Sieving the sands of time

As a rule of thumb, grain size – the diameter of individual grains of sediment in ‘clastic’ or sedimentary rocks – reflects the energy of the environment a rock forms in

Because of this association between grain size and the energy of a sedimentary environment, 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.

What’s more, the degree of size-sorting (a.k.a. how uniform the size of the grains is) can also help indicate the energy of the depositional environment. Well-sorted sediments tend to indicate higher energy depositional settings. Think about what happens if you vigorously shake a bowl of popcorn (no, not that much).

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 grain size to decipher past environments.

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

In my research, I was able to use grain size differences to decipher layers of rock that formed from the high-energy heart of river channels to the low-energy,  distant floodplains of a forested landscape that persisted in the South Polar circle roughly 100 million years ago. From looking at these characteristics of the grain sizes (along with other clues), I reconstructed the ecology of different plants fossilised within these ancient sediments.

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