156
We found an access point down onto the landward edge of the marsh and proceeded to our target location. At the time we made a mental note that the graveyard
would be a good target for our exit point to ensure we would be able to find the car.
An hour or more later, after walking through dense marsh grasses and crossing
boot-sucking tidal channels, empty at low tide, we arrived at our chosen site for
the day.
Based on a few hours of site reconnaissance and investigation, the stratigraphy at
this marsh site revealed evidence of at least two or three potential large earthquakes
(Fig. 10.4). On the west coast of the U.S., ‘ghost forests’ are sometimes associated
with earthquake deposits; the forests are ghost-like because of the dead, sunbleached trees that stick up out of the modern marsh (the tree roots are especially
obvious along creeks, river channels, and even some beaches; Fig. 10.5). These
‘ghost forests’ represent a vertically displaced, dead ecosystem, inundated by subsidence associated with an earthquake. Prior to the earthquake, continued convergence between the locked plates causes the overriding plate (North American plate)
to deform and uplift, raising the coastal forest to a higher elevation. Upon strain
release (the earthquake), the overriding plate subsides, causing the coastal forest to
fall to a lower elevation, where seawater inundation creates a marsh or tidal flat.
This in turn kills the trees and salt-marsh sediments builds up around them. After
decades and centuries all that remains are often the weathered white trunks and
roots of ‘ghost forest’. At this particular site there was no obvious ‘ghost forest’ but
the stratigraphy was a stark reminder of how tectonically active this coastline is and
the repeating abrupt changes from dark brown rooted peat (upper salt marsh) to grey
clayey-silt (tidal flat sediments) showed earthquake-related elevation changes of
Fig. 10.5 Neskowin, Oregon ‘ghost forest”. Former coastal forest stumps are now in the tidal zone
following land subsidence caused by a great earthquake in AD 1700. (Image from https://www.
shareoregon.com/things-to-do/en/listings/124939-neskowin-ghost-forest)
A. D. Hawkes
We found an access point down onto the landward edge of the marsh and proceeded to our target location. At the time we made a mental note that the graveyard
would be a good target for our exit point to ensure we would be able to find the car.
An hour or more later, after walking through dense marsh grasses and crossing
boot-sucking tidal channels, empty at low tide, we arrived at our chosen site for
the day.
Based on a few hours of site reconnaissance and investigation, the stratigraphy at
this marsh site revealed evidence of at least two or three potential large earthquakes
(Fig. 10.4). On the west coast of the U.S., ‘ghost forests’ are sometimes associated
with earthquake deposits; the forests are ghost-like because of the dead, sunbleached trees that stick up out of the modern marsh (the tree roots are especially
obvious along creeks, river channels, and even some beaches; Fig. 10.5). These
‘ghost forests’ represent a vertically displaced, dead ecosystem, inundated by subsidence associated with an earthquake. Prior to the earthquake, continued convergence between the locked plates causes the overriding plate (North American plate)
to deform and uplift, raising the coastal forest to a higher elevation. Upon strain
release (the earthquake), the overriding plate subsides, causing the coastal forest to
fall to a lower elevation, where seawater inundation creates a marsh or tidal flat.
This in turn kills the trees and salt-marsh sediments builds up around them. After
decades and centuries all that remains are often the weathered white trunks and
roots of ‘ghost forest’. At this particular site there was no obvious ‘ghost forest’ but
the stratigraphy was a stark reminder of how tectonically active this coastline is and
the repeating abrupt changes from dark brown rooted peat (upper salt marsh) to grey
clayey-silt (tidal flat sediments) showed earthquake-related elevation changes of
Fig. 10.5 Neskowin, Oregon ‘ghost forest”. Former coastal forest stumps are now in the tidal zone
following land subsidence caused by a great earthquake in AD 1700. (Image from https://www.
shareoregon.com/things-to-do/en/listings/124939-neskowin-ghost-forest)
A. D. Hawkes