skip to main |
skip to sidebar
Marrowstone Island's East Beach Park is a wonderful place to watch ships cruising in and out of Admiralty Inlet. It's also a good starting place for a walk to Nodule Point and the beaches at the south end of the island. But it's a long walk, so I used my bike as a shuttle, allowing me to make my July 4th 5-mile hike a one-way excursion.
The east side of Marrowstone Island is a continuous bluff from the south end almost all the way to Marrowstone Point at the northern tip, except for East Beach, where a broad sandy foreshore has built seaward of the low bluff. It's basically a northward trending spit that diverges from the coastline, then reconnects a short distance north.
I like the terms fringing and looped barriers for these things, but whatever you call them, these wide places in the beach are pretty common on the Salish Sea. Like most barriers, they come in many sizes and shapes. They often form where an abundant sediment supply coincides with a small bend in the shoreline (they occur downdrift of the bend). Sometimes they are a little hard to recognize, since they were prime development spots and are often covered with homes.
The gravel ones tend to be fairly persistent; the sandier ones are much more dynamic. This combines a little of each. The beach and the bars on the lower beach are constantly changing -- the little concrete wall that protects the parking lot has been undermined at times in the past, but this year the beach is way out in front of it.
Google Maps: Aerial View
Ecology Coastal Atlas: 2006 Oblique Aerial Photo
At a relatively high tide, walking the beach at West Point below Discovery Park one might be struck by the relief - a steep gravel beach and a much higher and steeper bluff. But when the tide is 3' below Mean Lower Lowe Water and one walks out to the receding water's edge, the high bluff and the thin strip of gravel sort of fades in the distance.
Previous posts:
November 2009
February 2012
West Point: Coastal Care -- November 2011
AERIAL VIEW
These broad sand flats south of West Point are spectacular. Spring time low tides are often an excuse for bus loads of school children to decimate large populations of vulnerable fauna, but really, the best part of these midday spectacles is walking barefoot through sand and eelgrass on gently rolling sand bars. You feel you could almost walk across the Sound - as long as you ignore the fact that the water is almost 1000' deep between here and Bainbridge Island.
The south side of Discovery Park shares the same deep-seated landslide complex that characterizes most of the southwest shore of Magnolia - the difference here being that the slide is sort of doing what it always did, whereas elsewhere in Magnolia, great efforts have been taken to stabilize the slope enough to build homes on top of it.
But here in the Park, we get a glimpse of a big landslide in action. The slope is sliding on clay at, or slightly below beach level. This is a common scenario for these deep slides and the symptoms are often similar.
Previous Posts:
Discovery Park: February 2010
West Point: February 2012
The beach (way out onto the terrace) is a mosaic of subtle ridges of cobble and clay, likely artifacts of past movement on the slide as the coastline has retreated. Although there are some intact outcrops of clay on the beach, most show signs of uplift, tilting, internal deformation, and a general tendency towards soft and gooey.
The eroding bluff at the toe of the slide consists of sandier material and large wood that has been incorporated into the slide mass as it has gradually moved towards the beach.
Magnolia Bluff is both some of Seattle's premier real estate and one of Puget Sound's best known deep-seated landslide complexes. The homes along Perkins Lane are held in place with expensive public drain systems (some dating to the WPA in the 1930s) and at least in some cases, expensively engineered foundations and retaining walls. Occasionally, these things aren't enough and the homes don't stay put.
AERIAL VIEW
The views are spectacular, but the beaches leave something to be desired. There is very little sand and gravel - most of it where seawalls are sufficiently far back to allow a remnant of the original upper beach or where sediment has been trapped on the updrift, southeast side of old groins. Most of the beach is coarse gravel and cobble, too stable to get rolled around under most wave conditions. It is unlikely that this beach was so starved of finer beach sediment 100 years ago, since the eroding bluffs should have provided plenty of it. But today, it's really hard to figure out where that sediment would come from.
Maury Island cuts eastward across the north-south grain of the Puget Sound landscape, begging questions about both why it's here and its effect on glacial processes 16,000 years ago. Point Robinson lies at its eastern tip, collecting sediment that is transported up the southern shoreline of the island (where it merges with a much smaller contribution from the northern shore of the island).
AERIAL VIEW
Point Robinson is a small cuspate foreland and like many other similar features, its location and its morphology lend themselves to a lighthouse.
Point Robinson: January 2010
Tramp Harbor lies on the eastern shore of Vashon Island, just south of Ellisport and Point Heyer (KVI Beach), and just north of the Portage, the narrow isthmus (historically, a tombolo) that connects Vashon and Maury Island.
AERIAL VIEW
Like many other early roads on Puget Sound, Dockton Road (George Edwards Road on some maps) was built on the beach. It was fairly simple to construct above normal high tides - often on piles, later on fill - and it was certainly easier than the alternative, which was building roads through steep, unstable, and heavily forested terrain.
These shots capture a simple point about bulkheads and beach fill. While most walls bury the uppermost beach and tend to isolate the marine and terrestrial environments, some transform the beach much more than others.
These walls all differ in their waterward extent, which is as much about the amount of fill behind them as about the character of the wall itself. The first picture shows the base of the old wharf/pier, a rectangular mole extending to mid-tide (not counting all that rock dumped in front). Not much beach!
This next one shows the roadway itself, protected by an old wood bulkhead. The upper beach is missing - along with the drift logs and the natural bluff toe. Old riprap and quarry spalls cover what's left of the beach.
The following photo shows a section of wall that was replaced a number of years ago. Still no upper beach, but the design reduces the impact on the beach somewhat, and some of the rock debris on the lower beach has been cleaned up.
The last picture is a residential rock seawall at the north end of the bay. It is fairly typical of modern era bulkheads on Puget Sound. It was built closer to the toe of the bank, so its immediate impact on the beach is certainly less than the ones described above. But it still eliminates the upper beach, so no drift logs and none of the natural riparian zone that would have once been observed. They lock up sediment on the beach and prevent new sediment from being delivered by erosion. And unlike roads on the beach, these residential structures are much more widespread and are still being built. Regardless of their individual impacts, their aggregate influence on Puget Sound beaches can be significant.
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.
AERIAL VIEW
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.
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.
AERIAL VIEW
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.
AERIAL VIEW
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.
Marrowstone Island's East Beach Park is a wonderful place to watch ships cruising in and out of Admiralty Inlet. It's also a good starting place for a walk to Nodule Point and the beaches at the south end of the island. But it's a long walk, so I used my bike as a shuttle, allowing me to make my July 4th 5-mile hike a one-way excursion.
The east side of Marrowstone Island is a continuous bluff from the south end almost all the way to Marrowstone Point at the northern tip, except for East Beach, where a broad sandy foreshore has built seaward of the low bluff. It's basically a northward trending spit that diverges from the coastline, then reconnects a short distance north. 
I like the terms fringing and looped barriers for these things, but whatever you call them, these wide places in the beach are pretty common on the Salish Sea. Like most barriers, they come in many sizes and shapes. They often form where an abundant sediment supply coincides with a small bend in the shoreline (they occur downdrift of the bend). Sometimes they are a little hard to recognize, since they were prime development spots and are often covered with homes.
