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Smithsonian National Museum of Natural History Logo The Department of Paleobiology - The Life of a Vertebrate Fossil
Level 2 - Track Down Fossils - How Old - It's Relative
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So you think you've found a fossil. Your first duty is to carefully document the context of the fossil, before you take it out of the rock. This means that you find and record clues about your fossil's age and the environment where it was buried. You will need to label the fossil with a field number and record information next to this number in your field notebook or laptop. Clues to look for are:

  • Depth - in general, the lower something is in an outcrop, the older it is.
  • Matrix - the rock above, below, and around the fossil.
  • Other plant or animal fossils nearby.
  • Volcanic rocks near the fossil.
  • Geologic map of the area to tell you the area's age.

Department of Paleobiology staff member, Bob Purdy, in a limestone quarry in South Carolina.

STOP: It's the law. The Law of Superposition says that the oldest sediments and fossils are laid down first and the youngest last. BUT: Sometimes the oldest layers are on top. This happens when:

  • Rock layers are pushed up and folded over (folding).
  • Rock layers crack apart and one part drops lower than another, or the two parts slide past each other (faulting).

Which is older?

  • This horse skull found in sediments at the top of the limestone quarry?

View of the underside of a Pleistocene-age horse skull, showing the palate and upper teeth.

  • Or this whale skull found 15 meters (about 50 feet) below the horse skull?

Basilosaurus, skull of an archaic whale. It is 4 feet long.

  • The rocks at this quarry were laid down according to the Law of Superposition and are not faulted or folded.

ANSWER: Since the sediments have not been disturbed, we can use the Law of Superposition to determine that the whale skull is older than the horse skull. However, we cannot say how much older without more evidence about how long it took to accumulate the 15 meters of sediment that separate them.

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