There are two locations for this earthcache. The first, a
point along a beach side bike path, is an example of the
current shoreline where you will learn to recognize the rocks
that are found on a marine terrace. And the second, a road cut
in Grant Park, will show you a marine terrace that has been
shoved up a couple of hundred feet above sea level.
The current marine terrace can be seen at the first coordinates.
A marine terrace is a flat area at about sea level often at the
base of cliffs. The marine terrace typically slopes down into the
ocean at an angle of 1 degree. This flat area is constantly being
widened by erosion at the base of the cliffs. Ocean waves erode
material at the base of the cliff. One of the most effective
mechanisms of erosion occurs when strong winter storm waves pick up
the rounded cobbles at your feet and pound them against the cliff
base.
Image source: SDNHM
This process creates notches that undercut the cliff. When the
notch gets deep enough, a portion of the cliff becomes unstable and
collapses. Waves then pull the collapsed material off the beach and
redistribute it.
Typically these beaches have quite a bit of sand on them that is
moved around by the seasonal variability in ocean wave strengths.
However, this beach has very little sand due to a combination of
its shape (it sticks out into the ocean and thus waves hit it more
directly) and the sand that usually is delivered to the beach by
the Ventura River has been blocked by dams on its tributaries (See
Matilija Dam Removal Earthcache).
Older marine terraces will have similar features, an abrasion
platform and cliff, however these features will be at a higher
elevation and likely covered by eroded material. This can be seen
at the listing coordinates.
The road cut in Grant Park shows a distinct layer of rounded
cobbles above a sandy material. These cobbles were once the
abrasion platform for an ancient shoreline. The cliffs that were
above are likely on the switchback of the road above and have been
eroded down into a steep slope.
Now the question comes up of how did these ancient shorelines
get so far above sea level. There are three possibilities.
- Sea levels were much higher in the past and gradually came
down.
- The land was lower and gradually came up
- A combination of the two
First we can look to see if sea levels were higher when these
sediments were deposited. The age of the rocks in the Ventura area
have been estimated using a layer of volcanic ash from a volcanic
explosion in Yellowstone Park about 600,000 years ago. Since there
is about 3,000 feet of additional sediment above this layer of ash,
the youngest rocks must be much younger, n the order of 200,000
years old. The history of sea level during that time shows that sea
level was not some 200 feet higher, so the first possibility is not
a likely explanation of how a marine terrace got to be so far up on
the hillside.
That leaves the explanation that the land was lower and
gradually came up. In the gullies that have eroded back into the
road cut, you can see that the contact between the cobbles and the
sand under it is sloping at a greater angle than about 1 degree.
This is evidence that the sediment has been tilted. In fact these
sediments are part of the
Ventura Avenue Anticline (Earthcache). Tectonic forces have
been pushing sediments in this area up into an arch. That accounts
for some of the elevation gain. Movement along faults accounts for
the rest.
From the switchback or Padre Sierra’s cross you can look west
across the Ventura river and identify a series of elevated marine
terraces based on the topographic pattern from the shore line
inland of gently sloped flat areas, then a short steep cliff, then
another gentle slope.
Logging requirements:
Send me a note with :
- The text "GC1955N Ventura’s Elevated Marine Terraces" on the
first line
- The number of people in your group.
- at the first coordinates, is there a sea cliff and if there is
how high is it?
- Compare the size of the cobbles on the current abrasion
platform to the elevated platform
- How thick is the elevated abrasion platform (the cobble
layer)?
- What is the approximate angle that the elevated platform is now
tilted?
The above information was compiled from the
following sources:
- Robert P. Sharp and Allen F. Glazner, Geology
Underfoot in Southern California, Mountain Press Publishing
Company, 1993
- San Diego Natural History Museaum (SDNHM), San
Diego Ancient Shorelines, Thomas A. Deméré, Ph.D., Curator of
Paleontology, illustration by Bradford O. Riney,
http://www.sdnhm.org/research/paleontology/sdshoreline.html
- Wikipedia, Wave-cut platform
http://en.wikipedia.org/wiki/Wave_Cut_Platform