Unconformities
The concept of an unconformity arises from two of the oldest principles of geology.
The Law of Original Horizontality: Layers of sedimentary rock (strata) are originally laid down flat, parallel to the Earth's surface. That's the law of original horizontality.
The Law of Superposition: Younger strata always overlie older strata, except where the rocks have been overturned. That's the law of superposition.
So in an ideal sequence of rocks, all the strata would stack up like the pages in a book in a "conformable" relationship. Where they don't, the plane between the mismatched strata—representing some sort of gap—is an unconformity. There are four main kinds of unconformity.
The Angular Unconformity
The most famous and obvious kind of unconformity is the angular unconformity. Rocks below the unconformity are tilted and sheared off, and rocks above it are level. The angular unconformity tells a clear story:
First a set of rocks was laid down.
Then these rocks were tilted, then eroded down to a level surface.
Then a younger set of rocks was laid down on top.
The Disconformity
Now, omit the second step: strata are laid down, then a period of erosion happens, then more strata are laid down. The result is a disconformity or parallel unconformity. All the strata line up, but there is still a clear discontinuity in the sequence—maybe a soil layer developed on top of the older rocks, or a rugged surface where they were eroded.
The Paraconformity
If the discontinuity is not visible, it is called a paraconformity. These are harder to detect, as you might imagine. A sandstone in which trilobite fossils suddenly give way to oyster fossils would be a clear example.
The Nonconformity
There is a body of rock that is not sedimentary, upon which strata are laid down. Because we aren't comparing two bodies of strata, the notion of them being conformable doesn't apply. This kind of junction between two different major rock types is a nonconformity.
Logging as a find:
Please send me a note with the following:
•The text "EC 1000 Steps of Unconformity" on the first line
•The people in your group you are submitting answers for
1) What kind of unconformity do you see in the ridge to the north?
2) Go to the additional waypoint. Is this the same type of unconformity?
3) Is the second waypoint a bigger or smaller example of unconformity?
(Optional) 4) Any pictures of your adventures you can post in your "found" log
Happy (earth)caching!!
Sources:
-http://www.geocaching.com/geocache/GC543MV_crystal-cove-a-gap-in-time?guid=be199379-a7c1-4834-8b05-6b90c3570d4a
-http://geology.about.com/od/geoprocesses/a/unconformities.htm