Here I’m going to gather what I’ve found.
Portland is not really an island but is reached over a narrow causeway from Chesil Beach. It is a huge block of limestone, measuring 4.5 miles by 1.75 miles and rising to a height of 400 feet above sea level in the north. The famous Portland Stone quarried here has been used for many well-known buildings. These include both our own St Paul’s Cathedral and the United Nations Building in New York. Many of the quarries here are owned by the crown as Portland is a Royal Manor.
The stone was quarried centuries ago.