PILLOW LAVA IN THE SAN JUAN ISLANDS

By Dave Tucker   January 24, 2012

Classic pillow structure at Richardson. Notice how it drapes over the pillow at lower right. Photo provided by Michael Yeaman. Click to enlarge any image.

This is the third installment in a series of field trips to pillow lavas locales in Washington State. This guide directs you to pillow locations on San Juan, Lopez, and Cypress islands in the San Juan archipelago. The first two are reached via Washington State ferries from Anacortes or Sydney, but Cypress Island is accessible only by private boat. Geology in the San Juan Islands is extremely complex and still controversial. A recent field guide by Ned Brown, Bernie Housen, and Liz Schermer at WWU’s Geology Department is listed in the references below.

I wrote about two other sites elsewhere on the Northwest Geology Field Trips website. These trips visit the Crescent Formation pillows along the road to Hurricane Ridge in Olympic National Park, and the mind-blowing, textbook examples at Cape Disappointment at the mouth of the Columbia River. I wrote a primer on the formation of pillow lava here .

LOPEZ ISLAND

Geology of southern Lopez Is. Black fault separates untis JKl from Jl and Jf. Based on Brown and others, 2007. Click to enlarge.

I know of good-quality pillows exposures in two locations: Richardson on the south shore, and Watmough Bay in the southeast corner. This route is well-suited for a one-day bicycle tour.

Richardson is a small community on the south shore of Lopez Island. A small exposure of pillow basalt is found on the beach. There is more to see, and access is simpler, if you plan your trip for low tide.

Directions: From the ferry landing at Upright Head, take Ferry Road south. At 2.2 miles, turn left onto Center Road. Continue to a T in the road at 7.7 miles and turn right on Richardson Road. After making a few bends, this road dead ends at some tanks and a dock on the shore in another 1.9 miles. Other than the public road, all land and tidelands in the area are privately owned. In my experience people have been amicable and visitors have tread lightly and with discretion, understanding what a special spot this is.

My friend Ginger responds naturally to the pillows at Richardson. The drill hole at left center is probably from somebody’s paleomagnetic studies (pre- 1994). Yours?

Google Earth image of Richardson. Look for pillows just north of the dock, and up the shore from there.

The elongated pillows are immediately west of the end of the dead end road, which is the site of a much-loved and famed grocery/general  store that burned in October 1990. The pillows are only a dozen feet or so from the roadside. They are mostly covered at high tide. Donn Charnley, geology instructor at Shoreline Community College,  says he has never been challenged by anyone, though this site is not public. Walk along the shore west of the road to find pillow exposures like the one in the photo at left. A map appears on page 168 of Brown et al. (2007). You can also find pillows on the southeast side of the point below the tanks.

Low tide at Richardson reveals very fine pillow lava. Note the elongated shapes. Photo submitted by Michael Yeaman.

The elongated shape of these pillows is remarkable, and provides an opportunity to see how successive lobes of lava flowed over each other. The circled pillow in the photo below drapes over the pillow beneath. Also, these flows appeared to have been moving in what is now a north to south direction. However, since these rocks have been subducted and subjected to who knows what amount of rotation, these compass direction may bear no relation to the original sea floor lava flows. If a reader out there knows who did the paleomagnetic study that left the holes in the picture with my friend Ginger, please let me know. The orientation of the magnetic minerals in the pillows could shed light on the orientation of the pillows when they were cooling beneath the briny deep.

The pillows are part of the Lopez Structural Complex, a Jurassic-aged thrust fault zone consisting of marine sedimentary and volcanic rocks. The rocks are part of the very complicated set of terrane slivers comprising the San Juan Islands. Rocks of these ‘exotic terranes’ have been stacked and sliced by faulting, and accreted to the margin of North America. The geology is quite complex- see Brown et al. (2007) for details on the Lopez Structural Complex. An Ar-Ar age obtained from mica in one of the pillows gave an age of 124.43 ± 0.72 Ma (Brown et al., 2007).

Lopez Island. Watmough Bay and Chadwick Hill is in the southeast corner. Richardson is on the shore of Mackaye Harbor to the west.

Watmough Bay and Chadwick Hill are part of a preserve in the San Juan County Land Bank system. Read more about this location here. A map of trails at the preserve and more information is here. Pillows are well exposed on the shoreline cliff  below Chadwick Hill on the north shore of the tranquil bay.

Directions: From the ferry landing at Upright Head, take Ferry Road south. At 2.2 miles, turn left onto Center Road. Continue to a T in the road at 7.7 miles and turn left onto Mud Bay Road (turn right to get to Richardson). At 12.2 miles, turn right onto Aleck Bay Road. At 12.75 miles, continue straight onto Watmough Head Road. Turn left at 13.75 miles (may be signed to the preserve). Park at the end of the bumpy drive, 13.95 miles from start. There is an outhouse here. It is a 1/4 mile flat walk to the shore. You surely can not miss the cliff from the beach! If you are coming from Richardson, backtrack on Richardson Road just a bit and go right (east) on Vista Road. In 1.3 miles this joins Mud Bay Road. Go right here 2.4 miles to the turn at Aleck Bay Road and proceed as above.

The 300-foot cliff above Watmough Bay is entirely pillows. Photo courtesy Donn Charnley.

The Geology: Donn Charnley provided some of this information and the photos for this part of the trip. He lives nearby, and knows the geology in the area particularly well. He has taken generations of students and geo-enthusiasts to this site over the last 4+ decades. The 300-foot-high cliff on the north shore of the bay is made entirely of pillows. The pillowed lava apparently erupted at a mid-ocean spreading center sometime in the Jurassic, similar to activity at the Juan de Fuca Ridge off our coast today. These rocks, superficially similar to the pillowed basalt at Richardson, actually lie across a fault, and belong to a different geologic unit, the Lummi Formation. These rocks are somewhat older than the Lopez Complex at Richardson, but are still Jurassic.

These pillows are on the dihedral at top center in the preceding photo. These are 100+ feet above the beach, and can only be reached via difficult rock climbing! Courtesy Donn Charnley.

SAN JUAN ISLAND

Pillows are exposed along the rocky shores of Limekiln State Park. They are not nearly as obvious or pristine as the Lopez island examples, and it may take a little imagination to spot them. Don’t make a special trip to see the pillows here, but if you are out at the point for, say, whale watching, enjoy the rocks, too. There is limestone which is worth seeing. This site will be described in detail in my book, Geology Underfoot in Western Washington. This site is also described in Brown et al. (2007), and in Hiking Guide to Washington Geology by Bob Carson and Scott Babcock (Learn more about this fun and essential book by scrolling down elsewhere on this website.)

Shattered pillow breccia in beach bluffs south of the Limekiln Lighthouse.

The San Juan County Land Bank manages preserves to the north and south of the State Park: Limekiln to the north and Deadman Bay to the south. Both are well worth visiting for geological and scenic treasures. Maps and more info are available if you click on the links to the left of the map at the Land Bank’s website.

From Friday Harbor get to Limekiln State Park by driving west across San Juan Island. Follow the signs, or get a map and pick your own route. You’ll need a Discover Pass to park. Walk the trail behind the outhouses to the shore.

The best pillow exposure I found at Limekiln. See text for approximate location.

Follow foot paths to the south to see pillow breccia in the shoreline rock outcrops. Pillow breccia is fragmented pillow lava, which either fell apart as it flowed down steep submarine slopes or was broken up during flow by the explosive interaction of incandescent lava and water. To see the best pillow examples, walk north beyond the lighthouse. Stay right along the shore. A track leads to a small point below a bench. Like I said, it takes a little imagination (or experience, whichever works best) to recognize these as pillows.  The pillows and the interbedded limestone pods (look for them south of the lighthouse) make up the Deadman Bay Volcanics, which are Permian seafloor deposits, accreted to North America during the very complex tectonic events that brought many of the North Cascade rocks and other San Juan Island units to this location from exotic locales far away and long, long ago (sorry, I couldn’t resist).

CYPRESS ISLAND

Eagle Cliff from the west. Photo (C) Bud Hardwick.

This wild island can only be reached by private vessel. There is a popular kayak beach and paddle-in camp at Pelican Beach on the northeast shore. Eagle Cliff, site of the pillows, rises 752′ above the waves. It is reached by way of a 1.3 mile trail leaving from the Pelican Beach campground. I don’t know for certain what geologic unit the pillows are assigned to, or their age, but looking at map relations, they are probably part of the same Lummi Formation as the pillows at Chadwick Hill above Watmough Bay and pillow breccias on nearby Lummi, Sinclair and Vendovi islands. Find a description of the Eagle Cliff trail written by Bud Hardwick here, and a DNR map of the extensive Cypress trail system here. The ever-helpful Donn Charnley tells us that the Cypress pillows are on the east side of the Eagle Cliff massif a couple of hundred yards or so below the top.  Watch for wonderful examples of N-S glacial striations on rock surfaces, including some very deep grooves. Thanks to Bud Hardwick for supplying some of his Eagle Cliff photos. He has many more on his SkyDrive photo site, on the web here.

Watch for sectioned exposures of pillow lava along the Eagle Cliff trail. Bud Hardwick photo (C).

Most of Cypress Island is a protected natural area. It has unique geology for the San Juans, as most of the island is serpentinized ultramafic rock. The magnesium-rich soil that erodes from this rock produces a rare plant community. Some special rules apply for users of the island’s recreational facilities:

• Visitors should stay on trails for the majority of travel on Cypress Island NRCA
• Off-trail travel in the Cypress Highlands NAP is by permission only
• Eagle Cliff Trail is closed from February 1 until July 15 to protect nesting raptors
• Dogs must be leashed at all times
• No dogs or other pets (not even if leashed) are allowed on Eagle Cliff Trail or at Cypress Lake
• Camping only allowed at established campgrounds located at Cypress Head or Pelican Beach and only from Memorial Day weekend through Labor Day weekend
• Cypress Island is open for Day use only from Labor Day weekend until Memorial Day weekend
• Mooring buoys (located at Cypress Head, Eagle Harbor, and Pelican Beach) are open to overnight use year-round

Scoured pillow lava on Eagle Cliff. Green color is from low grade metamorphism of platioclase in the rock to chlorite and other green minerals. Donn Charnley photo.

REFERENCES

Brown, E.H., Housen, B.A., and Schermer, E.R., 2007, Tectonic evolution of the San Juan Islands thrust system, Washington, in Stelling, P., and Tucker, D.S., eds.,
Floods, Faults, and Fire: Geological Field Trips in Washington State and Southwest British Columbia: Geological Society of America Field Guide 9, p. 143–177,
doi: 10.1130/2007.fl d009(08).   This paper and the accompanying field trip guide are available for purchase (pdf) from GSA for $25 here. The entire guidebook with 10 field trips is available in the Bellingham and WWU libraries.

Deep glacial grooves just below the summit of Eagle Cliff. Photo (C) Bud Hardwick.

A fascinating and very dated (1927) publication on San Juan geology is on a University of Washington website. This is the delightful PhD thesis by Roy Davidson McClellan, The Geology of the San Juan Islands. Most of McClellan’s classic is forever doomed by his ignorance of plate tectonics, which inform all modern theories of San Juan geology. McClellan calles all the rocks we now understand to be pillowed basaltic lava flows the “Eagle Cliff porphyrites”. (‘Porphyry’ refers to the ‘prophyritic texture’: coarse minerals visible within a fine grained matrix, typical of many lava rocks.) McClellann, who like most geologists didn’t know what pillows were in 1927, figured they were some sort of very odd and very shallow intrusion. Go to the igneous rocks section of his dissertation and scroll down to read about the ‘porphyrites’. The first photo in that section clearly shows pillow lava ‘on the east side of Davis Bay’- which is near the Richardson pillows. The other photo, at Cape St. Mary on Orcas, is just northeast of Chadwick Hill.