The Geologic time scale was completed in the mid to late 1800s. What methods were used to date the various subdivisions?

  • 2
    $\begingroup$ This would be a great idea for a book. Here is a short summary : ucmp.berkeley.edu/exhibit/histgeoscale.html. $\endgroup$ Mar 6, 2019 at 17:53
  • $\begingroup$ too broad. stratigraphy? $\endgroup$
    – user12525
    Mar 6, 2019 at 17:57
  • $\begingroup$ The geologic timescale is updated all the time, it is not completed in any sense. Please show some minimal attempt to research this on your own. $\endgroup$
    – John
    Jun 29, 2022 at 2:38

1 Answer 1


Here is an answer that matches the wording in this question.

Nice summary from http://hyperphysics.phy-astr.gsu.edu

Principles for Relative Dating of Geological Features From over two hundred years of careful field explorations by geologists, a number of practical principles for determining the relative dates of geologic features have emerged. The assignment of numerical ages to these relative dates had to await the development of radiometric dating.

  • Law of Superposition**: For sedimentary rocks, each bed is older than the one above it and younger than the one below it.
  • Principle of Original Horizontality. Layers of sediment are generally deposited in a horizontal position. This is useful even if beds of sedimentary rock have been subsequently tilted.
  • Principle of cross-cutting relationships. A fault or intrusion is younger than the rocks affected by it.
  • Inclusions: the rock mass containing the inclusions is older than the rock providing the included material.
  • Unconformities: interruptions of sedimentation with removal of material by erosion and then a resumption of deposition can place rock strata in contact that have a gap of time and material between them.
  • Principle of fossil succession: fossil organisms succeed one another in a definite order. The fossils observed help to identify the time period in which the organism lived.

Does anyone know if there a published book that gives a comprehensive history of geologic time scale (looking for more than summary that is presented in countless text books)?

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    $\begingroup$ Maybe this book? The Map That Changed The World by Simon Winchester $\endgroup$
    – Gimelist
    Mar 6, 2019 at 21:31
  • $\begingroup$ You should probably elaborate on that **. Sedgwick got himself in trouble because he didn't understand the effect of overturned folds. $\endgroup$
    – Spencer
    May 26, 2022 at 15:05

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