Fragrance Lake geology guide

By Dave Tucker  December 14, 2012

Fragrance Lake. A shelf of till-draped sanstone makes a good swimmin' hole. Click to enlarge any image.

Fragrance Lake. A shelf of till-draped sandstone makes a good swimmin’ hole. Click to enlarge any image.

The trail to Fragrance Lake in Larrabee State Park is one of the most popular in the Bellingham area. Even in the foulest, darkest weather, a hiker or runner is likely to meet someone else. The trail is particularly popular with dog owners, and I often feel like I’m breaking some sort of rule because I’m unaccompanied by one of the furry beasts. People familiar with this trail may think: “Geology? On this trail? It’s just a bunch of pebbles and mud and crappy moss-draped sandstone. What is this guy talking about?” The trail does, of course, pass through geology. Every unpaved surface on the planet’s crust does. This hike’s geology may not be the “spectacular, get out the camera and rock hammer” variety, granted. But if you focus on the details, there are things to see and think about.

Getting There

This map is drawn on a LiDAR image. The syncline fold in the Chuckanut is evident. The heavy red line marks the axis of the fold. The red 'T' shapes indicate strike and dip of bedding.

This map is drawn on a LiDAR image. The syncline fold in the Chuckanut is evident. The heavy red line marks the axis of the fold. The red ‘T’ shapes indicate strike and dip of bedding. The trail is shown in orange. Click to enlarge.

The Fragrance Lake trailhead is on Chuckanut Drive (Washington State Route 11) 6 miles south of Fairhaven (the south side of Bellingham). One way distance to the viewpoint is 0.9 miles; the lake is reached in 1.9 miles. Elevation gain to the lake is 775 feet; to the ridge crest above is another 150 or so. The trail is entirely in Larrabee State Park, and a Discover Pass is required to use the parking lot. The trail is popular, and on sunny weekends, especially in winter, the small parking area is liable to be jammed. There is overflow parking a couple hundred feet further south. The most sporting access is by way of the Interurban Trail from Bellingham. Hike or ride your bike 6.6 miles to the intersection with the Fragrance Trail. A basic hiking description is on the WTA website, as well as trip reviews. Trail directions are given assuming you are hiking UP the trail. The trail is moderate, but in some places the grade is steep, but short. About a dozen benches along the way excuses to stop and rest. TRAIL MAP: Trails of the Chuckanut Mountains by Ken Wilcox. Skookum Peak Press.

Trailside Geology

The trail tread is till. Stones are enclosed in a matrix of clay, silt and sand.

The trail tread is till. Stones are enclosed in a matrix of clay, silt and sand.

The geologically astute will immediately notice that the trail surface is glacial till. This is the case the entire way. However, their is plentiful admixture of sand eroded from the Chuckanut bedrock, and the trail is well designed, so it is rarely very muddy. A rich variety of rocks is represented by the rounded pebbles and cobbles in the tread. Look for quartzite, which is a sure-fire indicator of an interior-of-BC source. (Read about this quartzite elsewhere on the website.) These very hard, fine grained quartoze stones are so smooth that mud doesn’t stick to them in the rain. They are usually clean and even a little shiny and they tend to really stand out. some people call these ‘agates’, but they are really quartzite, metamorphosed quartz-rich sandstone from eastern British Columbia. Plutonic rocks are also plentiful, mostly black-and-white speckled granodiorite and tonalite, carried out of the Coast Mountains. I had to really look hard, but I did spot a few dark, nondescript volcanic cobbles. Their porous surfaces tend to collect a thin coating of mud, so they are inconspicuous. There are also scattered foliated metamorphic rocks in the till; a few nicely foliated gneisses and schists really jumped out as different from the others. The cobbles have been brought from the north. Their shapes indicate they were rounded in vigorous mountain

Qurtzite cobbles in Vashon till. I plucked 'em out of the trail.

Quartzite cobbles in Vashon till. I plucked ’em out of the trail.

streams tributary to the Fraser River, bouncing along the stream bottom, banging in to each other, and gradually assuming a rounded shape as edges were smoothed off. This is not done by the water per se, but by the other sediment carried by the running water. The rounded stones were then pickup up by the advancing ice as the great glacier flowed out of the highlands. Erosion via glacial transport does not make stones round, but may grind off one or more sides to form planar polished surfaces on an otherwise

A plutonic rock in the till.

A plutonic rock in the till.

previously rounded stones. Other stones in the glacial till are not rounded. They either were not transported very far by running water prior to entrainment in the glacier, were plucked out of rock surfaces by the movement of the ice, or fell onto the glacier in landslides and surface sluffs, and were never dragged along the glacier’s bottom, or sole.

There is plenty of blocky sandstone from the Chuckanut Formation in the trail tread. These are usually pretty angular, as they were quarried locally by the Puget lobe glacier of the last Vashon glaciation. The sandstone easily weathers into sand, and won’t make a rounded pebble or cobble- too soft and granular for that.

wentworth_scale

Digression: pebbles and cobbles. These are grain-size categories, in the continuum from clay to boulders. Pebbles, in the ‘Wentworth scale’ , are defined as sedimentary grains between 4 and 64 mm (between a pea and a tennis ball). Cobbles are larger than pebbles, but smaller than boulders, which in the Wentworth scale are over 256 mm in greatest dimension.

The first point of interest is the largest glacial erratic I have seen on this trail. All the stones in the till, except the Chuckanut, are erratics, since they do not match the local bedrock. The 5-foot-wide is by far the largest. The angular boulder is on the right shortly after the trail begins switchbacking. It is dark, and no surface is clean enough to really judge the rock type. I broke open a flake lying next to it, and it appears to be a metamorphosed plutonic rock, orthogneiss, similar to the nearby Arroyo erratic above Chuckanut Creek.

The odd horizontal scar on this 8-foot-diameter Douglas fir fills teh ntoch left from an unfinished effort to cut the tree down in the early 1900s.

The odd horizontal scar on this 8-foot-diameter Douglas fir fills the notch left from an unfinished effort to cut the tree down in the early 1900s.

I know this is a geology field trip, but I have to point out a remarkable tree. The entire trail passes through lush second growth forest. A few of the largest trees survived early-20th Century logging, but most are less than 100 years old. One of the survivors is an 8-foot-diameter Doug fir on the left, about 20 feet off the trail, about 1/10 mile before the junction with the viewpoint. It may be easier to watch for this tree on your way back, since the junction is the best landmark. A deep notch was cut into the tree on the  side facing the trail but the loggers didn’t finish- maybe it was quitting time, or the loggers went on strike, or whatever. The notch left by the saw healed and is marked by different-looking bark in an oddly geometric horizontal scar on the tree about 8 feet off the ground. I have hiked this trail uncounted times, but just noticed this as I investigated this geohike on December 13, 2012.

The viewpoint is a recommended short, level side trip, as there are no other vistas along the trail. Chuckanut Bay lies below you, with the open waters of Bellingham Bay beyond, and the San Juan islands.

Two metamorphic cobbles: gneiss (top) and quartzite.

Two metamorphic cobbles: gneiss (top) and quartzite.

The first rock outcrops are beyond the junction, and after crossing a divide between two small drainages. Sandstone, with some small pebbles mixed in, dips 35° to the northeast. We are on the outer limb of the great synclinal horseshoe of Larrabee Mountain. The original flat-lying Chuckanut flood plain deposits have been folded into a U, then tilted downward to the northwest. The layers in the Chuckanut get progressively younger as we move inward on the horseshoe. Note the big cliffs above you- the trail will now climb fairly steeply to get on top of them. You won’t see them as you do this, however, due to the forest. So, make the uphill grunt.

About 0.4 mile past that initial outcrop, the trail goes through cuts in a pair  of side-by-side cut logs. Just beyond on the next switchback to the left, I found a 10″ cobble of gneiss, with swirled veins of quartz. There are several of these along the trail, but this is the easiest one to point out.

Fragrance Lake from the top of the trail along the top of the sandstone cliffs.

Fragrance Lake from the top of the trail along the top of the sandstone cliffs.

Reach a bicycle barrier and a branch trail that climbs higher on Chuckanut Mountain. Continue straight towards the lake. On the right is a milepost sign. An informal but well used track goes left here, and climbs to the top of the vertical rock cliffs above Fragrance Lake. It loops back to the lake’s outlet. There are a couple of places with clifftop views of the lake, but no geology of note other than– more till.

So, if you opt to continue on to Fragrance Lake, another trail branches left and crosses the marsh on planks. This trail passes directly between the lake and the base of the rock cliffs and loops around the lake to return to where you are standing. Since the topic is ‘geology’, I’ll direct you left over the planks. Walk along the cliffs; they rise straight out of the lake for around 160 feet, with short overhangs and plenty of cracks. I have climbed these, but long ago and we had to drill some holes for bolt anchors. I was always nervous depending on those stubby steel bolts in that soft sandstone. But, I was young then, and braver. You’ll see in a few places that the bedding planes are  more or less horizontal, but the angle of dip isn’t evident because you are walking right along the strike of the beds. (Here is a primer on sedimentary rock structures.) They actually dip to the northwest, into the cliffs, around 20°. You are crossing the hinge, or axis, of the Chuckanut Mountain syncline (see the LiDAR image in the map above). Here is where the dip of the Chuckanut strata flattens out from the northeast dips we hiked up through. Beyond the lake to the northeast, the strata begin to dip the other way, toward the southwest. So, we walk across the U. The cliffs are pocked with shallow scoops and some smallish deep holes. Not sure what all of these are, but a little coal is left around a few, so some are where sticks caught in the river sediment had been coalified. They soft coal that they became has weathered away. Why are these sandstone cliffs so prominent? There’s nothing like them on the opposite side of the lake. Why is there a depression in the mountain with a lake sitting in it? Here comes my interpretation. it is admittedly an arm-waving hypothesis. Others may disagree, and I encourage you to write in with your comment.

The cliffs from the lake side trail.

The cliffs from the lake side trail.

For starters, we have a thick layer of relatively resistant sandstone in the Chuckanut. Note that this cliff faces more or less in the direction the Vashon glacier, and its several Pleistocene predecessors, was flowing. Also, the Chuckanut tends to be well fractured because as it was being deformed into the syncline it was high in the cold, brittle part of the crust. So, the rock didn’t actually ‘fold’ like a piece of paper does; rather, it tended to fracture on both large and micro scales to accommodate the deformation. Now picture this sandstone while it was still below the surface, uneroded but already folded. The sandstone of these cliffs continued to the southeast; the Chuckanut strata just below this cliff-former currently end at the big cliff below Chuckanut Mountain’s peak (I don’t know how far the Chuckanut once extended prior to folding; there are outcrops near Mount Vernon.

Aerial view looking SE. Fragrance Lake nestles at the foot of steep cliffs. The underlying Chuckanut strata culminate at Chuckanut Mountain's summit, upper right.

Aerial view looking SE. Fragrance Lake nestles at the foot of steep cliffs, lower left. The underlying Chuckanut strata culminate at Chuckanut Mountain’s summit, upper right.

The result of all this is that the repeated continental glaciations over the past million years could pluck away the rock at the top of what is now Chuckanut Mountain, gradually biting off large chunks defined by fractures and carrying them off, moving the cliff back to this point. The lake is hemmed in by topographic depression defined by the folded sandstone underlying the cliff-forming unit.

Walk around the lake. It’s a great place to swim in the summer.

7 Responses

  1. […] written up a geology guide to the popular Fragrance Lake trail. You’ll find it here. Even if the hike is mostly in glacial till, there are interesting stones in it, and the Chuckanut […]

  2. Thanks, Dave! I always take out-of-town guests on this hike (whether they like it or not), and now I can regale them with even more geologic insights.

    • Eric,
      You mean some people don’t like this hike? Amazing. I have hiked it uncounted times, and admittedly hadn’t thought much on the “same old-same old” geology until recently. Now realize there is no such thing as that kind of geology.
      dt

  3. Dave,
    Thanks. All of these structural observations add to the delight of this great little trail. It will never be simply moss-and-leaf-covered-slopes for me when I head back up to enjoy the views. Geology rocks!
    If you know what I mean.
    Bruce

  4. -Where in B.C. does quartzite come from?-Roadside Geology of Southern British Columbia only references a few places.
    -Where might THIS quartzite have come from?
    -What are the temp/pressure conditions to create quartzite?
    -Why was quartzite not created anywhere in North Cascades?

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