Simplify the following text for a Grade 9 native English-speaking person.
Original: About 100 BC , iron bars began to be used as currency , while internal trade and trade with continental Europe flourished , largely due to Britain 's extensive mineral reserves . Coinage was developed , based on continental types but bearing the names of local chieftains .
Simplified: About 100 BC , iron bars began to be used as currency , coinage was developed , based on continental types .