Rock Trail geology, Larrabee State Park

By Dave Tucker, April 11, 2014; Updated April 18, 2021.

The first portion of this new trail is well named. The Rock Trail descends past high walls of  sandstone of the Chuckanut Formation (for background on the Chuckanut, go here). The cliffs reveal some fine stratigraphic detail; a contact between conglomerate and sandstone, thin sandstone beds highlighted by mineral concentrations, and solution pockets (tafoni). Along the way there is ample evidence for  mass wasting, both large- and small-scale. The hike is entirely in the forest. This guide describes a one-way trip from the trailhead to the junction with the South Lost Lake trail. Do the trip on a sharp, clear day in the winter and you’ll be rewarded with eastward views through the trees to Mount Baker and the Twin Sisters Range. Round trip distance is 3.8 miles; the trail is downhill from the get-go, so you begin with an elevation loss to the lake of 690′; you’ll gain every inch of that on the way back.
Getting there:

Red line marks the new Rock Trail. Contour interval is 20′. Note scale in lower left.

The trail is brand new (spring 2014)  and is not shown on any maps (yet). It was build with countless hours of volunteer labor by volunteers from the area and the good folks from the Washington Trails Association. With the map at left you shouldn’t have any difficulty transferring the trace to the USGS Bellingham South 1:24,000 topo quad. Leave Chuckanut Drive (WA 11) north of the Larrabee State Park entrance (south of Bellingham) on the Highline Road, which becomes the gravel Cleator Road. It is 3.6 miles (5.8 km) to the  Cyrus Gates Overlook at the end of the Cleator Road in Larrabee State Park. The trailhead dives into forest at the south end of the parking lot. There is a toilet at the trailhead.

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 Geology: The road climbs up through the interior of agreat, north-plunging syncline. The summit of Chuckanut Mountain just above the trailhead is the highpoint of the horseshoe. (A short trail climbs straight to the top, where there are views). The Eocene Chuckanut formation consists of alluvial deposits that were deposited on a flood plain prior to uplift of the Cascades- the sediments were carried from the Rockies to this point in meandering tropical rivers. The western shore of North America was somewhere to the west, probably not too far, but the transition from terrestrial alluvial rocks to marine rocks is nowhere preserved. Subtropical fossils, including palms, are preserved elsewhere in the Chuckanut, but the sandstone along the Rock Trail is too coarse to preserve any organics other than a few minor bits of wood. The hillside the trail descends to get down to the Lost Lake Trail has a number of large scale ‘steps’- beds of resistant massive sandstone form cliffs, and intervening layers of eroded softer shales and thinly bedded sandstone form the flattish benches between the cliffs. This is shown quite well in the bottom right corner of the LiDAR image- the trail is on this steep nose.

The first  of several stairways on the Rock Trail.

Shortly after you start your hike you’ll cross a popular mountain bike trail that descends the ridge- the ridge marks the crest of the western limb of the syncline. Watch that you don’t get run over and continue straight to the top of a remarkable set of stairs. Descend these. The trail now sidehills southwest below the rim, parallel with the NW-dipping beds that underlie Chuckanut Mountain.

Bud and the thin beds at the overhang.

One quarter mile (400 m) from the trailhead you come to a wonderful exposure of thin cross-bedded sandstone under an overhang. The cross beds contrast sharply with the massive unlayered sandstone below. Note that the contact is erosional- the river deposited the massive layer during steady-state conditions. Then there was a brief period of erosion, perhaps during one or more flood events. Thin beds overlie this disconformity (a break in continuous deposition), highlighted by concentrations of dark biotite mica crystals. Put your face right up against the rock to examine these thin beds very closely. Alternating dark and light sandy layers represent short-term, minor, fluctuations in river energy. Note how the thin beds pinch out on the left: the current scoured sediment from the river bottom, before again depositing

The thin beds, Red knife for scale.

thin beds. Repeated scour and deposition resulted in these subtly cross-cutting beds.

Reach the next point of geologic interest in 0.1 mile (160 m). Here are conglomerate beds rich in 1-3″ angular pebbles. The grain size contrasts sharply with the sandstone on top, and indicates a sudden change in river dynamics. The base of this pebble bed abruptly decapitates down-to-the left sandstone cross beds. At the time the pebbles that would eventually be compressed into conglomerate were deposited, the river had enough oooomph to scour into its own bed, removing earlier-deposited cross beds, roll pebbles along its bed, and swirl away sand and finer grains, leaving only pebbles. Pebbles were concentrated and left as a souvenir of this burst of higher energy flow. Perhaps there was a flood, that undercut and collapsed an older bank of pebbles into the stream near this point. The surge of water passed, and once again was less energetic. Even sand grains were too much for it, and they were deposited on top of the pebbles.  The composition of pebbles like these provided evidence for the provenance, or source area, for the Chuckanut sediment- rock types match those on the west flank of the Rocky Mountains.

Tafoni Wall, striped by winter-time tree shadows, is a highlight of the Rock Trail.

Now the trail passes through scattered large blocks, some the size of a garage, that have fallen from the cliffs. Gravity happens, but hopefully not when you are in the way. The last high cliff face is reached 1/2 mile (800 m) from the trail head. The base of Tafoni Wall is pocked holes weathered into the rock. The largest of these are two feet across and deep enough to squeeze people inside- see how many kids you can cram into one.

Large tafoni along the Rock Trail. Enlarge to see faint horizontal bedding in the sandstone. Tafoni seem to align with the bedding.

These holes are called ‘tafoni’, a term derived either from the  Corsican noun for ‘window’ or a verb, ‘to perforate’. They are forming now – this is not some remnant of older geologic conditions. A chemical weathering process called ‘solution’ dissolves the microscopic mineral cements that hold the sand grains together in the sandstone. Falling rain picks up carbon dioxide to make carbonic acid which dissolves the cements: H₂O + CO₂ —->> H₂CO₃. This is natural  ‘acid rain’. It has persisted for eons, since long before air pollution though at reduced levels to the infamous industrial-spawned variety. George Mustoe at WWU’s Geology Department determined that, at least in the moist temperate Chuckanut environment, this weathering process involves algae that colonize rock surfaces. Once algae form their micro-thin film on rock surfaces they inhibit the absorption of water and so retard solution. Algae may or may not be readily apparent on rock surfaces, but Chuckanut cliffs are pervasively colonized – you can see the green film on most rock surfaces unless they are freshly broken. Acidic solution attacks an entire rock face, which may weather away evenly. When something happens to damage the surface and remove or kill the algae, solution immediately attacks the cement, and the sand grains fall off of steep surfaces. This ‘something’ could be freeze-thaw expansion and fracture of the rock, or getting smacked in a rockfall, or a hard freeze that locally kills the algae, or a particularly permeable contact between rock layers. Maybe snow piled against the cliff bottom in cold winters plays a role in inhibiting algal growth- the better developed tafoni are at the base of the cliff. Each falling sand grain leaves a tiny pit in the rock which can be exploited by solution. The holes, only millimeters across, grow inward as solution migrates into the rock ; visible growth of tafoni has been measured photographically over a few short years. The algae can reestablish on the sunny surfaces at the opening of the developing hole. Once algae recolonize the rock surface, solution is again retarded. But the algae may not grow well on shaded rock surfaces beyond the entrance to a solution hole, so acidic water can dissolve cement more readily there and the holes migrate inward. The sandstone in the Chuckanut Formation is permeable rock, so tafoni can grow laterally behind the algae-protected surface, and the developing tafone (singular of tafoni) is larger on the inside. The process is complicated and fraught with variables. Tafoni.com is an entire website devoted to tafoni.

Growth of tafoni is probably accelerated by the influence of salt spray – check out the beautiful patterns on the Larrabee shore. I write about them on the  honeycomb’ weathering  page of this blog.  I devote an entire chapter to the process in the Larrabee Park chapter of my book (which should be out next fall). The book is described in the ‘My book’ tab on this website. The surface of a rock can be shown to change in photographs taken 10 years apart.

The pale sandstone slab with the cross beds beyond ‘Tafoni Wall’.

Tafoni Wall is the last rock large exposure along the trail. From here is it another 0.4 mile (600 m) to the junction with the South Lost Lake Trail. Several hundred yards beyond the wall, and a short distance beyond a narrow wooden foot bridge, watch for a very clean 20-foot sandstone slab on your left several hundred yards beyond the wall, as the trail traverses a bench. There are some beautiful cross beds in this rock- get up close to look them over.

Cross-bedded stream sand in the ‘pale slab’.

Consult a trail map to know what to do from the junction- left turn takes you to the Lost Lake Trail 1.5 miles from the start of the Rock Trail. A right takes you on a long sweep to the Fragrance Lake area.

One of many large fern draped fallen blocks. Kim Brown provides scale if you click to enlarge .

References:

Mustoe, G. E., 1982, The Origin of Honeycomb Weathering, Geology Society of America Bulletin, v. 93, p. 108-115. This paper is available in full online.

Mustoe, G. E., 2010, Biogenic origin of coastal honeycomb weathering, Earth Surface Processes and  Landforms. DOI: 10.1002/esp.1931

Tafoni.com