The bluestones ( some of which are made of dolerite , an igneous rock ) , were thought for much of the twentieth century to have been transported by humans from the Preseli Hills , away in modern-day Pembrokeshire in Wales .
The bluestones ( made of dolerite , an igneous rock ) , were thought for much of the 20th century to have been brought from the Preseli Hills , 160 mi away in modern-day Pembrokeshire , Wales .