Simplify the following text for a Grade 7 native English-speaking person.
Original: The finished slates were sent by packhorse to the wharf at Pennal , transferred to boats for a river trip to Aberdyfi ( then known as Aberdovey ) , and then finally loaded into seagoing vessels , a complex and expensive transportation arrangement which limited the quarry 's output .
Simplified: The finished slates were sent by packhorse to the wharf at Pennal . They were then transferred to boats for a river trip to Aberdyfi ( also known as Aberdovey ) .