In , a young doctor named Nicholas Steno was invited to dissect the head of an enormous great white shark that had been caught by local fisherman near Florence, Italy. Figure While it may seem obvious today, most people at the time did not believe that fossils were once part of living creatures. The reason was that the fossils of clams, snails, and other marine animals were found in tall mountains, miles from any ocean. Two schools of thought explained these fossils. Some religious writers believed that the shells were washed up during the Biblical flood.
Relative dating — Science Learning Hub
The most obvious feature of sedimentary rock is its layering. This feature is produced by changes in deposition over time. With this in mind geologist have long known that the deeper a sedimentary rock layer is the older it is, but how old? Although there might be some mineral differences due to the difference in source rock, most sedimentary rock deposited year after year look very similar to one another.
The Law of Superposition states that in a layered, depositional sequence such as a series of sedimentary beds or lava flows , the material on which any layer is deposited is older than the layer itself. Thus, the layers are successively younger, going from bottom to top. The convention in geology is to number the layers beds within a sequence such that the oldest layer has the lowest number. In the illustration, layer 1 was deposited at time 1.
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