San Juan Islands Geology

Plate Tectonics, Ice Ages, and the San Juan Islands

200 million years ago the San Juan Islands, and Washington State in general, were largely represented as a couple of micro-continents (think Japan, New Zealand, Vancouver Island), and as scraps of land drifting around in the immensity of the east Pacific Ocean.  Meanwhile, an oceanic ridge in the Atlantic continued to enlarge that ocean's basin, forcing the North American continent westward where it collided with, and floated over, the Pacific oceanic plates. As the heavier oceanic plate impacted the relatively light continental plate it subducted (sank) underneath it. It doesn't take much to imagine the incredible forces at work here. In fact, if you are standing on top of any hill in the San Juan Islands all you need to do is look to the east. The mighty Cascade Range, with its massive volcanoes still steaming, bear testimony to the immense power of plate tectonics. The entire mountain range was created by the impact.

Soon the micro-continents that were once drifting out in the ocean were seen out on the horizon from the North American coastline. In a geological time frame it did not take long before they docked, one after the other, into the newly formed Washington coastline. As each micro-continent approached it would obliterate a trench that formed in front of its advance. In the trench would be a mish mash of sedimentary rocks, as well as ocean bedrock, that was stuffed into it by the subduction. (A useful way to visualize this process is to take a deck of cards and separate them into two stacks. On one stack spread peanut-butter evenly on top, these cards represents the oceanic plate. The peanut butter represents oceanic crust and sedimentary rocks. Now take that stack and slowly shove it underneath the other (continental) stack. Do it at a bit of an angle. Notice how the peanut-butter gets jammed- no pun intended- into the shallow "V" created where the stacks meet? Well, the shallow "V" is the trench, and the little pile that is formed is the mess from which much of the San Juan Islands eventually eroded out of.)

Standing again on a hill in the San Juan Islands, look to the west. See Vancouver Island? Well that's another micro-continent that is heading for North America. Although it is not likely to crush the San Juan Islands, it will definitely alter the landscape somewhere near Alaska. But don't hold your breath, it will take another 10-15 million years before this happens.

As you travel through the San Juan Islands, get to know the rocks. You will see that while the majority of the rocks are remnants of oceanic bedrock, there are many examples of sedimentary rocks as well. Some of these sedimentary rocks were exposed to great heat and pressure while deep in the trench and are consequently greatly altered and deformed.

Geologists have divided the San Juan Islands into three terranes, or areas of related rocks, called the San Juan terrane, the Decatur terrane, and the Lopez terrane. In addition to this there is the Nanaimo Formation that extends up into the Canadian Gulf Islands.

1) The Nanaimo Formation is mainly represented in the northwest part of the San Juan Islands. Stuart Island, Waldron Island, Sucia, Matia, and Patos Islands, as well as a handfull of smaller rocks and islets are in this formation. This formation is much younger than the other terranes in the San Juan Islands and consequently they are much less deformed. Soft sandstone and conglomerate rocks perhaps laid down in an ancient river delta, or shallow bay are the predominate feature. The sandstone is similar to the Chuckanut sandstones near Bellingham and may be considered part of the same formation.

2) The San Juan terrane is the largest, and quite possibly the messiest. It contains a chaotic assortment of limestone that was deposited in shallow water, mudstone and chert that appear to have accumulated on the deep ocean floor, and scraps of oceanic crust. The rocks vary in age by at least 200 million years, and the only way they could now be together is through having been swept into a trench. San Juan Island, Shaw Island, Orcas Island and the Wasp Island group are the main members of the San Juan terrane.

3) The Decatur Terrane - The Decatur terrane is a bit more coherent. It seems to consist mostly of rocks formed during late Jurassic and early Cretaceous time, about 150 million years ago. Those rocks include thick sections of sandstones and pebble conglomerates scrambled in with slabs of oceanic crust. It seems reasonable to suspect that the sedimentary rocks may have been deposited on the edge of the oceanic crust. Then the sinking ocean floor stuffed both into the trench. The north half of Lopez Island, Blakely Island, Decatur Island, most of Cypress Island, Guemes Island, and Fidalgo Island, and the Burrows and Allan Islands group are all included in this terrane.

4) The Lopez Terrane - The Lopez terrane also consists of ancient oceanic crust, pillow basalts, and sedimentary rocks. The great temperatures and pressures created in the depths of the trench converted most of the sedimentary rocks to dark slates, which break into thin slabs. Kayaking around the Cone Islands, near Cypress Island, is a great way to see these slate rocks. South Lopez Island, and the extreme south edge of Decatur Island also have nice representative rocks exposed.

As the contents of this trench was pushed upwards by the subduction it eventually reached the surface. And just like everywhere else on the planet, as soon as rocks see the light of day they begin to be torn down by the forces of erosion. Rain, snow, wind, and -particularly in the San Juan Islands- glaciers begin to shape the land.

Ice Ages and the San Juan Islands

The San Juan Islands owe their current form to the last ice sheets that covered the area until about 15,000 years ago. These immense sheets, over 5000 feet high in places, plowed over the northern third of Washington State. The weight of the ice pushed the Earth's crust down many hundreds of feet and the relentless advance of the glaciers took advantage of any weaknesses in the rocks of the San Juan Islands. The combination carved away any sharp crags in the summits of the hills and gave them the rounded look associated with today's San Juan Islands postcards. As you are kayaking around the San Juan Islands, look around to the surrounding foothills. The upper reaches of the glacial ice sheet can be determined by where the peaks get a craggy, shark's tooth look to them. Notice how all of the San Juan Islands are well rounded at the summits? The islands were entirely buried in the ice.

Nobody knows for sure how many ice ages there were as each ice sheet pulverizes or covers evidence of previous events. We do know that several came and went during the period of time known as the Pleistocene. This was a time of incredibly unstable weather and temperatures worldwide that lasted 2-3 millions years, and is possibly still happening now (probably aided by the burning of fossil fuels over the last century). We do know that the latest ice age covered the San Juan Islands until the ice began to melt away 15-20,000 years ago. As the ice retreated to the north it put the finishing touches on the contemporary look of the San Juan islands in the form of huge amounts of sand, gravel, rock, and clay being deposited with the meltwater. The mix is known as "glacial till" and it is a very important landform throughout the San Juan Islands and the Puget Sound region. In the San Juans glacial till is generally thickest on the south sides of the islands. A visitor to San Juan Island and Friday Harbor can visit South Beach in American Camp Nat'l Historic Park and walk directly on the till. The beach itself is formed by the huge bluff (Mt. Finlayson) that rises above the water there. The southeast side of Lopez Island has some terrific sand bars and bluffs to walk along. The entire south shoreline of Cypress Island is also comprised of glacial till. Everywhere we go we see reminders of a much colder time in our recent past. In addition to these huge deposits from glacial outwash are massive boulders that were left behind by the retreating ice. These granite rocks, known as "glacial erratics", were plucked up and transported in the ice as it advanced to the south. Some were carried hundreds of miles from their parent stone formations. The bane of the farmer ploughing his fields, and a natural jungle-gym for children, "erratics" bleach bright white in the sunlight and are gorgeous examples of the powers of nature. Look around as you travel through the San Juan Islands, you'll be sure to see them.

At the end of the last Ice Age the temperature was much cooler in the San Juan Islands and Vancouver Island area. Large mammals like Wooly Mammoth and Wooly Bison roamed in extensive Garry Oak woodlands. These woodlands were interspersed with large grasslands and probably filled with wildflowers in the spring. As worldwide temperatures warmed coniferous trees such as the Douglas fir began to colonize the areas and displaced the Garry Oaks. Now only remnant populations exist here in the San Juan Islands. Occasionally, exciting discoveries of mammoth bones eroding out of the post-glacial bluffs remind us of these days in our not so distant past. Ironically, even though sea levels rose significantly during the warming period, the shorelines of the San Juan Islands were not inundated. This was largely due to "glacial rebound", the lifting of the Earth's crust after the tremendous weight of the ice sheet was removed. Even today shells can be found high in the forested slopes of the San Juans.

San Juan Islands geology is fascinating and complex. It is also fairly accessible to look at for visitors to the San Juan Islands. As you visit the islands take some time to get out on the water. Preferably a small boat like a sea kayak. You'll be amazed at what you can learn!

Sea Kayaking with Anacortes Kayak Tours

Sea Kayak Tours to Burrows Island

San Juan Islands Sea Kayaking, the Outer San Juans

Multiday Sea Kayaking in the San Juans