Simplify the following text for a Grade 11 native English-speaking person.
Original: 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 .
Simplified: The Pennsylvanian is the Upper Carboniferous period in the strata of North America . It comes after ( above ) the Mississippian , and before the Permian period .