Geology Guide to Cougar Divide, Mount Baker

Copyright Dave Tucker. August, 2012

Geology after Hildreth and others (2003a). Map topography from Google Maps. Click to enlarge any image.

Renowned for its views of the north side of Mount Baker and eastward into the Cascades, Cougar Divide is also a place to observe volcanic geology significant to the eruptive chronology of the Mount Baker volcanic field. The rocks all predate the young Baker cone; one dike is the oldest dated volcanic rock in the entire volcanic field (Hildreth and others, 2003). The north-south ridge runs parallel to, and only a 1.5 miles east of, the oft-crowded Skyline Trail on the other side of Deadhorse Creek. The following description is an excerpt from a layperson-oriented geology guide I’m writing about Mount Baker. Source for most of the geology description is Hildreth and others, (2003); all italicized geologic units are taken from that essential paper. Note use of standard geology shorthand:  ‘ka’ and ‘Ma’ to refer to thousands and millions of years respectively. 105 ka = 105,000 years; 1.2 Ma = 1,200,000 years.

Weight % SiO2: basalt < 52; andesite 52-63; dacite 63-68; rhyodacite 68-72; rhyolite >72. No basalt is known from the area.

Mount Baker from Forest Road 33. Mazama Glacier lies at the head of Bar Creek (left). Copyright Dave Tucker.

Geologic significance of Cougar Divide: Rocks along the ridge are the remnant of a focus of eruptive activity that lasted for over a million years, from before the collapse of Kulshan caldera (1.15 Ma) until at least as recently as 105 ka. As such, volcanism persisted here for longer than at other large eruptive centers in the Cascades such as Mount Rainier or Mount Adams, and is far and away the longest-lived focus of activity in the Mount Baker region.  While most eruptive centers in the Baker volcanic field ‘specialized’ in andesite eruptions, the Chowder Ridge focus, as Hildreth and others (2003) refer to this area, erupted dacite and rhyodacite. The latter is on the divide between dacite and rhyolite, and is not found at Baker beyond the area of Kulshan caldera. Consequently eruptions fed by the dikes producing the lava flows were likely more explosive than the andesitic eruptions of Baker and the Buttes. The rhyodacites seen on this geology hike appear to be leakage from the magma that underlay the Kulshan caldera. Their presence here, several kilometers west of the caldera margin, suggest that the rhyodacite magma body was greater in extent than the collapse area of the caldera (4.5 x 8 km). A rock hammer and hand lens will be useful on this field trip.

The andesite of Deadhorse Creek is very vesicular, making it distinctive among the rocks on the ridge.

Getting there: Turn south off the Mount Baker Highway (Washington 542) at the Wells Creek/Nooksack Falls junction, (Forest Service Rd 33, gravel). This is 40.6 miles (65 km) from I-5, 26.5 miles (42.5 km) east of the junction with Highway 9, and 7.5 miles (12 km) east of Glacier. (See the Wells Creek volcanics field trip and the Pinus Lake lava flow trip for for nearby field trips on this website.) The road is gated just beyond the falls between November 1 and July 1, making for a mighty long walk to Cougar Divide (but a great ski!). Drive the gravel road for 12.2 miles (19.5 km) and go straight at a junction with a road branching right (4580’). At 12.8 miles (20.5 km) and after a couple of twists and turns, park at the unsigned and unofficial trailhead in a clearcut (4930’). N48.86771 W121.80630. As of this writing (August, 2012) the road is not brushy.

Some waypoints along the road are described. Distances are in miles from Mount Baker Highway; parenthetical mileage is the distance from the preceding waypoint. To convert miles to kilometers, multiply by 1.6.

2.0                 Road 3010 to Pinus Lake lava flow branches to the left at a sharp switchback to the right (2040’). The geologic guide for this recommended side trip follows the Cougar Divide description.

4.4 (2.2)      View upstream of Mount Baker’s north face. The snowy dome to the left (east) is Epley Portal at the head of Ptarmigan Ridge. There are many more fine views of Baker as the road climbs.

5.3 (0.9)      Cross Wells Creek, view upstream to 90-foot-tall Wells Creek Falls (2520’). Note that the road beyond this point is maintained by Citizens for Public Roads, a volunteer non-profit group.

6.1 (0.8)      Bridge over Bar Creek, draining Mazama Glacier. (2637’). This braided stream is the principal water course in the valley, rather than Wells Creek, and thus the main trunk stream draining north into the Nooksack at the foot of Nooksack Falls should be called Bar Creek, rather than Wells Creek. The road beyond the bridge is maintained by a group of citizen volunteers rather than the Forest Service. It is usually drivable all the way to the to the Cougar Divide trailhead, 6.7 miles beyond the Bar Creek bridge.

The road now climbs for nearly 4.3 miles across landslide debris fallen from lava flows high above on the valley wall. The road is rocky but not particularly steep. It is almost always drivable with an SUV but use caution in a low-clearance sedan. There are wonderful views to the north, east and south. The denuded slopes of Barometer Mountain (5763’; 1756 m) rise across Wells Creek. The mountain is underlain by marine sedimentary rocks of the Nooksack Formation. The slope burned in the 1960s and is kept largely treeless by avalanches in the winter. It has become prime mountain goat breeding grounds.

8.4 (2.3)      Stop for views across Bar Creek. A lava flow caps the 4000’ ridge east of the creek. This large flow, the 70 ka andesite of The Portals, descended 3 miles from a vent at 6800’ on the skyline of Landes Cleaver, the western extension of Ptarmigan Ridge. At least five andesite lavas have survived severe glacial erosion; the remnants comprise Landes Cleaver and are over 2000 feet thick above the Mazama Glacier, which certainly did not exist in its modern form at that time: Mount Baker did not exist for another 30,000 years or so.  The Portals andesite flows ponded against thick ice, as demonstrated if you can get up close enough to see the very small, chaotically-oriented ice-contact columns in the lava cliffs. What was the source of this ice? The ice was conceivably flowing off the Black Buttes, but the age of the Portals andesite also corresponds with the Double Bluff Glaciation, the second to last great ice advance out of the British Columbia interior. Glacial ice filled the Puget Lowland, and must have invaded valleys and covered ridges in the northern Cascades.

10.4 (4.3)    At the end of the long northerly switchback is a small outcrop of an andesite lava flow along the road to the left (4200’). This is the andesite of Dobbs Creek and is about 105 ka. It is one of several remnants of valley-filling lavas that erupted from a swarm of dikes below Hadley Peak in the headwaters of Dobbs Creek. The remnant of this lava flow is 200 feet thick and is the source of the landslide debris you have driven through since crossing Bar Creek. It was plastered against the side of Cougar Divide a half-million years after the ridge-capping dacite lava flow you’ll encounter on the trail.

12.2 (1.8)    Go straight at junction with a road branching right. (4580’.)

12.8 (0.6)    After a couple of twists and turns, park at the unsigned and unofficial trailhead in a clearcut at 4930’. 48° 52.063’N 121° 48.378’W.

The Cougar Divide Trail

Scott Linneman stops swatting skeeters long enough to model at some platy jointing in the dacite of Cougar Divide. Copyright Dave Tucker

Rating: moderately difficult; distance to end: 2.7 miles to a saddle at 5800 feet. Reach the crest of Chowder Ridge in an additional 0.85 miles and 1000′ vertical.

The unmaintained trail heads south across the clearcut and through a narrow belt of trees, then climbs in earnest through a meadow. The boot-beaten trail is rough and rutted from erosion. Two ash layers are prominent in the trail. The gray, sandy Mount Baker Layer BA, erupted from Sherman Crater 6500 years ago, is just below the root mat. It tends to wash out onto the trail surface. Below BA, Layer O from the Mount Mazama eruption and Crater Lake caldera collapse (7.2 ka) is the orangish layer in eroded trail ruts. Layer O is very fine-grained, reflecting sorting of grains over the 400 miles the ash cloud traveled to this point. The particles are too small to be felt when you rub a sample between your fingers. Contrast this with the much coarser BA, here only 6.5 miles from its source at Sherman Crater.

The view from the first crag, in the dacite of Cougar Divide. Epley Cleaver and the Portals are the glaciated ridge at left center. Copyright Dave Tucker

In one half mile, reach the ridge crest at a rock-lined tarn. The rock here is the dacite of Cougar Divide, 613 ka. Like all the other lavas preserved along Cougar Divide, this flow probably erupted from the Dobbs Creek dike swarm 2.5 miles or so to the south (see the map, above). The dacite lava now caps the ridge crest, although it probably flowed on a valley floor, an example of reversed topography. At a fork in the trail by a tarn, go left to enjoy the views from a rocky knob, then go back to the junction and follow the trail left (south). You’ll soon come to a steep drop off and a descent of 30 feet or so through the dacite cliff. The ridge crest is split by many narrow north-south depressions and gullies, running parallel to the ridge crest. The relatively weak, thin-bedded Nooksack Formation sedimentary rock, the bedrock below all the Cougar Divide lavas, spreads outward under the mass of the volcanic rocks, and erosion in the Wells and Deadhorse valleys to either side of the ridge removed much rock from the valley walls that would otherwise buttress the divide. Consequently, fissures parallel to the ridge crest have developed. Watch for more of these gravitational spreading features all along the trail.

Many long swales run parallel to the ridge crest, resulting from gravitational outward spreading of the overburdened weak Nooksack rocks beneath the lava flows. Photo copyright Dave Tucker

The trail stays mostly on the crest of Cougar Divide. The tread may divert around newly fallen logs, so stay alert for changes in direction. About 1/2 mile beyond the junction by the pond, is a short but steep descent in the forest, The rock on that descent is the rhyodacite of Cougar Divide. This is probably the remainder of a small lava dome, dated to 1.015 Ma. You may see the holes left from seven cores taken from this rock for a paleomagnetic study (Hildreth and others, 2004). Shortly beyond this descent,  pass another small pond on the left in a saddle.

The andesite of Deadhorse Creek, going back down. The exposure is small, but easy to spot on the way in. Copyright Dave Tucker.

After the pond, the trail climbs again, leveling out at a small, outcrop of andesite lava blocks that is very vesicular (andesite of Deadhorse Creek, 192 ± 8 ka). Source of this isolated lava scrap is uncertain, but most likely it is from one of the Dobbs Creek swarm. Beyond here for quite a way, the only exposed rocks are thin, tilted beds of Nooksack Formation shale, easily eroded and topographically subdued. It is a good idea to examine hammer-broken bits of this rock, so you can tell it apart from the next geologic point of interest. A fresh surface is dark gray and very fine grained, no grains evident even with a hand lens.

Scott Linneman marks the mid-point of the rhyodacite dike running across the ridge crest. The camera is at the north end. This subtle exposure is the oldest dated rock in the Mount Baker volcanic field. Copyright Dave Tucker.

Now the trail follows the ridgecrest, mostly. There are two relentlessly steep but mercifully short stretches before you reach meadow glory with expansive views in all directions. Keep going along the open ridge crest if you want to reach a significant site in the Mount Baker volcanic field.  At the top of another short but steep grade, just past a snow patch that typically lasts through the year (but in hot or dry years is a large pond), the trail again reaches the ridge crest in a small clump of stunted trees (48° 50.415′ N 121° 49.613’W). The unremarkable-looking rock crossed by the trail at this point is significant.  Use your hammer to get a fresh sample and examine it under your hand lens. This is a rhyodacite dike containing dark hornblende and biotite crystals that intruded the Nooksack rocks, and is the oldest measured rock in the entire Mount Baker volcanic field, 1.293 million years old (rhyodacite dike of Cougar Divide). It predates Kulshan caldera by about 100,000 years, but is compositionally very similar to the thick tuff that filled the caldera. The reservoir of magma below the caldera may have leaked to the surface via this dike. Only this 100-foot-wide dike remains from what may have been a sizable eruption of magma onto the now-eroded surface. If you are unsure that you have found the dike because you are unfamiliar with such rocks, continue along the trail,  Once beyond the dike, all the rock you’ll find until the big saddle is Nooksack, which looks really different from the dike. Again, use your hammer and lens to learn the help recognize the dike rock. Backtrack to the area of the dike, or stop on your return so you can be sure to find it.

This is a closeup of a sample from the rhyodfacite dike of Cougar Divide. The circled minerals are 1 mm across- the black circle marks a hexagonal biotite grain, the red circle marks an elongated amphibole (hornblende).

This is a good lesson in how detailed geologic maps are made. When Wes Hildreth and his assistants walked over hundreds of miles of terrain in the Baker region, they examined every rock exposure they found. Since Hildreth’s work for USGS was focusing on the volcanics in the area, all igneous rocks were collected, numbered, described, located on a map, and sampled for thin sections, geochemistry and potential age determination. Only by careful examination of every outcrop were obscure, isolated rock units like this dike discovered, and their significance eventually determined.

Fairly typical Nooksack Formation along Cougar Divide. Note the close-set metamorphic “slaty foliation” that splits the rock into thin, fraible ‘flakes’.

Acres of flowers and meadows on the crest of Cougar Divide. Did I say I like this hike? The snow fingers mark the Dobbs Creek headwaters, site of many dikes. Chowder Ridge and Hadley Peak at the head of Hadley Glacier, and Mount Baker beyond. Copyright Dave Tucker.

Continue south along the ridge-top meadow. From the dike to the saddle at the head of Cougar Divide are outcrops of Nooksack Formation argillite. Scan the basin to the south, at the head of Dobbs Creek, for pale tan dikes cutting the dark Nooksack rock. There are many of them, but you’ll need to look carefully. About a dozen dikes (undated) ranging from rhyodacite to andesite have been mapped in the basin. About the same number are on the far side of the skyline ridge. These are probably the feeders for the lavas along Cougar Divide.

Pale dikes intrude Nooksack Fm. at the head of Dobbs Creek. Click to see enlarged view.

From the crest of Chowder Ridge, Baker looms above all. In the middle distance, the glaciated pointed peak is The Bastile, remnant of an andesite volcano (322 ka).

You can continue beyond the saddle, climbing upward through a shallow intrusion, the rhyodacite of Chowder Ridge, dated at 1.018 Ma. This shallow body intruded the Nooksack rocks, and is shot through with younger dikes. It is a 1000′ elevation gain to reach the crest of Chowder Ridge (~6800′), and you may need to cross a steep snow patch to get there. Glorious views stretch in all directions. This is Nooksack Formation argillite, and the plentiful marine fossils give the ridge its clever name.

 

 

 

REFERENCES

Hildreth, W., Fierstein, J. and Lanphere, M., 2003, Eruptive history and geochronology of the Mount Baker volcanic field, Washington: Geological Society of America Bulletin, v. 115, p. 729-764.

Hildreth, W., Lanphere, M., Champion, D.E. and Fierstein, J., 2004, Rhyodacites of Kulshan caldera, North Cascades of Washington: postcaldera lavas that span the Jaramillo: Journal of Volcanology and Geothermal Research, v. 130, p. 227-264.