Thursday, July 11, 2013

Nodule Point

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)

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