Illustration from Steno's 1667 paper comparing the teeth of a shark head with a fossil tooth

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Nicholas Steno (a Dane, 1638-1686) was one of the first scientists to recognize that fossils were the remains of living organisms, but his most important contributions to geology were several defining principles for stratigraphy (the study of the layering in sedimentary rocks):

  1. Superposition: older rocks occur below the younger rocks.  When geologists depict time, they usually use the "rock column", and the oldest time goes on the bottom of the depiction.  Where superposition is violated, and older rock lie atop younger, geologists look for an explanation is terms of structural disruption of the rocks through folding or faulting.
  2. Original horizontality: sedimentary rocks are originally deposited in horizontal layers.  While there are exceptions (cross bedding in dunes, deltas, alluvial fans, and undersea fans), other features in those rocks will reveal the original tilting.  Most sedimentary rocks that dip have been folded or faulted by tectonic forces.
  3. Lateral continuity: layers normally persist in the horizontal direction.
  4. Cross-cutting relationships: something which cuts across a sedimentary layer is younger than the layer.

Last revision 7/16/2015