Simplify the following text for a Grade 13 native English-speaking person.
Original: The `` Purbeck Marble '' of the Jurassic period and the `` Sussex Marble '' of the early Cretaceous period , which both occur in southern England , are limestones containing the tightly packed remains of the pond snail Viviparus .
Simplified: The `` Purbeck Marble '' of the Jurassic period and the `` Sussex Marble '' of the early Cretaceous period which both occur in southern England are limestones containing the tightly packed remains of the pond snail Viviparus .