Laid down some 135-140 million years ago during the Jurassic period when the rise and fall of the sea level deposited sediments on the sea floor the stones lie in four layers:- by type from top to bottom, - Lower Purbeck (or "rubble" to the Quarryman), Portland Stone, Portland Sand, and Kimmeridge Clay. During the Cretaceous period, which followed the Jurassic, the continent of Africa bumped into Europe and the rocks around Weymouth were forced up into a bump. The Isle of Portland is the remnant of the southern slope, eroded over millions of years.

The Jurassic period was rich in sea life and many creatures were fossilised in the sediments, including the famous ammonites (to be found adorning many garden walls on the Island) as well as spiral shaped snails and other shellfish. Between the Jurassic and the Cretaceous periods the sea level fell and forests grew on the exposed land. The fossilised stumps of these trees can be seen in Kingbarrow Quarries and at the Portland Heights Hotel.

During the ice ages some 125,000 and 210,000 years ago, the sea levels rose between 22 and 52 feet leaving raised pebble and shell beaches in the south of the Island.

Portland stone has played an important part in the life of the people of the Island; they quarry it, live in houses made of it and export it around the world. Many of London's finest buildings, including St Paul's Cathedral, are witness to the enduring and aesthetic quality of these Jurassic sediments.