Friday, July 04, 2014

Otter Rock

Welcome to Oregon! This will be the first of many posts from beaches on the Oregon and northern California coasts that I'll dribble out over the next week or two. I should probably be more reserved, but a week of good weather on a spectacular coast leave me little choice! Okay - it's still a little gray in these pictures, but it gets better!

There are a few common themes that will dominate this series. One is the persistent and variable marine terraces that define much this coast. Another is the compartmentalization of the coast into cells - ranging from long strands to tiny pocket beaches - separated from one another by resistant rocky headlands. Within these broad themes are an endless variety of small stream mouths, elegant barriers, mixed sediment beaches, and human interactions. And all sorts of bedrock features, like seastacks, eroded platforms, and occasional punch bowls.

Otter Rock and Otter Crest are perched on a relatively narrow marine terrace just south of Cape Foulweather. This is the most recent, and most obvious, terrace, along this coast (not counting the future terrace forming at sea level today).  It probably dates to a prolonged high stand of the sea about 80,000 years ago. Note that sea level at that time was roughly what it is today and that the raised elevation of the terrace reflects subsequent uplift of the coast.  It turns out that variability in the height of the terrace along the coast is an excellent indicator of the local uplift rates.

The picture shows the nice angular unconformity developed on top of the tilted sedimentary rocks. This planar surface was the wave cut platform - akin to the modern one in the surf zone. When the terrace was abandoned (by falling sea level and the uplifting coast) it was covered with a think sequence of beach, dune, and alluvial deposits, which is the lighter band on top.

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