Tuesday, May 28, 2013
Seattle's Blue Ridge neighborhood lies on the hills that rise from the beach north of Ballard. The community even has its own private beach access, but Sunday's low tide (-3.6' MLLW, so one of the year's lowest) made this a much more public beach, with a lot of folks walking between Carkeek Park and Golden Gardens.
A couple of small streams - not sure what distinguishes a stream from a stormwater outfall in these urban areas - emerge from culverts beneath the tracks at the Blue Ridge access. The larger one takes a sharp, but brief, right hand turn, before turning again down the beach. I guess this suggests drift, or at least recent drift of the sandier material, is northward, although I think the larger pattern is the other way, since this area gets some shelter from the south.
The railroad dominates the upper beach - pretty much replaces it, actually - except for a narrow backshore right around the stream mouths. But the lower beach goes out forever, or so it seemed today, when low tide sort of merged into the gray day.
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.