Iceberg Point, Lopez Island: a Geologic Field Guide

By Dave Tucker

October 25, 2013

The navigation marker at the tip of Iceberg Point protrudes westward into the Straits of Juan de Fuca. In the distance is Cattle Point, at the southeast tip of San Juan Island. click to enlarge any image.

ICEBERG POINT protrudes westward from the south end of Lopez Island in the San Juan Islands. The isolated point is part of the San Juan Islands National Monument. The geology consists of young glacial deposits lying above slightly metamorphosed and sheared sandstone and mudstone. The point is a great destination for a part-day hike, with dramatic views across the water, windswept trees, and good rock exposures. You can spend a lot of time looking at details in the rock cliffs. There is always the chance to spot marine mammals nearshore, too. (Maybe you’ll be as fortunate as these folks!) Iceberg Point makes a fine destination for a one-day bicycle visit to Lopez.

GETTING THERE

Bedrock geology for the southern end of Lopez Island, adapted from Brown and others, 2007. See references below.

Iceberg Point (48° 25.319’N  122° 53.688’W) is at the opposite end of the island from the ferry dock. 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. At 10.5 miles, turn right onto Mackaye Harbor Road. Stay on this road to the Agate Beach County Park parking lot 12.2 miles from the ferry dock. Agate Beach is covered in pebbles, and makes for a nice destination in its own right. If you are driving, park and walk south on the road (it becomes Flint Road) for 0.3 miles. Cyclists can continue further on bicycles. Watch for a trail leading off to the right opposite Bat and Ball Lane. The track is marked with a sign painted on a rusty cross-cut saw indicating the way to Iceberg Point. This is the only public access to Iceberg Point. Walk this trail for 1/2 mile through the forest onto the open grass and bracken-covered rock backbone of the point, and beyond a few hundred yards to the extreme west point beside the automated navigation light. Total walking distance is a bit over a mile from Agate Beach. There are quite a few trails branching off, mainly to the south and the rocky bluffs above the Strait of Juan de Fuca. It doesn’t really matter which way you choose to go, so long as you can remember how to find your way back to the main trail. The geology hike I describe below begins at the light house and follows the shore south and back east. Note that Iceberg Point is DAY USE ONLY- no camping and definitely no fires. The point is directly accessible via your own boat; kayaks can be launched at Agate Beach park.  (SPECIAL REQUEST: If you know of an alternative public access to the eastern portion of Iceberg Point, please let me know via comment, with directions. To the best of my knowledge, all land beyond the trail head is private.)

GEOLOGY GUIDE

Sandstone at Icebert Point. That’s my patient wife Kim providing scale.

Once you leave the forest, you will be hiking on glacially-rounded rock knobs, signature of the most recent advance of the Vashon Ice Sheet. Hike on to the automated navigation light and foghorn at the western tip of Iceberg Point. Rocks here are interbedded sandstone and mudstone (shale) in the Lopez Structural Complex. The complex is a a major thrust fault zone that runs roughly east-west and includes the southern edge of Lopez Island and the north tip of Cattle Point over on San Juan Island. The 2.5 km wide thrust zone includes a variety of oceanic rocks that were brought together during subduction of the Farallon Plate (the Juan de Fuca plate is a remnant of the Farallon) between 110-80 million years ago. The rocks are believed to be Jurassic to Early Cretaceous (perhaps 150-100 million years old). The rocks in the structural complex include pillow lavas at nearby Richardson and Watmough Bay (click the link to learn about these pillows elsewhere on this website). All these rocks were brought together during faulting as terranes broke off the subducting oceanic plate and were attached (or ‘accreted’) to North America. The fault zone is a mish-mash of rocks that may not have been related or even close together prior to accretion.

Shearing is evident in the appearance of sandstone at Iceberg Point.

At Iceberg Point, the rocks are blocks of meta-sandstone within a broad zone of parallel shear structures. These shears are close-set faults generated by the compressive forces that ripped the rock apart during subduction and accretion . The blocks were once-coherent strata that were broken apart and shoved past each other on a small scale by the shearing. You can get a sense of this pervasive shearing near the navigation light, expressed by the long parallel fractures running through the rocks. The interbedded shale was weaker rock, so the shear zones tended to run through the shale; consequently, shales were pervasively broken up during this faulting. This friction led to fine-scale crushing and shearing of the shale into small pieces, a process geologists call ‘cataclasis’ (from Greek: cata + clasis, literally ‘entirely breaking’). The diced and minced shale weathers and erodes into the deep east-west fractures and the little indentations at the point.

Click to enlarge this detail from the preceding photo to see the many veins crosscutting the sheared sandstone.

Look for swarms of more or less parallel white quartz veins. These generally  crosscut the shear fractures at various angles. Quartz veins like these grow in fractures that develop as rocks are pulled apart. Since the veins cut the shear fractures, they indicate that after the rocks were compressed and sheared, the crust was ‘extended’ to the NW-SE (using current directions; it isn’t known what directions the actual extension occurred) while these rocks were still far below the surface. There is considerable debate among structural geologists who struggle to interpret the complex tectonic history preserved in San Juan rock units, and I won’t go into it all here. You’ll probably enjoy the scenery far more than that long and certainly confusing discussion! See the reference below for an overview of the subject.

A deep glacial groove runs left-to-right in the center of the view. This is just south of the foghorn/light. That’s a granitic erratic from Canada in the foreground,

The last continental glaciation left glacial till on top of the bedrock, and deeply grooved the rocks at Iceberg Point. A track runs along the shore south from the light. A prominent rocky point a couple hundred yards to the southwest with a nice layer of cobbles, sand and clay lying unconformably on the bedrock. The trail runs right along it and you can’t miss this example of a geologic unconformity. Unconformities mark a time break in the geologic record caused by erosion. The hundred-million-year-old Lopez Complex rocks were uplifted and eroded for who knows how long prior to being covered by the 12,000-year-old glacial till. Whatever rocks lay above the Lopez rock are also completely eroded. (Here is how to visit another unconformity I describe on the Olympic Coast that will be a chapter in my book.) A number of erratics lay strewn about the landscape. Of particular note are several conspicuous white granitic boulders along the south shore of the point 500 yards east of the southwestern point. From a distance they look like white sheep

The southwestern point. Kim stands on the trail atop of the till above the unconformity.

A spaghetti-network of boot-beaten tracks traverse the bluffs eastward above the southern shore. These are clearly visible on Google Earth. You can continue on these for close to a mile, but you’ll eventually have to find your way north to the access trail. A good landmark for doing this along the south shore is the 6-foot-high white concrete Iceberg Point Reference Marker, perched on the crest of the bluffs. The monument bears a bronze plaque noting the distance and azimuth to ‘Turning Point 7’, a sharp bend in the US-Canada boundary almost due west of the marker. From the monument, walk one of a couple of trails bearing north or northwest for 300 yards or so to the trail.

Erratics along the southern shore of Iceberg Point.

The geologically-inclined (and who else would be reading this screed?) with more time on their hands may wish to continue to Watmough Bay or Richardson to see shore-side pillow lava in the Lopez Structural Complex.

Iceberg Point Turning Monument 7.

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: Geologic Field Trips in Washington State and Southwest British Columbia: Geological Society of America Field Guide 9, p. 143-177.