Simplify the following text for a Grade 12 native English-speaking person.
Original: The bluestones ( some of which are made of dolerite , an igneous rock ) , were thought for much of the 20th century to have been transported by humans from the Preseli Hills , 250 k away in modern-day Pembrokeshire in Wales .
Simplified: The bluestones ( some of which are made of dolerite , an igneous rock ) , were thought for much of the 20th century to have been brought from the Preseli Hills , 160 miles ( 250km ) away in modern-day Pembrokeshire , Wales .