PEBBLES AT THE PASS
By Terry Slotemaker and Gene Kiver, photos by Gene Kiver
In Deception Pass State Park there are miles of pebbly beaches such as at West Beach, North Beach, Rosario Beach, Bowman Bay and Hoypus Point. If you are at the beach on an average to low tide day when the rocks are visible you will see a variety of beautiful and interesting pebble-sized rocks. You and your kids or grand kids might be challenged to wonder: What kind of rock is it? How did this rock form? Where did it come from? How did it get here? It would be helpful to take a copy of this article with you to the beach. Pebbles are small rocks. The ones described range in size from approximately golf ball to chicken egg.
To learn the kinds or names of the pebbles, you can match the pebbles you find with the pictures. The answers to the questions about the fascinating origins and journeys of the rocks are in the supporting information.
There are three basic types of rocks: igneous, sedimentary and metamorphic. The first three sections in this article define and describe the rocks. A fourth section defines and describes a few minerals.
Igneous rocks formed from molten rock called magma that forms very deep in the earth. If the magma erupts onto the surface it is called lava. Igneous rock includes the oldest and youngest rocks on earth.
If you see a salt and pepper looking rock, pick it up and look at it closely. If the black and white specks, the minerals, in the rock are grainy it is likely granite or another granitic rock. The minerals from which granite is composed are glass-like quartz; white, creamy or pink feldspar and dark minerals such as biotite mica.
The granitic rocks on the beach were originally part of a large mass of molten rock that formed deep in the earth along the west coast of Canada. Because molten rock is hotter and more liquid than the surrounding rock, it is less dense and rises very slowly towards the surface. During this process that takes millions of years it cools and hardens. Because the magma cools slowly, individual crystals grow large and are easy to see.
Movement of the earth’s plates also uplifted the granitic rock as the ocean plate and the North American plate collided forming the Coast Range in British Columbia. Eventually the surface rocks were exposed by erosion. During the last glacial age several advances of slowly moving ice thousands of feet thick broke off large and small chunks of rock from the Canadian mountains, ground them into pieces and deposited them on Whidbey and Fidalgo islands and southward as far as Olympia, Washington! Glacial meltwater streams, sea currents and waves moving sand and gravel further rounded and smoothed the rocks. Many were transported from area bluffs to the Park’s beaches.
Basalt is a common igneous rock that formed when iron-rich magma erupted onto the earth’s surface as lava and cooled. Basalt varies from very hard to soft and from a rusty color to dark gray and blackish with a greenish tinge. The two forms pictured here are scoria and spotted basalt. These pebbles likely were transported by glaciers from a volcano in Canada.
Sedimentary rock is formed from fragments of pre-existing rock and can include tiny particles, sand, pebbles, cobble and sometimes boulders. Water or ice transported the fragments and deposited the material in a stream, bay, or ocean, or on land. Sand, silt and clay particles could also be transported by wind. The deposited fragments formed sediments thousands of feet thick. The weight of the overlying sediment compressed lower levels that hardened by compaction, cementation or chemical action.
The sedimentary rock on the beach possibly formed in deep deposits somewhere at the bottom of the Pacific Ocean or as deltas where rivers entered the sea. For millions of years the deposits rode northeasterly on an ocean plate that collided with North America. As with the igneous rock, the movement of the earth’s plates uplifted the sedimentary rock as the ocean plate and the North American plates collided. Glaciers, running water, waves and sand and gravel interactions rounded the pebbles. Many of the dark rocks you see on the beach are sedimentary rocks or metamorphosed sedimentary rocks. Many show layers formed when they were originally deposited.
Breccia – Breccia resembles conglomerate except that most of the individual fragments appear broken and angular rather than rounded. Breccia can be formed when angular rock fragments slide down the edge of a volcano or cliff into sediments which later harden.. Breccia can also be formed at or near faults.
Metamorphic rock was changed from previous igneous, sedimentary, or metamorphic rock as a result of heat, pressure and/or chemical action but not by melting. As with the igneous and sedimentary rock, glaciers transported the pebbles to northwest Washington.
All rocks are composed of mixtures of different minerals. A mineral is any naturally occurring solid substance with a definite crystal structure and chemical content. It is the basic stuff from which rocks are made. All rock is made from minerals but rock formations can also be host rocks in which new minerals form as a result of metamorphism or from fillings in rock fractures and cavities.
There are many more pebbles than described in this article about which you may be curious.
A field guide to the Identification of Pebbles by Eileen Van der Flier-Keller is a handy fold out chart, easy to take with you, with color photos which can help you identify pebbles.
Washington and Oregon Rocks & Minerals by Dan R. lynch and Bob Lynch is a small easy to understand book that contains color photos and information about rocks and minerals.