This photograph shows rocky outcrops in the water at Rosario Beach, in Deception Pass State Park. This is part of the San Juan Islands near Anacortes, Washington. Rosario Beach contrasts with nearly all the beaches farther south on Puget Sound because much of the shore consists of solid rock. Farther south, most of the bedrock by the shores of Puget Sound is covered by thick deposits of glacial drift, so most of the beaches have only gravel, sand, and mud--no steep, rocky shorelines. The outcrops of bedrock near Rosario Beach give us a chance to look at pieces of oceanic lithosphere--parts of an ophiolite--that have been accreted to the edge of North America.

Ribbon Chert

The thicker beds in this photograph, the ones with white veins cutting across them, are ribbon chert. Ribbon chert forms from deposits of dead plankton (small organisms that live suspended in ocean water), with hard parts made of silica. These accumulate on the ocean floor into layers of chert. Such layers of chert are called ribbon chert. Select the image to see a larger view. Use your browser's back button to return to this page.

It is clear from the photograph that the originally horizontal beds have been tilted so that they are nearly vertical. Along with the nearly vertical dip of the beds, small folds and faults are also visible in this outcrop.

To put together the history of the area, geologists analyzed microscopic fossils from the chert and used radiometric dating methods to determine the ages of the igneous rocks associated with the ribbon chert. The fossils are Jurassic species of radiolaria (or radiolarians, one-celled organisms that had mineral exoskeletons and lived in shallow ocean water). The igneous rocks have an age of roughly 150 million years, also Jurassic. Both lines of evidence indicate that these sediments accumulated on the ocean floor about 150 million years ago, during the Jurassic period.

Argillite Beds (Vertical)

The nearly vertical beds of shale and siltstone are near the ribbon chert in the previous photograph . The shale and siltstone have actually undergone a slight amount of metamorphism, turning the shale into a harder rock called argillite. Despite the slight amount of metamorphism, the rocks can still be recognized as layers of mud and silt that were deposited on the same ocean floor as the ribbon chert. Select the image to see a larger view. Use your browser's back button to return to this page.

Basalt Pillow

These rocks are pillow basalt on the cliff above Rosario Beach. They can be seen in contact with the ribbon chert and shale on the side of the cliff. Originally, the pillow basalt was probably beneath the sedimentary rocks. New oceanic crust forms with pillow basalt as its upper layer. As time goes by and the oceanic crust moves with its tectonic plate, sediments accumulate on top of the pillow basalt. Select the image to see a larger view. Use your browser's back button to return to this page.

Serpentinite

Some components of an ophiolite are missing in the Rosario Beach area. An important missing piece is the ultramafic rock that comes from the upper mantle part of oceanic lithosphere. Many square miles of serpentinite, metamorphosed ultramafic rock from the mantle crop out within a few miles of Rosario Beach. . However, faults and other types of non-ophiolite rocks are in-between Rosario Beach and the large outcrops of serpentinite. It seems that the bedrock around Rosario Beach has been broken apart along faults.

Till on Rosario Isthmus

In this photo taken behind Rosario Beach, part of the ground is covered with glacial till from the last glaciation of the Puget Sound area. Compared to the age of the chert and shale from the ocean floor (approximately 150 million years), the glacial till is of recent vintage, only about 15,000 years old. Select the image to see a larger view. Use your browser's back button to return to this page.

Glossary terms that appear on this page: glacial drift; lithosphere; ophiolite; ribbon chert; silica; dip; fold; fault; igneous rock; radiometric; shale; siltstone; metamorphism; pillow basalt; crust; tectonic plate; ultramafic; mantle; serpentinite