocks of basement rock that the Hidden Lakes intrusion invaded, which fell into the magma as it ascended.

Weathering pockets in Hidden Lakes stock granodiorite below the lookout. That’s my pal Scott Linneman puzzling over this feature.

The trail ends at the popular lookout (6890′), perched precariously amid big blocks. Enjoy the view!

The geology is described in Tabor and Haugerud’s

Blocks of wall rock caught in the granodiorite. The bruised finger nail has healed, thank you.

wonderful Geology of the North Cascades. However, they make an important error. From the lookout, they direct the geophile to “descend with ease along the ridge to the north” to observe the contact between the granitic rock of the Hidden Lake Peaks with brown schist. But they fail to mention that you first have to hike 500 vertical feet cross-country up to the summit of the northern Hidden Lakes Peaks (7080′) from the saddle before making that descent; the trail to the lookout won’t get you there. I haven’t made that ascent and descent to the contact, though it sounds like a great geo-trip!