Uniformitarianism - can be paraphrased as the present is the key to the past.
Superposition - Principle which states that the oldest layers are those that were deposited first, and hence deepest and that the youngest layers are stratigraphically higher.
Original Horizontality - sedimentary beds are deposited approximately horizontally. It is only after deposition that the beds may be tilted or folded.
Original Lateral Continuity - at least on a local scale, beds will be laterally continuous. This principle is used to correlate beds on opposite sides of a valley.
Cross-cutting Relationships - if unit A is cut by unit B (A may be a dike intruding the sedimentary beds of unit B), then A must be younger than B.
Inclusions - very similar to the Principle of cross-cuting relationships in that inclusions (fragments) of A included in unit B indicate that unit A is older than unit B.
Walther's Law of Facies - when depositional environments migrate laterally, sediments of one environment will lie on top of sediments of another environment. Can be paraphrased as "what you see laterally, you should see vertically."
Faunal Succession (guide/index fossils) - fossils will evolve
with time so that during a particular time period certain fossils may be
used to indicate that time.
Unconformities
Conformable vs. Unconformable - units are conformable if they have no break in time across them. If there is a break in deposition, then an unconformity is present.
Angular unconformity - a break in deposition caused by beds having been folded or tilted. This will cause the beds above and below the unconformity to lie at different angles, hence its name.
Nonconformity - an unconformity between either igneous or metamorphic rocks and sedimentary rocks.
Disconformity (paraconformity) - an unconformity that at first glance doesn't look like an unconformity. Beds above and below the unconformity are parallel to each other. However, based on guide fossils or dating, there is a distinct break in deposition.
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URL http://www.ux1.eiu.edu/~cfjps/1300/principles.html
Revised 9/6/99
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