Simplify the following text for a Grade 12 native English-speaking person.
Original: In Europe , the Mississippian and Pennsylvanian are one more-or-less continuous sequence of lowland continental deposits and are grouped together as the Carboniferous system , and sometimes called the Upper Carboniferous and Lower Carboniferous instead .
Simplified: The Mississippian is the lower part of the Carboniferous period in geology .