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It's exactly one mile from Lip Lip Point southwest to Kinney Point. The largely glacial bluffs start and end high, perched on the 2-meter thick layer of the Tertiary sedimentary rock we've been following around the island. The bluff drops to a mere 5-meters or so in the central area, where the glacial till swings down to beach level (following a valley in the bedrock surface?)
AERIAL VIEW
This line of bluffs runs in a straight line. More intriguingly, it's almost perpendicular to the maximum fetch (Edmonds is 30 kms away to the southeast). It's not the only long straight stretch of bluffs in Puget Sound that lines up this way (Scatchet Head 2010). Perhaps this is happenstance, but I think the same search for balance that causes pocket beaches to become swash-aligned should also favor bluffs to do so.
Continuing my July 4th walk around the south end of Marrowstone Island. These are interesting bluffs and ones that most folks will never see, so there are several more short posts coming.
Lip Lip Point is where the eastern shore of Marrowstone begins to turn west. It is underlain by more of the Tertiary sandstone we saw at Nodule Point (previous post), which outcrops on the beach, but the bluff is almost all Pleistocene. This is a remarkably exposed location - large fetch from the south and nothing to keep the beach sediment from skittering north -- normally a recipe for rapid erosion. But the point is anchored by the bedrock.
The bluffs in each direction from Lip Lip Point are quite different. There's a big divot in the coastline to the north, between here and Nodule Point. It appears to correspond to a segment of bluff where the bedrock dips below beach level, leaving easily eroded outwash at the base of the bluff. But in the other direction, to the southwest, there is a remarkably long, straight stretch of coastline, with the Tertiary forming a significant bulkhead. See the next post for the rest of the story.
Google Maps: AERIAL VIEW
Ecology Coastal Atlas: 2006 Aerial Photo
From a distance, the eastern and southern shores of Marrowstone might be mistaken for just another long stretch of coastal bluffs carved into Pleistocene sediments, but when you look closer you see that the Vashon till and advance outwash are perched on top of Tertiary bedrock. This Eocene sandstone and mudstone can be followed at the base of the bluff, with a couple of gaps, around most of the south end of the island.
Last year, I paddled south along the west side of the island. The sandstone first appeared as imbricated clasts at the base of the glacial till north of Mystery Bay and as solid bedrock at Griffith Point on the south side of the bay (Mystery Bay 2012).
Walking south from East Beach, the pattern is similar. You begin to notice tan sandstone clasts in the bottom of the till, all imbricated as if scraped off the bedrock by a big sheet of ice moving south (that's what I'm picturing, but I'm not sure it's right). A little farther south you notice that the beach is underlain by a sandstone platform and then the bedrock gradually makes an appearance at the base of the bluff. I suspect the presence of this sandstone revetment may explain the change in the orientation of the coastline in this area, as this stuff probably erodes much more slowly than the outwash farther north.
Nodule Point is named for the volleyball-size concretions (others compared them to bowling balls, I wanted to be different) that weather out of the sandstone. See the links at the end for a more thorough treatment of the geology.
The spherical concretions are reported to be the same basic lithology as the surrounding sandstone, but with the addition of calcareous cement. Small concretions appear to be nucleated on small rip-ups of dark gray mudstone - so I assume the larger ones are, too, but I didn't try to break any open. The concretions were pretty pervasive throughout the sandstone, although they varied in density and size from one place to another. They range from baseballs to basketballs. Many eventually weather out completely, and gather in clusters near low tide.
A little farther south there are two sandstone fins that stick out onto the beach like groins (they are readily visible in the aerial imagery - linked below). The 5-m wide trough between them used to be a diabase (basalt) dike, probably intruded into the sediments during the time that the Crescent Volcanics were making their appearance in this region. The heat of the dike baked and hardened the adjacent 1-2 meters of sandstone. When the more easily weathered volcanic rock eroded out, it left the two hardened spines.
Some of the many photos I took last Thursday are posted here, but I also uploaded many more to:
Nodule Point Photos (4 July 2013)
Google Maps: Aerial View
Ecology Coastal Atlas: 2006 Aerial Photo
For more about the geology of Nodule Point and Marrowstone Island, check out:
Dan McShane, Reading the Washington Landscape
Dave Tucker, Northwest Geology Field Trips (based on Dan's blog, but with some additional geologic background)
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.
It's exactly one mile from Lip Lip Point southwest to Kinney Point. The largely glacial bluffs start and end high, perched on the 2-meter thick layer of the Tertiary sedimentary rock we've been following around the island. The bluff drops to a mere 5-meters or so in the central area, where the glacial till swings down to beach level (following a valley in the bedrock surface?)
This line of bluffs runs in a straight line. More intriguingly, it's almost perpendicular to the maximum fetch (Edmonds is 30 kms away to the southeast). It's not the only long straight stretch of bluffs in Puget Sound that lines up this way (Scatchet Head 2010). Perhaps this is happenstance, but I think the same search for balance that causes pocket beaches to become swash-aligned should also favor bluffs to do so.
