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Relative Time Relative Time • When looking at exposed rock, the layers are always sequenced from oldest to youngest. • Relative dating does not tell how old the rocks are, only the order in which they formed. Original Horizontality • Sediment is deposited in horizontal or nearly horizontal layers. • If the layers are tilted or folded, it took place after the horizontal layers were already formed. Original Horizontality Superposition • Within a sequence of undisturbed sedimentary rock, the layers are older on the bottom, and younger on the top. Cross-cutting or Intrusions • A disrupted pattern is older than the cause of the disruption. • Ex) Faults, dykes, canyons, batholiths, laccoliths, sills. Intrusions Faunal Succession • Fossil species succeed (follow) one another in a definite and recognizable pattern. • Because we can track the changes of a particular species through the fossil record, faunal succession is evidence supporting the Theory of Evolution. Unconformities • Unconformities are the exceptions to the four principles of relative time. • There are three different types of unconformities. Disconformity • Older rocks have eroded away, and a new layer has formed on top. This is the hardest type of unconformity to detect. • It is usually found only by studying fossils from the layers above and below the unconformity. Angular Unconformity • A new layer of rock is deposited over a layer that has been tilted or folded and then eroded. a. New sedimentary rock is formed. b. Something folds or tilts the layers. c. Erosion occurs. d. A new layer of rock forms over the erosion surface. Nonconformity • An erosion surface on metamorphic rock has been covered by younger sedimentary rock. • This shows that erosion has occurred for a very long time before a new rock layer formed because metamorphic rock is usually found deep in Earth’s crust.
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