Tuesday, July 08, 2014
The theme of this post is sea stacks (or seastacks). From Cape Arago north, the terrace has been composed of Tertiary sedimentary rocks, but moving south we encounter Jurassic metamorphics at the coast. Here in Bandon, they are scattered across a beautiful broad sandy beach.
Sea stacks are simply the leftover bits of a receding coastline. They can occur on any rocky shoreline, but I suppose they are more common in places where geologic differences allow some areas to erode much more easily than others.
This can happen in heavily jointed or fractured areas, but it can also occur where chunks of more resistant lithology are surrounded by softer rocks. In California, this can occur when resistant blocks of Franciscan form knockers that get left behind. Or in other places where volcanic rocks are surrounded by more erodible sediments. You can probably get stacks even in uniform geology, but it seems like it would be less likely.
I have no idea what controls the formation of these particular stacks - differences in lithology within the metamorphic rocks?
Another interesting observation about this area. Komar and colleagues observed in the 1990s that slower cliff erosion rates in this area (evidence includes gentle vegetated slopes and intact talus) was consistent with modern uplift along this shoreline, which tends to reduce wave action at the toe.