Green Geology

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Site of Special Scientific Interest (SSSI)

Isle of Portland

Text and photographs:  Naomi Stevenson

Text Naomi Stevenson

Photos  © Naomi Stevenson/Natural England except: Bomber Command Memorial, Roach stone © Naomi Stevenson

Page updated June 2013

South of Weymouth in Dorset, Portland is strictly-speaking a ‘tied island’ connected by a spit linking the eastern end of Chesil Beach and the northern tip of the island.  Chesil Beach is a tombolo that was pushed northwards as sea levels rose at the end of the last ice age ans is a very interesting geomorphological featuire in its own right..  A road and utilities run across the connecting spit which has been well built-up.


The geology of Portland records a gradual change in the local environment, from deep seas to coastal swamps with a fall in sea level at the end of the Jurassic period.   Portland’s oldest rocks, at the base of its visible succession, consist of fossiliferous Upper Jurassic Kimmeridge Clay, which have yielded a variety of dinosaurs and marine reptiles.  The clay was followed by the Portland Beds - dolomitic Jurassic sands laid down as the seas shallowed and limestones laid down in clear, warm seawater.  These limestones also contain Titanites ammonites, huge numbers of Portland Screw Aptyxiella portlandica, Trigoniid bivalves and occasional reptile remains. Portland Screw and trigoniids are very common in the Roach Stone; the Grove Whitbed is stuffed with the remains of clams.


Stone from the Portland Beds has been and continues to be a prized building stone.  Sir Christopher Wren owned a quarry on Portland, and used the stone to build St. Paul’s Cathedral in London.  Portland stone has been used for many of the great Whitehall offices, for all British War Graves, and for the newly-built Bomber Command Memorial in Green Park, London, as well as for many buildings further afield.  Quarrying has been a major influence on the landscape of Portland; even Pulpit Rock, at Portland Bill, is the remains of a sea arch that was quarried away. A group of the former quarries is now a Quarry Park with sculptures from various artists, including the famous "Falling Man" by Anthony Gormley.


The Purbeck Beds, deposited after the Portland Beds, mark a clear change in environments and are a mix of terrestrial, lagoonal and marine sediments.  They contain dinosaur footprints, the remains of a prehistoric forest, and other fossil plants such as the tree now on display outside the Portland Heights Hotel, and stromatolites which formed around tree trunks.  All of these beds now dip to the south, having been tilted as part of an anticline folded during the Tertiary Alpine Orogeny - hence Portland's sloping plaeau landform.


At the southern tip of the island, there are deposits of periglacial head and near to these are raised beach deposits recording sea level changes resulting from previous climate change.


A spectacularly large Quaternary landslip at the north of the island extends around Portland’s coastline, thinning out as the plateau dips towards the sea.


Click to enlarge

Cheyne Weare