Share this page


Boulder/Cobble Shoreline

Ancia Road, South Pender Island

Razor Point, North Pender Island
Bouldercobble Boulder Cobble .1.Razor Point North Pender Island


Boulder/cobble shorelines are shallow sloping sediment beaches which are moderately resistant to erosion. The beach slope and form are determined by wave and tide action. On moderate to high wave energies, the smaller cobble material will be mobile, while in more sheltered areas the beach material will be stable. This shore type is common in Baynes Sound. 

The boulder/ cobble shore type is 9% of the total shoreline in the Islands Trust area based on the twelve major islands that had shoreline mapping completed.  Denman Island has the most at 29%, while Lasqueti Island has the least at 3%.   4% of South Pender Island's shoreline has been mapped as the boulder/ cobble shore type.

Physical Features 

Boulder/cobble shorelines at moderate and high wave exposures may be subject to erosion during storms, however, the finer sediments in the mixture are more easily transported and can accumulate in pocket beaches between more stable shore types if longshore transport is present. Boulder/cobble shorelines may be stable or dynamic and are likely to change on a human time scale. 

Boulder cobble beaches often occur at the base of unconsolidated bluffs or banks as 'lag' deposit where larger materials are left on the beach after finer sediments have washed away.  

Biological Features

On moderate to high energy boulder/cobble beaches, the beach may be scoured and bare of attached algae and invertebrates, with mobile invertebrates like shore crab and sea stars living under the boulders and cobbles. In lower wave energies in particular, burrowing animals such as clams and various species of worms are present in the finer sediments.  

On sheltered shores and bays, upper intertidal zone communities commonly include rockweed, barnacles and blue mussels, along with other invertebrates.  

Bull kelp may be found in areas of moderate to high wave exposures or higher tidal currents where substrate is immobile (i.e., boulders). Benthic kelps are also present in the lower intertidal zone at all but the most protected exposures, and mobile invertebrates such as sea stars and crabs may be common. 

Eelgrass, a shallow water flowering plant, is often present in protected wave exposures in pockets of finer substrates of sand or mud or seaward of the boulder/cobble beach. A highly productive community, eelgrass beds provide important nursery areas for many species of fish, including herring and juvenile rockfish.  

Shallow nearshore water can be important feeding areas for diving sea ducks and other waterfowl.  

The coastal riparian vegetation, dune grasses and other salt-tolerant plants adapted to the dynamic environment of the upper intertidal zone transition between marine and terrestrial habitats play an important role in stabilizing the upper beach as well as contributing to productivity.

Low Rock And Boulder Cobble WEB

Page last updated: 01/10/15
© Islands Trust. All Rights Reserved.