MARTIN SHIELDS / SCIENCE PHOTO LIBRARY MARTIN SHIELDS / SCIENCE PHOTO LIBRARY
Fern fossil imprint in shale juxtaposed with modern day fern fronds. The distinctive white outline on black shale is caused by aluminum silicate (also known as pyrophyllite) and is characteristic of fern fossils found in coal mines in the St Clair, Pennsylvania area. The fossils are from the Pennsylvanian Epoch of the Carboniferous Period, 300 million years ago.
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