Monday, May 06, 2013
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.
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!