Simplify the following text for a Grade 10 native English-speaking person.
Original: In the period before the Roman supremacy in southern Italy , Nuceria Alfaterna appears to have been the chief town in the valley of the Sarnus , Herculaneum , Pompeii , Stabiae and Surrentum all being dependent upon it .
Simplified: Nuceria Alfaterna appears to have been the chief town in the valley of the Sarnus , Herculaneum , Pompeii , Stabiae and Surrentum all being dependent upon it .