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-
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-
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-
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-
Photo credits: P. Loubere