boulder cobble 1  220x136  hhWhat is a boulder/cobble shoreline?

This type of shoreline is found on rocky coasts where the local geology supplies a source of sediment. The dominant sediment takes the form of boulders (rock greater than 25 cm in diameter) or cobbles (rounded rock 5-25 cm in diameter) that may overlie smaller sediment (sand/gravel) or a bedrock shelf. Boulder and cobble shorelines support a varied assortment of organisms that may be mobile or attached to rocks.

Where are boulder/cobble shorelines found around Victoria?

Boulder/cobble shorelines are found in a few locations near Victoria, for example Mt. Douglas beach and Sombrio beach, and some locations along the Victoria waterfront at Dallas Road.

How are boulder/cobble shorelines formed?

Boulder and cobble shorelines are formed over thousands of years. The boulders and cobbles come from erosion of bluffs or cliffs that are composed of glacial till. Glacial till is ground-up rock that was pushed in front of a glacial ice sheet as it advanced, or sediment that was deposited by rivers formed when the ice sheets melted. The force of waves rearranges the rocks on these shorelines and gradually wears them down.

What lives on boulder/cobble shorelines?

The life on these shorelines depends on both the size of the rock and the degree of exposure to waves. Shores with large, stable boulders and/or low wave exposure support fairly diverse communities of plants and animals. Shores with smaller cobbles that are more exposed to waves, on the other hand, support fewer species, since the rock is constantly being moved around. The bio-bands on boulder/cobble shorelines are usually not as obvious as on rocky shorelines since the physical environment is more varied.

In the backshore zone (above the high tide line) of this region the native vegetation includes oceanspray, Oregon grape, snowberry, Douglas-fir, arbutus, and Garry oak. Animals that reside here include eagles, ravens, great blue herons, river otters, mink, racoons and mice.

In the intertidal zone (between the high and low tides), large boulders can support attached species such as green, brown or red algae, mussels, barnacles, periwinkles and limpets. Sea anemones, chitons, sea cucumbers, nudibranchs, sea urchins and sea stars can live in moist crevices and tide pools. Shore crabs and hermit crabs scuttle among the rocks and tidepools. Burrowing species such as clams and worms live where there is soft sediment. Birds such as gulls, crows, sandpipers, oystercatchers and great blue herons forage along the shores.

Since boulder/cobble shorelines are quite varied, they may host patches of eelgrass, which roots in muddy sediment, or kelp, which attaches to stable rocks. Some animals graze on this vegetation, but more importantly, both eelgrass and kelp provide many types of marine life with habitat, among the various parts of the plants.

Some of the animals found in the subtidal zone (below the low tide line) along boulder/cobble shorelines include fish such as sculpin, sole, flounder, smelt and greenling; mammals such as harbour seals; and birds such as buffleheads and cormorants.

What threatens boulder/cobble shorelines?

Boulder/cobble shorelines, though not as stable as rocky shorelines, are still relatively resistant to erosion. However, if upland vegetation is removed, bluffs can become exposed to the forces of rain, wind and waves, leading to gradual erosion or slope failure.

When seawalls are constructed along these shorelines, the natural movement of sediment, by the action of waves, is interrupted. The remaining beach often becomes steeper and coarser as waves reflect off the seawall and scour away the smaller pebbles. If the shoreline is filled in and reinforced with a vertical seawall, a large percentage of the intertidal habitat is lost. Such changes have serious impacts on marine life.

Oil spills can cause lingering damage to these shorelines, since the oil can become trapped among the sediment.

Sediment and pollution can enter the marine environment from runoff in the watershed, and cause damage to marine plants and animals. Excess sediment may arise from poorly planned logging or construction activities, as well as from impervious surfaces that cause rainwater to rush into streams and lead to erosion.

How can I help protect boulder and cobble shorelines?

For information on protecting shorelines, please visit our How Can I Help section.

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© Image courtesy of L. Townsend

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