Wednesday, August 11, 2010
The northeastern tip of Salt Spring Island, along Trincomali Channel, consists mainly of sandstone ledges and boulders derived from them, but there are also beaches where the bedrock allows and where sediment is available. The aerial image shows how the rocky points trap pocket beaches and barriers across the mouths of the small valleys.
On Tuesday, I paddled around from Southey Point. On Thursday, I came back by foot to explore a couple of these small beaches. They are composed of sandstone gravel and pebbles and a high proportion of broken shell. Some of the shell is washed up from the lower beach, but some of it is eroded out of the bank, which either due to waves or previous inhabitants, is thickly layered with clam shells. The sandstone makes beautiful skipping stones, the best of which are always concentrated high on the berm. It has something to do with their shape and preferential transport by the swash.
The largest beach shelters a high salt marsh and tiny estuary, with the small stream trickling out where the spit pushes it against the bank on the western edge. Occasional high tides have pushed a wedge of gravel into the estuary, forming a small flood-tide delta. The stream drains out across a broad intertidal delta-fan, with lots of evidence that the stream and the wave-built gravel bars shift around. This is a big pile of sediment in a place without an obvious source of new sediment, except for the clam shell. I suspect the stream and the waves have been pushing the stuff back and forth on the beach for millenia - the stream carries it down during low tides (alluvial fan), the waves carry it back up during storms (swash bars).