The San Juan Islands of Washington State are an archipelago of over 400 islands and rocks situated in the Salish Sea. This collection of islands, famous for its orcas and quiet harbors, holds a complex geological history spanning hundreds of millions of years. This history involves the slow collision of distant landmasses, immense crustal compression, and powerful erosion from colossal ice sheets.
The Foundation: Accretionary Terranes and Exotic Rocks
The bedrock forming the San Juan Islands is fundamentally different from the continental crust of North America. This foundation is composed of fragments of crust known as accretionary terranes, which are pieces of ocean floor and distant landmasses scraped onto the continent’s edge. This process occurred from the Paleozoic through the Mesozoic Eras, as the oceanic Farallon tectonic plate slid beneath the North American plate at a subduction zone.
These incoming materials, often referred to as exotic rocks, traveled thousands of miles across the ancient Pacific Ocean. The islands contain remnants of ancient island arcs and deep-sea sediments. On San Juan Island, for instance, one can find slices of ophiolite, which are sections of ancient oceanic crust and upper mantle rock uplifted to the surface.
This chaotic mixture created a geological zone known as a mélange, a large-scale rock body composed of blocks of various origins embedded in a finely ground matrix. Specific terranes, like the Deadman Bay and Decatur terranes, were assembled in this manner, containing pillow basalts and limestones with fossils that originated near the equator. The largest component is the Wrangellia terrane, a massive piece of crust that underlies much of Vancouver Island and extends beneath the San Juan Islands. These collisions created the San Juan Islands–northwest Cascades thrust system, a complex stack of rock sheets piled one upon the other.
Tectonic Forces: Compression and Uplift
Once these exotic terranes were plastered onto the western edge of the continent, they were subjected to intense geological forces. During the Mesozoic Era, particularly the Late Cretaceous period, the relentless movement of converging tectonic plates caused deep crustal compression. This powerful squeezing shortened the continental margin, forcing the stacked rock layers to fold and buckle with great intensity.
This compression resulted in the formation of the San Juan Thrust System, where enormous slabs of rock were pushed up and over one another along fault lines. This folding and faulting caused materials that were once flat-lying layers to be tilted nearly vertically in some places. The accumulated pressure and heat from this tectonic activity led to the uplift of the entire region, raising the deep-sea and island arc rocks far above sea level.
This period of mountain-building created a continuous, mountainous highland that existed before the ice ages began. The underlying structure of the present-day islands, characterized by complex faulting and steeply dipping rock layers, is a direct result of this ancient tectonic squeezing. This pre-glacial topography provided the foundation that would later be modified by massive ice sheets.
The Great Sculptor: Glaciation and Ice Age Erosion
The mountainous terrain created by tectonic forces was reshaped during the Pleistocene Epoch by the repeated advance of continental ice sheets. The most significant event was the Vashon Glaciation, a stage of the larger Fraser Glaciation, which saw the Cordilleran Ice Sheet advance southward from Canada. This immense sheet of ice, which was over a mile thick in some areas, flowed directly over the region now occupied by the San Juan Islands.
The weight and movement of the ice carved deep valleys and troughs into the underlying compressed bedrock. The ice followed lines of weakness in the folded and faulted rock, deepening them into the wide, U-shaped channels that separate the islands today. The Strait of Juan de Fuca, which bounds the southern edge of the archipelago, is a prime example of a massive valley carved by this glacial scouring.
As the ice moved, it plucked away blocks of rock from the down-current (southern) faces of the highlands, resulting in the steep, jagged cliffs visible on many islands. Conversely, the up-current (northern) faces were smoothed and polished by the abrasive action of the ice. When the ice finally retreated between 15,000 and 12,000 years ago, it left behind deposits of unsorted sediment known as glacial till, as well as large, foreign rocks called erratic boulders. The ice had effectively drowned the old mountain range, leaving only the highest, most resistant peaks exposed as the modern island chain.
A Modern Archipelago: Post-Glacial Rebound and Geography
The final configuration of the San Juan Islands was settled immediately after the ice sheets’ retreat. The removal of the enormous weight of the ice, which had depressed the Earth’s crust, initiated a process called isostatic rebound. The land began to spring back upward, a vertical movement that occurred rapidly at first, sometimes exceeding half a foot per year.
This land rebound competed with the simultaneous global sea level rise caused by the melting of ice sheets worldwide. Initially, the uplift of the land outpaced the rising sea, leading to the formation of raised shorelines and wave-cut terraces visible on some islands. However, as the crustal rebound slowed, the rising ocean waters began to dominate the process, leading to the slow submergence of the landscape.
This submergence created the intricate, island-dotted geography that exists today, where the highest points of the old, eroded mountain range became the islands. The deep channels separating the islands are remnants of the glacially-carved valleys, now filled with seawater, creating the complex bathymetry of the archipelago. The islands are the unsubmerged peaks of a tectonically-formed mountain range that was first scoured by ice and then partially drowned by the returning ocean.