The Pennsylvanian is in the ICS geologic timescale the youngest subperiod or upper subsystem of the Carboniferous period. It lasted from roughly 318.1± 1.3 to 299± 0.8 Ma (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 state of Pennsylvania, where rocks with this age are widespread.