Monday, May 06, 2013
Washington Park is a rocky headland at the northwest corner of Fidalgo Island. Here at Green Point, the bedrock forms a distinct terrace a couple of meters above high tide-- in this case a gently rolling glacial surface, not a marine terrace. The bedrock is mantled by glacial drift - which forms a low bluff. As they tend to do, pocket beaches have formed on both sides of the headland, the glacial gravel unable to escape the potential energy well (butchering the physics again) created by the bedrock points and the local wave regime.
In the previous post, we saw rocks that formed at the top of an ophiolite sequence - the basalt, ribbon chert, and fine grained sediment that one finds on the ocean floor (or that one would find on the ocean floor, were one to actually visit it). In Washington Park, the bedrock is composed of peridotite and dunite, now partly altered and metamorphosed to serpentinite. These rocks formed at the base of the oceanic crust and form the bottom part an ophiolite sequence. These are ultramafic rocks, which means they are rich in iron and magnesium, and very low in silica. Some people saw the thin bands of altered chromite. I was intrigued by a vein of heavily weathered, coarse-grained pyroxene, or at least I think that was what I was seeing. I'm a beach guy - so my mineral identification skills are as rusty as these rocks.
One of the lessons of Eric's field trip today was that although Fidalgo Island presents a beautiful series of oceanic rocks, it does not represent an entire classic ophiolite sequence (the middle part is missing). And the abundance of felsic dikes and silica rich (relatively) inclusions and clasts suggest an island arc origin rather than a mid-ocean ridge. Another lesson was that the rocks in this little corner of the world are sliced up at many different scales and there remain tough questions about the spatial relationships and the relative timing of emplacement and whether these rocks can be lined up with other similar rocks in the region.
In the true British tradition, I see most of the beach work I do as physical geography. Today's field trip was real geology!
My previous posts from here have been about the beach (September 2009, March 2012). But Rosario Head, which defines the southern side of this pocket beach, is a classic destination for geologists trying to figure out how Fidalgo Island was actually built.
Rosario Head is a spectacular exposure of deep-ocean sediments - very old ones. Pillow basalts, erupted beneath a Jurassic sea, cherts deposited from a rain of radiolaria in deep water away from any significant sediment sources, and black argillites. These seabed rocks are typical of the upper portion of ophiolites, which are basically preserved sections of oceanic crust.
A vast majority of the world's past ocean bottoms have been dragged by subduction back into the mantle and recycled, but on complicated plate margins like ours here in the Pacific Northwest small slices of these oceanic rocks can get thrust up onto the edge of the continent and preserved. Preservation is a messy exercise, however, and more often than not the rocks are sliced and diced at many different scales -- like these.
Much of Puget Sound is nothing but Pleistocene - with Vashon glacial deposits on the surface and older glacial and interglacial sediments peaking out from the lower portions of some of the bluffs. But at Rocky Point (2009), on the west side of Whidbey Island, and in Skagit Bay (Craft Island 2011, Hope Island 2012), the Mesozoic re-emerges from the basement to form rocky islands and headlands.
Deception Pass, separating the north end of Whidbey Island from Fidalgo Island, is a topographic gap in these older metamorphic rocks, which make up much of the San Juan Islands to the north and west.
Deception Pass moves a lot of water, including much of Skagit Bay and Saratoga Passage and the currents are fast. Steep rocky cliffs plunge into deep water in two channels, split by Pass Island. But there is sediment moving at depth and USGS work has shown a submarine delta of sorts west of the entrance (I know I've seen some bathy from here - but can't find it online). There are pocket beaches on both the Whidbey and Fidalgo sides of Deception Pass, probably consisting of sediment derived from the erosion of overlying glacial drift, although the ones on the south side may also include sediment that has made it around the corner from the sediment rich beaches on northwest Whidbey.
Thursday, May 02, 2013
I stop briefly at this Kitsap County Park almost every time I drive to Jefferson County or elsewhere on the Peninsula, since it's right off the main road just before you get to the Hood Canal Bridge. In the mid-1990s, I came here even more often, sometimes on weekends with a small child in tow, to informally monitor the beach after the old riprap was pulled out and replaced with a small gravel beach. It's done remarkably well, given the nourishment occurred seventeen years ago (October, 1995), but the beach face has gradually eroded landward and some serious thought should be given to adding some more gravel, tweaking the backshore vegetation (maybe lowering the backshore six inches or so), and dealing with the old drain pipe that's seeping onto the beach. A solution for the failing rockwork at the eastern end would also be nice, but trickier.
As mentioned previously, Salsbury Point has suffered a number of insults - the floating bridge has modified the wave regime, perhaps cutting off the supply of sediment from the south. The boat ramps have isolated it from the beach to the east. And of course, this was once a narrow recurved spit with a small tidal lagoon behind it, although one would is hard pressed to see much evidence of that today!
Previous Mentions: Salsbury Point
The western end of the Hood Canal bridge is built on fill and extends across the beach. The result has been some fairly obvious changes to sediment transport and beach processes both north and south of the approach (Shine Tidelands 2007, Termination Point 2011). But here on the eastern end, the bridge passes over the beach and any effects on the beach are less obvious. I've speculated a little about the possible effect of the bridge on wave action north of the bridge in an earlier post on Salsbury Point (2007).
The beach continues unbroken beneath the bridge, although the character of the low tide terrace changes significantly along here and there is also a small accretional bulge in the upper beach a short distance north of the bridge. The low tide beach is likely a fairly robust feature of the landscape - it would be very difficult, short of dredging, to change the width of the low tide terrace very much. But the upper beach is a different story and I've always wondered whether the accretion might be in part to the falling off of wave action from the north due to the bridge itself. There was already a fairly wide beach here -- and a much older dock -- before the bridge was built in 1961, so this would require some serious sleuthing to sort out.
Under the bridge, WashDOT (I assume) has placed a series of nicely arranged logs at the base of the slope. This is one of numerous examples on Puget Sound where large wood is being used in a naturalistic way as erosion control. I'm often pretty skeptical of such approaches (long story, but sometimes I think they are neither very effective nor very natural), but in this case it's an interesting idea. Admittedly, I'm not sure how much of an erosion problem there was to start with at this location.
Sunday, April 28, 2013
Most of Indian Island is managed by the Navy as a convenient place to store WMD, but the bluff and the beaches south of the county road are public, accessible, and wonderful. Two weeks ago we brought the Beach Watchers class here -- on a beautiful afternoon after they had been cooped up in a small room watching slides all morning.
This one mile stretch includes the tombolo between Marrowstone and Indian Islands, a long stretch of till bluffs, and a beautiful spit that encircles a tidal lagoon. There are several different places you can park and get to the beach.
Net drift is northwest into Oak Bay, where sand and gravel once accumulated to form another long tombolo between Indian Island the the mainland. This was altered when they made the cut back in the early 1900s and the beaches have been sort of on their own ever since, although the east side (this one) and the west side have taken different paths.
We saw some fairly fresh failures in the glacial till bluffs, evidence of an oblique bar building at the base of the gravel beach, and some relatively recent overwash on the spit (recent means the last 6-7 years - February 2006 and December 2012 both had events that probably carried gravel over the berm, but there may have been others, too).
Indian Island (January 2007)
Marrowstone Isthmus (October 2010)
Indian Island, Eastern shore (April 2012 - posted in October)
This beach lies at the edge of the shipyard at the west end of Boat Haven marina. It's the jumping off place for the Larry Scott trial, which follows the old railroad grade along the bluffs and past the paper mill, before heading up the hill to the west.
This stretch of shoreline was once a barrier beach, separating Kah Tai Lagoon from Port Townsend Bay. Now it's the main road into town, a large marina and boat yard, and a lot of commercial real estate. There used to be a curved railroad trestle extending offshore to a barge dock, but the last portions were removed a few years ago and the small riprapped headland in these shots was where the trestle began.
The beach was once continuous along the northern shore of the bay, with longshore transport generally to the north and east. Sediment derived from bluffs in the vicinity of Old Fort Townsend would have passed here on its way towards Point Hudson and eventually to Point Wilson. In some communities, historical development completely obliterated the foreshore, but here in Port Townsend the old wharves and and made land simply broke the shoreline into a complex series of pocket beaches. They are still neat beaches - and wonderful opportunities - they're just not part of a larger system the way they once were.
What caught my eye was the contrast between the two adjacent beaches. The longer sandier beach on the east and the small, coarse gravel beach tucked up against the promontory. Is there some process keeping these beaches segregated and different - perhaps different responses to waves from differing directions? Or is this a historical artifact, just waiting for time to blend these beaches into a single curve? Maybe the beach is still responding to the change in wave regime that accompanied the removal of the old rail trestle.
Wednesday, March 27, 2013
For better and more up-to-date information on the geology of this landslide, see:
DNR's Ear to the Ground: Whidbey Island Coupeville Landslide
Early this morning (4:15AM, 3-27-2013), a very large landslide occurred up at Ledgewood Beach on Whidbey Island. This neighborhood is no stranger to slides, but nothing like this has happened in a long time. The slide was a large rotational failure within the much larger landslide complex that extends along this stretch of shoreline. It may have been 700-800' long and several hundred feet deep, with the head scarp stepping back several tens of feet into the 200' high upland surface. The failure surface - based on what we've seen on the beach elsewhere along here, but confirmed by today's observations - is a little below sea level.
KING5: Huge landslides damage, threaten homes on Whidbey Island
Previous Post on Ledgewood Beach (April 2006)
For a beach wonk, the toe was the coolest part. The shoreline along here was relatively straight prior to the slide and is now marked by a bulge that extends more than 100 feet into Admiralty Inlet. This is not just a big pile of debris from the slope above, but rather it is the uplifted toe of the rotational landslide. Distorted dark gray clay is pushed up along the base (the tide was too high to see the lowest portions of the toe), but as the clays pushed up, they simply raised the beach into the air. The toe bulge is mantled by a remarkably intact sand and gravel beach. Barnacle covered boulders sit on the surface. The driftwood still marks yesterday's shoreline, which just happens to be 20' higher and 100' seaward of where it was a few hours ago. I could still follow a discontinuous band of dead eelgrass and beach wrack, marking an earlier high tide. Sections of the raised beach were broken by tension cracks, or arrayed in a series of terraces (drift logs on the top step, sand and gravel on the middle steps, cobbles on the lowermost steps).
I've posted several entries recently about deep-seated landslides:
South Whidbey State Park, February 2013
Camano Island State Park, November, 2012
These geologic landscape are best reserved for open space or parks, not subdivisions. In this case, we were very lucky that only one or two homes along the lower bench were affected -- in other places along here it could have been much more serious.
Monday, March 25, 2013
This post encompasses a longer stretch of shoreline already addressed in the two previous posts and is mainly just an excuse to show the variety of seawalls I encountered on my Sunday afternoon stroll. Some of this shoreline has been developed for a long time, so some of these structures are very old. Some reflect recent tweaks to older walls. And some are relatively new replacements of bulkheads built decades earlier.
There is one common theme to all of these pictures. The upper beach - the part that we walk on at higher tides and relax on in nice weather - is gone. No dune grass. No drift logs. No madronas or big leaf maples hanging out over the beach, creating shade or a place to tie a swing. The walls, and the reclaimed land behind them, have created beachfront property while at the same time eliminating the beach!
For a shoreline once characterized by eroding coastal bluffs, spilling gravel and sand onto the beach, Seattle has remarkably few of these places left. They exist on the north and south sides of Discovery Park and there is a short stretch on the south edge of Magnolia Bluff. And there is this one elegant bluff left between the developed shorelines of Arroyos Beach and Seola Creek. Elsewhere they've been built on, excavated away, or locked behind seawalls.
Gravel was running in a continuous stream down the bluff while I watched - gradually adding to a talus cone on top of the drift logs. Next door, the remains of someone's bluff-top backyard perch had landed in a jumble. The views from the homes along Marine View Drive must be spectacular, made more vivid by the excitement of participatory geomorphology.
The beach south of the Arroyos, all the way down to Seahurst Park in Burien, is marked by a broad sandy low tide terrace. But to the north, the upper beach becomes coarser and the low tide terrace diminishes - it downright disappears in the vicinity of the previous post where the bathymetry drops off precipitously. Longshore transport here is to the north (southerly waves dominate along this shoreline), but I suspect it's hard for sand to make it past the Arroyos without getting lost into deep water close to shore. I wonder if that's why the beaches seem so different to the north?