July 1. The Pennsylvanian begins

Eastern Pennsylvanian showing coal areas (public domain, from an old text)

As we discussed at the start of the Mississippian Period last month, in the world of geology outside the United States, the Mississippian and Pennsylvanian together comprise the Carboniferous Period. In the U.S. the two are treated as full periods of geologic time, but the Pennsylvanian that begins today is the Late Carboniferous everywhere else. 

The late Carboniferous, or Pennsylvanian, is rich in coal-bearing rocks in many parts of the world, and that coal gives its name to the Carboniferous, meaning carbon-bearing. The Pennsylvanian in the U.S. is named for the state of Pennsylvania, where excellent exposures of coal of that age are found. The informal usage of the names Mississippian and Pennsylvanian received official recognition in the U.S. in 1906.

—Richard I. Gibson