JON WILSON / SCIENCE PHOTO LIBRARY JON WILSON / SCIENCE PHOTO LIBRARY
Shale. Shale strata in a cliff face. Shale is a sedimentary rock made from clay and silt deposited at the bottom of lakes and oceans. As subsequent layers were deposited on top, the lower layers were compressed into rock. Shale is a soft rock and breaks apart easily. Shale often contains organic material from creatures that died and were deposited at the same time as the clay and silt. This organic material can sometimes be extracted as crude oil. Photograph taken in Pembrokeshire, Britain, of shale from the Cambrian geological era (525-600 million years ago).
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