Urban Unconformity
A
cache
by Windkissed (Adopted from TerryDad2)
Hidden:
11/20/2006
Size:
 (Not chosen)
Difficulty: 
Terrain:
(1 is easiest, 5 is hardest)
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This location brings you to one of the last remaining exposures of
Timms Point Silt unconformablely overlain by Pales Verdes Sand in
San Pedro. It is right next to the street in a residential
neighborhood.
 An unconformity is a contact between rocks of
different ages that represent a period of geologic history
with no rock record. This can either be from no rock being
deposited or from rock being eroded away before more rock
forms on top of the eroded surface.
Geologists classify unconformities based on the orientation of
layers of rock above and below the contact. There is
paraconformities where the layers of rock above and below the
contact are roughly parallel to the contact. In a disconformity the
layers above and below the contact are also roughly parallel, but
the contact itself is not parallel to layers, but instead cuts
across the layers or cuts down into the rock like a river channel.
However neither of these describes the contact at this
location.
Here the layers of rock below the contact, the Timms Point Silt
are tilted at an angle and are cut of horizontally by the contact.
Above the contact is the roughly horizontal Pales Verdes Sand. This
type of contact is called an angular unconformity.
This configuration of rocks began in the Lower (?) Pleistocene
(about 700,000 years ago, although the literature does show a wider
range) with the deposition of the Timms Point Silt in about 151 to
600 feet of water. These silts were then buried and turned into
rock. Tectonic forces then brought the layered rock to the surface
tilting the rock in the process and where it was eroded. Then the
ocean covered the area depositing the Pales Verdes Sand in the
Upper Pleistocene (about 86,000 to 130,000 years ago) on top of
what was left of the Timms Point Silt. The unconformity represents
the time between the deposition of the Timms Point Silt and the
deposition of the Pales Verdes Sand, some where around 570,000
years. The rock that was deposited during this time period, if any
was, was eroded away leaving no geologic record of what happened in
the area.
Logging requirements:
Send me a note with :
- The text "GCZF18 Urban Unconformity" on the first line
- The number of people in your group.
- Take a close look at the contact and see if the contact is flat
or if there are holes in it.
- suggest possible reasons for your observation
The above information was compiled from the
following sources:
- Late Pleistocene angular
unconformity at San Pedro, CA, J. Douglas Yule, and Donald H.
Zenger, Department of Geology, Pomona College, Clarement, CA 91711,
Geolgocal Society of America Centennial Field Guide – Cordilleran
Section, 1987
Additional Information: There are additional exposures in
the alleyway west of the site that show the Lomita Marl. The San
Pedro Sand was exposed to the west, but has been removed by
construction. The Pales Verdes Sand is marine terrace sediment
derived from a transgression that carved out a number of marine
terraces in the area. The dates referenced above have been
simplified. The literature provides a much wider range of dates
based on a variety of methods. A variety of fossils are present in
these outcrops
Additional Hints (
Decrypt
)
Decryption Key
A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z
(letter above equals below,
and vice versa)
|
Gur rebfvba bppherq ba gur obggbz bs gur frn sybbe. Jung qb lbh svaq qvttvat gurer?
(Decrypted Hints)
Find...
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 November 11 by birdcachers05 (1891 found)
We decided to come down to the San Pedro/Palos Verdes area from San Bernardino to pick up some Earthcaches and a Virtual that are on the map in the area. The primary focus is to collect enough Earthcaches and Virtual caches to celebrate our 100th Earthcache find and 200th Virtual find in Hawaii next year. This particular trip landed us our 75th Earthcache find, which we chose the Portuguese Bend Landland to celebrate this cache milestone.Here we are in the middle of a residential neighborhood, and who would have guessed that we could have experienced this. We found a similar themed Earthcache near Vegas, but this one was much more impressive due to the development around the entire neighborhood. Really amazing. Thank you for bringing us here, and thank you for the cache!
[view this log] | October 19 by JRMan (1208 found)
Very cool layer of history. Thanks for bringing me here.
[view this log] | October 15 by Doggleboone (3221 found)
This was very cool. Wish we had a geologist's understanding of what we were looking at, but it was really cool even in our limited understanding. Thanks fro leading us to this spot, and for broadening our understanding of the earth we live on. Sending the info in private email.
[view this log] | September 15 by Team Weathergirl (2312 found)
Another very interesting spot I'd never seen when we lived out here. I've seen various unconformities in other earthcaches. It is interesting to see this one in the middle of an urban setting. TFTC.
[view this log]
| September 5 by chaosmanor (3352 found)
How cool is this?!? We've done 15 ECs before this one, but this has to be near the top of the list  Jim of the team (he was the one out with Webfoot today) took all of the geology courses offered at Riverside CC 20 years ago; I learned so much then, and it is always a blast to be able to use a little of that on these Earthcaches.In fact, caching with Webfoot is a real treat because (amongst other reasons, including a mutual appreciation for In-N-Out burgers  ) we tend to feed off each other (but not the burgers!!), and we both are jazzed by ECs. This cache was one of several focal points for our day, and it did not disappoint  I can't go into the details too much without giving away the answers; suffice it that we spent 15-20 minutes here, going over all the main features, and the various little details that can make these "geological virtuals" so intriguing. Answers have already been sent. Thank you very much for keeping this one going by adopting it from TerryDad2, and thanks to him for setting it up  It was a tough call which of today's caches we would use as our qualifier for the San Pedro Quad in the LA County challenge, as they all were neat, in different ways, but this one is going to be the one.
[view this log] | |
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Current time: 11/22/2009 3:30:06 AM
Last Updated: 11/19/2009 4:02:53 PM
Rendered: From Database
Coordinates are in the WGS84 datum
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