Discovering Beach Pebbles

I was walking on the beach on a sunny afternoon, listening to the repetitive sound of the waves and the slight crunching sound of my footsteps sinking into the pebbles. “People have stories, but do rocks?” I wondered out loud. I looked down at the pebbles, worn smooth and round by wave action and realized that all of these rocks must have come from somewhere, and may have traveled some distance to arrive at my feet.

Each pebble is remarkably unique. They may be striped, speckled, patterned or plain, and come in a range of colours: black, white, grey, brown, green, purple, pinkish and dull red. Surface textures vary also, some are smooth and fine-grained and others are rough or pitted. I often see people picking up pebbles, so I figure there must be others who find them as compelling as I do.

There are three types of rock, igneous, sedimentary and metamorphic. Sedimentary rock consists of layers of sand, gravel and mud that are sandwiched together and lithified (turned into rock) over time. Sedimentary rock sometimes contains fossilized remains of once-living creatures. Coal is made up of ancient plant material, and limestone consists or corals, shells and other ancient marine organisms. Other sedimentary rocks include sandstone, mudstone, conglomerate (gravel that is cemented together), chert (has a glassy, layered look) and shale (mostly from mud and clay). The effects of weathering and erosion on a soft rock like sandstone can create Swiss cheese like holes, potholes and other eye-catching formations.

When molten rock (magma) cools, igneous rocks are formed. Sometimes the magma cools very quickly at the surface, such as a lava flow from a volcano. These are called volcanic igneous rocks. Or, the magma may enter cracks beneath the earth’s surface and cool very slowly, forming intrusive igneous rock. Slow to cool magma grows the largest crystals. Mineral crystals that are trapped in the rock include things like quartz, feldspar, mica, hornblende and pyroxene.

The interlocking crystals give many igneous rocks a speckled look. One of the most common is granite, a black and white speckled rock. Some igneous rocks are fine-grained with invisible crystals, such as black basalt, which is wonderfully smooth if rubbed between one’s palms. Some igneous rocks have cavities that were filled with gas as the magma cooled, giving them a pitted look, and others have gas-cavities that were filled with minerals, which appear spotted with white. Large crystals, such as feldspar, in a fine-grained igneous rock can create lovely snowflake patterns. Generally speaking, any igneous rock with large grained crystals (usually feldspar or quartz) dispersed on a fine-grained groundmass are called porphyry (pore-fer-ee.) Igneous rocks are very common on our beaches.

Metamorphic rocks are pre-existing rocks that become changed by heat and pressure, or the chemical actions of fluids, deep in the earth. Igneous and sedimentary rocks are often transformed into metamorphic rock. Granite can become granite gneiss (look for granite that has a layered look to it), sandstone can change into quartzite, limestone becomes marble, and shale turns into slate. As a result of these transformations minerals may be re-organized into layers, or new minerals might grow. Schist is a metamorphic rock with visible, often “platy” minerals such as mica, chlorite, talc, hornblende and graphite that can sometimes be split into layers (schist is a greek word meaning “to split.”) Gneiss can be confused with schist, but often has dark and light bands of minerals.

Some beach pebbles are made up solely of minerals, such as milky white quartz, red jasper or green epidote. Human objects can mimic rocks, and can become similarly weathered by the elements into smooth shapes. Be on the lookout for pieces of worn cement, old brick, and glass or pottery shards. Over time, I hope that I can become more adept at spotting different types of rock and knowing more about where they came from, and what processes (heat, pressure, erosion), shaped them into their current form. Rocks do have a story to tell, and it is much more interesting than I thought.

A Field Guide to the Identification of Pebbles by Eileen Van der Flier-Keller, used as a reference for this article, is a laminated fold-out pamphlet with great pictures and descriptions some of BC’s pebbles.

Filed under Nature Writing | Tags: Beach, Pebbles, Rocks, Vancouver Island | Comments (2)