Black shale;
Ordovician, Wales
This is a view of a bedding plane surface of a black shale.
It formed by the deposition of very fine-grained sediment in
deep water. The rock splits easily into thin sheets along
the bedding planes. The black colour results from a high
content of organic carbon, which would not be expected to
accumulate in shallow, oxygenated water. The bedding planes
are covered in the fossil remains of graptolites, strange
colonial creatures that lived in compartments arranged along
a stem. This is Didymograptus, which has a double
stem, shaped like a tuning fork. Graptolites did not live in
the deep water but apparently floated in the open oceans,
falling to the sea floor only after they died.