Sieving the sands of time

Grain size refers to the diameter of individual grains of sediment in ‘clastic’ or sedimentary rocks.

As a rule of thumb, grain size reflects the energy of the environment a rock forms in. Because of this wonderful fact of physics, the proportion of different grain sizes in a fossil-baring rock can help palaeontologists piece together a picture of the environment their fossils formed in. Was it a quiet burial by a lakeside, or a tumultuous end in the pits of a mountainous river channel?

Generally speaking, higher energy water or wind currents are required to move larger grain sizes.

The degree of size-sorting a.k.a. how uniform the size of the grains is, can also indicate the energy of the depositional environment; well-sorted sediments tend to indicate higher energy depositional settings.

Sample e.g. with grain size graphic
A visual example of grain size analysis, with the number of grains (frequency, on the y-axis) in each size category graphed against grain size (x-axis). Peaks in the graph, as well as the splay of grains either side, hold the key for geologists using this technique.

Picture the kinds of environments you might see grains of these sizes laying around, and you’ll be on your way to thinking like a sedimentary geologist:

  • boulders, >256 mm
  • cobbles, 64-256 mm
  • pebbles, 4-64 mm
  • coarse sand, 2-4 mm
  • sand, 1/8-2 mm
  • silt, 1/256-1/8 mm
  • clay <1/256 mm

 

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