Portland Raised Beach

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The Pleistocene Raised Beach.
Part of the Pleistocene Raised Beach inside the Ministry of Defence Magnetic Range.

Portland Raised Beach (the west Pleistocene Raised Beach) is a raised beach, located at Portland Bill, on the Isle of Portland, Dorset, England; and is part of the Jurassic Coast. There are two major raised beaches located at Portland Bill, although the Pleistocene Raised Beach is often considered to be the more widely noted beach of the two.

The two raised beaches (sometimes labelled as the west and east raised beaches) at Portland Bill represent the significant changes fluctuations in sea level and climate change during the last Ice Age, and both largely consist of shelly and sandy pebble deposits.[1][2] The cause of the two fluctuations was twofold: the growth of the ice, which absorbed large quantities of water, and the weight of the ice in the north that caused the landmass to tilt.[3]

Background[edit]

Portland Raised Beach[edit]

The west Pleistocene Raised Beach is found close to Portland Bill Lighthouse and Pulpit Rock, and is situated next to the Ministry of Defence Magnetic Range, where part of the beach is located behind fencing. This lies in a shallow embayment known as White Hole. It has also been named Boxgrove Raised Beach. Considered an important feature in the area, the Pleistocene Raised Beach dates from a warm interglacial phase of the Pleistocene ice ages, approximately 200,000 years ago. The pebbles represent a beach, deposited when the sea was approximately 15 metres above the present level. The loam (sandy slit) above the raised beach was originally wind deposited, and is capped by a solifluction deposit or "head" formed during the last major glacial episode to have affected Britain – The Devensian. The head contains much limestone rubble broken up under severe arctic conditions. This sequence of deposits has yielded molluscs and the remains of other extinct animals.[4][5]

The raised beach sits on the top of the cliffs at Portland Bill, made of Purbeck beds. These are Cretaceous in age and are part of the Lulworth Formation. Below these are the Portland Beds, from the Cherty Member. Rocks from the Basal Shell Bed can also be seen in blocks fallen from the cliff. These are Jurassic in age. The beach itself is made up of Mesozoic rocks, as well as granite, bunter and quartzite pebbles (from Budleigh Salterton). Fossils can also be found here, as many of the rocks and pebbles are from the Portland Stone and Purbeck Beds. The latter now tend to be covered in rubble, but once made up the top most part of the quarry.[6] Human remains have been found on the site.

The rocks scattered around the area can contain fossils, although they are not very well preserved and only oysters and bivalves can be found, along with trace fossils.[7] The beach is on private land, as is the surrounding ex-quarried land next to the beach, although the public are allowed beyond the warning sign at their own risk. Interference with the deposits of the beach remain prohibited.[4]

Second Raised Beach[edit]

The second east raised beach is located near Cave Hole and the beach huts. The beach deposits are on a slope, with a stone ledge below. An old crane stands on the beach.[8] The beach is about 125,000 years old and has abundant mollusc shells such as species of Patella and Littorina and small bivalves that lived on seaweed. It has been much disturbed by cryoturbation (freezing and thawing) during the late Pleistocene ice age. Unlike the older Pleistocene Raised Beach further east no human remains have yet been found here.[1]

References[edit]

  1. ^ a b "Portland Bill - Geological Field Guide". Southampton.ac.uk. 2012-10-19. Retrieved 2013-07-12. 
  2. ^ "Quick facts about the Jurassic Coast". Jurassiccoastline.com. Retrieved 2013-07-12. 
  3. ^ Paul, Ensom (1998). Discover Dorset: Geology. Wimborne: Dovecote Press. p. 78. ISBN 1 874336 52 0. 
  4. ^ a b "File:Portland Raised Beach, Portland Bill, Dorset.JPG - Wikimedia Commons". Commons.wikimedia.org. 2013-07-07. Retrieved 2013-07-12. 
  5. ^ "Portland Bill, Portland, Dorset". Geoffkirby.co.uk. Retrieved 2015-01-26. 
  6. ^ "Portland Bill - Geological Guide for the Geology and Stratification". Portland.ukfossils.co.uk. Retrieved 2013-07-12. 
  7. ^ "Portland Bill fossils and fossil collecting". Portland.ukfossils.co.uk. Retrieved 2013-07-12. 
  8. ^ "Portland Bill". Heightshotel.com. Retrieved 2013-07-12. 

Coordinates: 50°30′58″N 2°27′33″W / 50.5161°N 2.4591°W / 50.5161; -2.4591