Friday Rock Blogging: Sedimentary Concretions

The Earth Science Picture of the Day is often light on both “earth” and “science” (it is reliably daily, and almost always a picture) but yesterday’s was a good one: bowling ball sized concretions in mudstone.

Concretions form within a sedimentary rock, after the material has been laid down. Water circulating within the rock will pick up and dissolve chemicals from the surrounding rock (or anywhere, really, ground water is kinda slutty like that); these chemicals then precipitate out around some small nucleus in the rock, like the chip of slate shown here.

Concretions are generally harder than the surrounding material, so are often exposed by erosion. They have historically been mistaken for fossils or alien artifacts – and of course some people never learn, but hey, everything is aliens when it’s on Mars, right?


  1. Schmerles wrote:

    thanks for the link to this image

  2. Craig McLanachan wrote:

    I enjoyed this very ‘busy’ concretion very much.I live in New Zealand on the east coast of the South Island.At Moeraki, Katiki Beach and Shag Point we have an excellent variety of fabulous concretions. They extend 50 km back from the coast into the Horse Range.
    They vary from setparian to one of the most famous, a plesiosaur encased in a boulder. I have been fascinated by these rocks for many years and believe we have the best variety of forms in the world.
    Thanks for your information, Craig.

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