Wednesday, August 21, 2013

Chocolate Beach

The Chain Islands run in a line out of Ganges Harbor and make a great island hopping kayak tour. All are rocky and there is neither sediment nor opportunity for beaches to form. With this notable exception. Here on Third Sister Island, shell debris has found a place to accumulate and it has formed a wonderful white beach consisting almost entirely of broken shell fragments.

From a distance, I expected it to be a tombolo, but it is actually just a low rocky area where a beach has been able to form on one side. If the process that formed it is still active, which I expect it would be, then this beach is fairly young. Otherwise I would expect the shell debris to continue to accumulate and to wash over and cover the rocky backshore. I was surprised it hadn't. I wonder if the perfect storm could actually clear the shell completely off this beach, requiring time to rebuild.

Mark and I did it on our own, but this beach is the destination for guided half-day kayak trips out of Ganges and maybe it's called Chocolate Beach because it's where the chocolate bars come out of the dry bags. I was curious about how these daily visits affect the beach itself? One of the lessons of Cama Beach was that without people, the berm consists of largely intact clam shell, especially after a big storm. Once you let people back in, it is all rapidly turned to crushed shell.

AERIAL VIEW (Google Maps)


I like that every time I come to Salt Spring, I find one or two new beaches. This one was unexpected and a new favorite. It's actually a couple of beaches along the exposed rocky shoreline at the south end of the island. The easternmost beach is a extremely well-sorted gravel pocket beach. The western beach is more complicated and is actually a small tombolo with fringing pocket beaches on each side.

These small beaches lie on land of the Tsawout First Nation and I really appreciated that they have allowed it to remain accessible. Hiswke - thank you! My apologies that my font options don't allow me all the proper accents/characters on the name of this site. This felt like a very special spot. And clearly, it had been special to many, many people before me, as witnessed by the anthropogenic stratigraphy of the exposed banks.

AERIAL VIEW (Google Maps)

The trees hanging over the rocky cliffs included both Madrone (Arbutus here in Canada) and Garry Oaks, which are fairly common in the Gulf Islands.

Price Road

Many small beaches on Salt Spring are swash-aligned pockets, completely contained by bedrock points.  Beddis Beach (Gravel Beach 2008), a short distance south of this one, is a great nearby example. 

This one off Price Road is not a pocket beach. The shoreline orientation changes and the beach becomes a little more swash-aligned, which allows a wider beach to build, but there's nothing at the distal end to keep it from spilling around the corner.  A small stream delta acts like a groin, and there's a small rock groin, too, but these simply the slow the beach down, they don't stop it.

The sediment on this beach appears to come from the shoreline to the south - eroding banks and a small stream mouth.

Farther downdrift the beach is coarse and narrow, except where a couple of groins attempt to trap the rapidly moving finer material. Half a mile farther north, where the coastline turns west again, the beach itself just keeps going strait, forming a small spit (a site for a paddle trip in a future year).

AERIAL VIEW (Google Maps)

I think one lesson here is that in this highly oblique wave environment (the west shore of Ganges Harbor), the width of the beach (the volume of the beach) is a function of the speed with which sediment passes through - which in turn is a function of the orientation of the beach.

Walker Hook

The northeast shore of Salt Spring Island is a relatively strait, steep, rocky edge along Trincomali Channel. Sediment sources are limited and wave action is highly oblique, so beaches are few and consist largely of narrow lenses of sand and gravel held by ledges of bedrock (Fernwood 2009) or associated with small stream deltas.

AERIAL VIEW (Google Maps)

But Walker Hook is a small rocky islet that lies just offshore and a tombolo has formed between it and the main island. The result is swash-aligned barrier facing into the southern waves with a protected embayment on its north side.

The island is public. The tombolo is not. So I paddled around from the public road end on the bay and landed discretely at each end of the beach to stretch my legs and take some pictures.

The beach is relatively sandy. Annuals were growing in the berm. Algae had washed up on the upper intertidal. And some sort of burrowing critters had left the mid-beach looking like it had goose bumps.

Like south-facing beaches anywhere on the Salish Sea, this one collects logs, but here someone has come along and stacked them like shingles. The backshore is high, suggesting it was filled in the past, and the imbricated logs act as a bulkhead.

Vesuvius Bay

Most of Salt Spring Island consists of folded sedimentary rocks of the Cretaceous Nanaimo group.  They range from conglomerates to finely bedded turbidites.

The structure of the folded rocks appears at various scales - from the overall shape of the Gulf Islands to small-scale deformation within individual outcrops.  Locally, they conspired with the glaciers to form narrow inlets, often with small pocket beaches tucked into their ends.

AERIAL VIEW (Google Maps)

The beach in Vesuvius Bay is a nice example of a small pocket beach perched on top of steeply-dipping Nanaimo silts and sands. The slowly eroding bedrock probably doesn't generate much sediment, so the beach probably accretes very slowly, if at all.

There are many other pocket beaches on Salt Spring - some I've visited before (Beddis Beach 2008, Southey Point 2010), some I'll visit in the next few posts.

Iona Beach

Iona Beach lies at the northwestern corner of Sea Island, near the northern edge of the Fraser River Delta and just north of the runways at Vancouver International (YVR).

It lies in the acute angle formed by two long jetties that open westward onto the Strait of Georgia. The northern of these trains the northernmost distributary of the Fraser; the southern contains two giant pipes that route Vancouver's sewage into the Strait of Georgia.

AERIAL VIEW (Google Maps)

The low tide beach at Iona is mucky, probably consisting of fine-grained suspended sediments from the Fraser.  Small runnels drain perpendicular to the beach, cutting miniature canyons into the more cohesive silts and muds.

The upper beach is sandy (no source of gravel here, except for whatever washes out from the riprap jetties) and there's a broad backshore that appears to be accreting (as its back edge gets overtaken by Scot's Broom).

In looking at the aerial image (but lacking any more careful research), it looks like dredged material from the north arm may be placed along the north jetty and that this material is then transported southeastward towards Iona Beach as a series of spits.

On this Saturday morning, the main attraction of this place seemed to be for cycling teams, bird watchers, and a couple of airplane buffs.

Friday, August 02, 2013

McWay Falls

A beautiful sandy pocket beach between rocky cliffs.  An elegant waterfall directly onto the sand.  But apparently there was little beach here at all until the big landslide just north of here during the 1983 El Nino.

You can see the eroding slope to the north in the photo below.  I'm not sure how much is the natural colluvium and how much is material trucked in to re-establish the roadway, but either way, the waves have reworked it and carried some it into the cove.

Google Maps:  AERIAL VIEW

I guess this is the classic Big Sur photo stop.  For me it was the last significant, and documented, stop for the day before hightailing it to the San Jose airport.

Lime Kiln

Lime Kiln Creek - in the 1880s, they mined and processed the limestone up the valley - drops steeply to the Big Sur coast, landing in this small pocket beach.  I was impressed by the boulders and cobbles strewn on the shore, but had trouble picturing them as a storm berm. 

Looking carefully where the stream flowed across the beach and the sand is washed away, you get the sense that these coarse deposits may underlie the whole beach.

The best I can figure, this cove is basically a sandy beach lapping onto a very coarse-grained alluvial fan. I bet those big rocks were carried by floods or debris flows down the creek (they don't look like the stuff that would have fallen from the nearby cliffs).  Waves may have rearranged them a little, but they're basically a terrestrial deposit.

Google Maps:  AERIAL VIEW

Big Sur

Traveling south past Big Sur, the granite of the Monterey Peninsula eventually gives way to Franciscan formation and other metamorphics - a significant step down in the quality of coastal building materials. Big landslides were simply not part of the Monterey landscape, but here the Coast Highway is challenged by one big failure after another, including this area near Lucia, where CalTrans engineers are undertaking some pretty amazing roadwork (at the Pitkin Curve Landslide, I believe).

USGS Big Sur Landslides

There have been some interesting debates along here about the disposal of landslide material during this kind of work, as the sediment creates turbidity and can impact marine life (the Monterey Bay National Marine Sanctuary is offshore). We have our own version of this debate in Puget Sound.  If the landsliding is largely a natural phenomenon (even if the repairs are necessitated by the road) that's been going on for millenia, how much effort should we expend to prevent sedimentation? Or is there something unique about the manner or rate of delivery that justifies caution?

Google Maps:  AERIAL VIEW


Some of Carmel is built on or behind sand dunes that appear to climb up the slope from the town's wonderful beach.  Some of it is built along a low cliff cut into an old terrace (as best as I can tell).

The beach was a long series of broad cusps, disappearing into a summer fog bank.

Google Maps:  AERIAL VIEW