By Dave Tucker and Doug McKeever February 25, 2010
The Pinus Lake lava flow forms a prominent north-facing cliff overlooking the Mount Baker Highway (State Route 542) just east of Nooksack Falls. The cliff features spectacularly long cooling columns. A talus slope at its base rises high above the Nooksack River. This trip goes to the brink of the vertical cliff, examining the lava in a few exposures along the way. From the top, you look down into the Nooksack River valley to see ant-like automobiles passing up and down the highway, nearly 600 feet (180+ meters) below you. The andesite lava flow is named for skinny kidney-bean shaped Pinus Lake, a tiny puddle on top of the plateau, well hidden within the thick fuzz of post-clearcut trees. We made this trip on our mountain bikes on a gorgeous Sunday afternoon, February 21, 2010.
Geology of the Pinus Lake lava
The lava forms a 1-km-wide isolated plateau set against the north flank of Barometer Mountain. It is a single lava flow, around 300-feet-thick (90 m), and is an ‘orphan lava’; erosion has disconnected it from its eruptive source, removing any tell-tale lava flow trail back to wherever that vent was.
The first paper to discuss this lava (Easterbrook, 1975; references listed below) averaged two analyses to give a K-Ar age of 400 ± 200 ka (‘kilo anna = thousand years); the definitive paper on Baker’s entire eruptive history (Hildreth and others, 2003), however, obtained a K-Ar age of 202 ± 9 ka. The more precise age obtained by Hildreth is likely more accurate, given advances in potassium-argon dating methods in the years between those two papers. The lava, therefore, is somewhat younger than the youngest date obtained from rocks erupted at the big volcano that preceded Mount Baker, the Black Buttes volcano, but is still much older than the young Mount Baker, which itself began to grow around 43 thousand years ago (Hildreth and others, 2003).
The Pinus Lake lava is andesite. Three samples analyzed in Hildreth and others (2003) had a range of silica concentrations: from 59.37, 60.29, and 62.17 weight percent SiO2. This range is somewhat unusual for a single lava flow (samples were collected within 100 m of each other), and is reflected in an unusual assemblage of minerals. There is the usual plagioclase feldspar, but with a variety of compositional subgroups, as well as the two pyroxenes found in most Baker-area lavas, clinopyroxene (very dark green to black) and orthopyroxene (beer-bottle brown). More unusual is the presence of both olivine (glass green) and hornblende (long black needles). The latter two are not commonly found in the same flow. It seems that wherever this lava flow came form, some different magmas were mixing; this may have been what drove the eruption of this lava.
Where was the eruption vent for the Pinus Lake lava?
The lava exposed in the high cliff above the highway is a single thick flow, with those remarkable long columns running right up the cliff face. Where did it come from? It does not extend up the nearby valley of Wells Creek. It might conceivably have come down Anderson Creek, 5 km (3 mi)to the east, then flowed down the Nooksack to at least this point (or farther). Wes Hildreth proposes a vent for this lava at the ‘Ochotona intrusive complex’, at the lower southwest corner of Table Mountain, 10 km up Wells Creek to the SE of the Pinus lava. This is a complex of andesite and dacite dikes, with compositions of the various rocks there not very different from that of the Pinus lava. The Ochotona complex has a single K-Ar date, with an age of 189 ± 11 ka, also not too different from the Pinus lava. Read about the interesting and nearly unknown Ochotona complex on page 745 of Hildreth and others (2003).
Emplacement of the Pinus Lake lava flow
The lava had to arrive at this point by either descending Anderson Creek, to the east, or by flowing northwest down Wells Creek to the North Fork. Wes Hildreth’s mapping shows that the base of the lava is perched 120 m (400 feet) above the Nooksack Valley. The elevation of the base of the flow tells us this was the elevation of the floor of the Nooksack at the time of emplacement, and that the river has cut downward 120 meters in the intervening 202,000 years, or about 0.6 mm per year. This is very close to the rate (0.55 mm / year) determined at the 3.7 million-year-old Hannegan caldera, up at Hannegan Pass (Tucker and others, 2007), and is believable for this energetic environment which has seen multiple glacial advances, as well. The high cliff walls of the plateau tell us that much of the lava has been stripped away even at this location; it very likely continued several km further down the Nooksack valley, though no vestige has been found.
Getting there: Drive Mount Baker Highway (Hwy 542) east of the thriving metropolis of Glacier- if you have the whole day, make a day of it and check out the Church Mountain landslide along the way. Turn right 0.6 miles east of mile post 40 (miles east of the junction with I-5) on the Wells Creek road (Forest Service Rd 33, gravel). (see the Wells Creek volcanics field trip). The road is gated just beyond the falls between November 1 and July 1; the trip is a nice stroll or an excellent low-gradient bike trip. Total return distance of the field trip I describe is 7.5 miles (12 km). All but the last 0.5 mile is drivable if the gate is open. But why pass up such a great bicycle trip if you like that sort of thing! All distances given below are from the gate. Take a hand lens to see the minerals at the quarry stop.
Maps: USGS Bearpaw Mtn. 7.5′ topographic, scale 1: 24,000. The USFS Mt. Baker Ranger District map (also topographic, scale 1:63,360) is adequate for the task. The newish Mt. Baker Wilderness-Mt. Baker National Recreation Area- Noisy-Diobsud Wilderness topographic map published by the USFS does not have enough detail for this trip.
The Field Trip
BEFORE you begin the trip into the lava flow, get a great view of the Big Cliff and its wonderful columns (see the photo taken from the highway, at the beginning of this guide). For this, stay on the highway for about 1 mile east of the turn off to Nooksack Falls. Watch for a river-side pullout beyond the cliffs of the Wells Creek Volcanics east of the Excelsior Trailhead. You can’t miss the imposing lava cliff rising high across the river. This is the same pullout used to visit the slate outcrop at the end of the Wells Creek Volcanics field trip.
Return west to the Nooksack Falls turn off. From the Nooksack Falls gate, head 1.4 miles up the wide, well-maintained Wells Creek Road 33 to an unmarked junction with Forest Road 3310. This is on a sharp switch back to the right with nice views out to the Nooksack Valley. You have been in the Wells Creek volcanic member of the Nooksack Formation, although decent exposures are few beyond the track up to the greenstone quarry featured in the Wells Creek Volcanics field trip. Turn left on 3310. It winds around through the second-growth, passing another (probably unsigned) junction (Road 011) at 1.8 miles from the gate. You may note another junction at 2.5 miles- stay left. This junction has a triangulated elevation of 2470 ft on the 7.5′ topo. You are now on Road 012 but there weren’t any signs when we went. The road gets a little rocky as you climb into the open and reaches the first exposures of Pinus andesite at a quarry on the left. UTM by my old GPS receiver is E588690 N541700 (NAD 1927 ± ca. 20 m is always used on this website to correspond with the USGS map grid).
The andesite in the quarry is not columnar and not as glassy as the rest you will see, and is the best place to see all the minerals mentioned above, including both green olivine and the long black needles of hornblende, and clots containing both pyroxenes and plagioclase. Plag also occurs as separate clear sparkling laths. The rock has small vesicles (gas bubbles), too.
Continue on another 0.2 miles to the junction with a spur diving left beside an outcrop of stubby columns (2.7 mi, 4.3 km, from the gate, elevation 2560; UTM E588920 N5417750). Turn left (north) here to go to the top of the big cliff. Return to this junction after visiting the Big Cliff for a nice digression.
To reach the Big Cliff, at the 2560′ junction follow the left spur road generally north. This track is not drivable, but is suitable for mountain bikes most, if not all, its length. It gradually deteriorates, but remains obvious, with freshly cut saplings helping to keep you on the route toward the end. Pass a few outcrops of very glassy columns up to 1-2 feet thick- the dark color of this rock makes it very difficult to see minerals other than whitish plagioclase. After 0.5 mile (0.8 km) the track suddenly peters out in a minor depression with a number of snags on the left, just beyond a rock knob on the right. This place is at UTM E588920 N5417750 (photo at right). Look carefully for a cedar tree on the left with three prominent woodpecker holes near the base. Scramble 15′ up the bank next to this tree, then go more or less true north, or left, at the top of the bank. Climb up a mossy rounded knob then stay on the subdued ridge crest through forest to the brink. As you get close, watch carefully for a few
small holes in the ground. You’ll know the brink when you get to it! Use appropriate caution, etc., etc., etc. According to the topo map, the vertical cliff is 240′ (73 m) high; 248o feet (756 m) above sea level. It is 600′ (180 m) straight down to the river from here. Nice views across the North Fork to Excelsior Ridge. Watch the cars go by on the highway. Their occupants are perhaps looking up at the cliff with its striking columns and wondering “how the hell do you get up THERE?”
Back at the 2560′ junction, continue east on the main road another few hundred feet to a radio repeater tower. The southern margin of the Pinus Plateau is there; the cliff is not quite as high but a good destination none the less. There is a view toward Goat Mountain to the east, and down to the rusting hulk of auto carcasses pushed off this cliff, just ‘for fun’, presumably. The very crest of Mount Baker’s summit plateau is visible to the south, over the skyline ridge of Barometer Ridge. If you rode your bike, get set for a really great descent to the falls.
Easterbrook, D. J., 1975, Mount Baker eruptions: Geology, v. 3, p. 679-682.
Hildreth, W., Fierstein, J., and Lanphere, M., 2003, Eruptive history and chronology fo the Mount Baker Volcanic Field, Washington: Geological Society of America Bulletin v. 115 p.729-764. The andesite of Pinus Lake is discussed on page 752. This is a must-own paper if you are seriously interested in Baker eruptive history and the rocks that were erupted.
Tucker, D. S., Hildreth, W., Ullrich, T., and Friedman, R., 2007, Geology and complex collapse mechanisms of the 3.72 Ma Hannegan caldera, North Cascades, Washington, USA: Geological Society of America Bulletin, v. 119 p. 329-342.