323 Million B.C.T. - The Pennsylvanian Subperiod Began

Around 323 Million B.C.T., the Pennsylvanian subperiod began.

The Pennsylvanian is the younger of two subperiods (or upper of two subsystems) of the Carboniferous Period. It lasted from roughly 323.2 ± 1.3 to 298.9 ± 0.8 million years ago. As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain by a few million years. The Pennsylvanian is named after the American state of Pennsylvania, where the coal-productive beds of this age are widespread.

The division between the Pennsylvanian and Mississippian subperiods comes from North American stratigraphy. In North America, where the early Carboniferous beds are primarily marine limestones, the Pennsylvanian was in the past treated as a full fledged geologic period between the Mississippian and the Permian. In Europe, the Mississippian and Pennsylvanian are one more-or-less continuous sequence of lowland continental deposits and are grouped together as the Carboniferous Period. The current internationally used geologic timescale of the International Commission on Stratigraphy (ICS) gives the Mississippian and Pennsylvanian the rank of subperiods, -- subdivisions of the Carboniferous Period.