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The field trip kick off

Group introduction to the coastal geology by Varn Brooks

Starting Point:

Beach access at the end of E. Beach Road just to the north of Nordland, Marrowstone Island

Then hiking south down the beach towards Nodule Point

Pleistocene sediments of the bluff scape

Diamict overlying Vashon glacial cross-bedded outwash (sediment is poorly sorted mix of silt to cobbles, a product of mass deposition

Erosional contact

Outwash sequence (Esperance Sand) produced by river and stream transport of sand from a glacier front (active during last glaciation which peaked around 20,000 years ago)

 

Slope fall, sand draped over the foot of the bluff

 

 

 

Base of bluff merges to beach and is worked over by waves at high tide and during storms

An information source for the field trip:

Dave Tucker's field trip guides at http://nwgeology.wordpress.com/the-fieldtrips/geology-trip-to-nodule-point-marrowstone-island%C2%A0washington/) 

 

Stratified outwash vs. Massive diamict

The difference between reworking and transporting vs. dumping

Nodule Point

Boulder conglomerate at the unconformity between the overlying Pleistocene and the underlying Scow Bay sandstone (mid-late Eocene; 45 million years old)

Boulders are mostly of the Scow Bay with this spot looking like the remains of  a scree slope deposit (rocks sliding down a hillside).

An unconformity is an erosional landscape surface leaving no deposit to record the passage of geologic time

 

 

 

 

Vashon Pleistocene outwash above

 

 

 

Unconformity

 

Scow Bay Sandstone below

The weirdness of the nodules at Nodule Point

The nodules are in fact concretions.  They formed from diagenetic chemical reactions which occurred in the pore waters moving through the sediments after they were deposited.  The reactions controlled the precipitation of minerals in spherical patterns giving differing cohesion to the rocks within the spheres and creating the weathering features that we see

Basaltic conglomerate or breccia

Stratigraphic position unknown, but adjacent to the dike at Nodule Point

The slot that used to be a basalt dyke

The walls here are of baked Scow Bay sandstone which is resistant to weathering.  The basalt mineral assemblage being more unstable at earth surface conditions has broken down more quickly than the baked rim.

The Scow Bay sandstone is arkosic with tuffaceous shale, suggesting a convergent tectonic setting.  It overlies and may interfinger with the underlying Crescent basalt lava flows.  These are part of a massive pile of ocean floor basalt volcanics which were incorporated into the margin of N. America in Miocene time (7-12 million years ago), leading to the uplift of the Olympic mountains.

Photo credits:  P. Loubere