This image was acquired from
wikimedia. It was marked as Public Domain or CC0 and is free to use. To verify, go to the source and check the information there.
Keywords from Image Description:
South African Geology Schwarz Fig . en Figure from South African Geology Schwarz Page Fig Spherical Limestone Septaria in Dwyka Shales The balls have broken across level with the surface and the interior has been removed by atmospheric agencies quicker than the harder rind The cracks produced by the drying of the concretion