A Field Guide to the Identification of Pebbles

by Eileen Van der Flier Keller

Rocks, Rocks, Rocks!!!

Rock is the foundation of the surface of the Earth. We live on it, mine it for useful materials, make roads with it, grow food on it and generally depend on it for many things. In British Columbia there are very few places where you don’t see rocks and much of our beautiful scenery depends on interesting rock formations. Some British Columbia rocks are as old as 2 – 2.5 billion years and others are forming right now.

Where, oh where are these pebbles from?

Pebbles and sand form when rock is broken down by surface processes such as rain, freeze/thaw, heating and biological activity. These broken up particles of rock are then moved by gravity, water, wind or ice.

If the pebbles come from the surrounding rocks, they will look the same and be the same colour. Around our beaches, many of the pebbles are eroded out of glacial deposits in the sea cliffs. These glacial deposits were dropped by retreating glaciers and ice sheets and contain all the rock fragments, sand and clay that the glacier carried. Pebbles may also be transported to the coast by rivers.

Some of the pebbles on our beaches have come a long way. For example, many of the granite and granodiorite pebbles we see around southern Vancouver Island come from the Coast Mountains.

While being transported by rivers or waves, the broken rock fragments get rounder and smoother. Even very angular debris that was carried and deposited by ice, once it is eroded from old glacial deposits and moved around by waves at the beach, will become rounder. Typically the further a piece of rock has been carried and the more it has been washed about, the smaller, rounder and smoother it will be.

There are three different groups of rocks: Igneous, Sedimentary and Metamorphic.