Sunset Bay Ghost Forest EarthCache

Sunset Bay Ghost Forest by Steve ‘n’ Nancy

For best viewing of this Earth Cache try to arrive as near to low tide as possible.  Please be aware that most all smartphones do not have a signal here.  It is best to save this from the Geocaching app to view off line.  Or you can do the original way by printing this page and taking it with you.

Answer the 3 questions below and message them to me within a week of claiming this cache.  There is an abundance of geology at this location.  No photos are required but if you see something interesting, we’d love for you to share your pictures.

Sunset Bay opens to the Pacific Ocean where the North American and the Juan de Fuca tectonic plates are locked together.  The Juan de Fuco Plate is being overridden by the North American Plate which is known as the Cascadia Subduction Zone.  As the plates exert pressure on each other the stress causes uplifts and falling when the pressure is released. In geology, this falling is known as subsidence.[1]

There have been repeated cycles of geographically rapid rising and falling caused by earthquakes. Native American stories include episodes of ground movement and giant waves that probably describe ancient earthquake and tsunami events. One example comes from a tale from the Hoh and Quileute tribes of Washington’s Olympic Peninsula. In this story, a battle between two supernatural beings, Thunderbird and Whale, caused the earth to tremble and the ocean waves to rise up.[2]

While the stress is building up the overriding North American Plate can rise at rates that can exceed 4 millimeters per year.   During the earthquake, the pressure will be released causing the overriding plate to fall back to its original elevation.  Because of the duration of these cycles the vegetation has time to become established as they rise slightly above sea level but then drown as it returns to its original level.  Oregon’s coastline reveals layers of peat, formed from the vegetation, alternating with mud-rich intertidal deposits.  You will also find some ghost forests, stands of dead trees rooted only a few feet above sea level and drowned by subsidence during a major earthquake.[3]

From the listed way-point, look to the south to where Big Creek feeds into the bay.    If the tide is low enough you will be able to see the stumps Sunset Bay’s Ghost Forest covered by moss.  At extreme high tides and storms DO NOT attempt to approach the roots.  But at mid to low tides you can walk out to and touch the roots of the Ghost Forest that was once growing here before the last big earthquake.  The Ghost Forest is Sitka Spruce that has been exposed in the tidal zone here at Sunset Bay.  Carbon-14 dating indicates these trees died some 1200 years ago, in a forest that was just above high tide during an interseismic period. After an earthquake, the land subsided below sea level and the trees drown.[4] They have since been exposed over several earthquake periods.

1.     Which two tectonic plates are meeting west of this sight in the Pacific Ocean?

2.     According to geological studies what age are these tree roots?

3.    At the south bay opening there is ridge that runs from the top corner at an angle down to sea level.  Estimate the Angle of descent.