Some of Dorset’s most ‘unique’ caves are located on Portland. Only a brief drive away from the caving haven of Mendip — the island is an excellent base for the beginner to experienced caver especially if they are also interested in other adventure activities such as snorkelling, trekking, diving, rock climbing, cycling or surfing.
This limestone island of 4 miles and only 1 1/2 mile wide with a limestone layer some 70 feet deep, offers three distinct forms of caves for the underground explorer -some with unusual features- varying in sporting and scientific interest.

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Portland boasts a number of dry caves and sea caves.

Cliff entrance to Sandy Hole Cave

Dry Caves: Portland’s oldest caves are the Water worn caves with near horizontal bedding ~ being the only resemblance to the Yorkshire Dales caves. Yet, even though large rift or mass-movement caves are fairly unusual in Britain, these appear to form the largest category on Portland, mainly following the massive jointing. Some of Portland’s cave systems are in excess of 1.5 miles.

Portland Sea Caves

Sea Caves: Numerous sea caves girdle Portland Bill where the limestone dips to sea level. Some of these can be explored by land while others can only be reached by sea. These are typically large joint or bedding-plane chambers, some of them utilising rifts.
Notes and Warnings: While quarrying has claimed its toll on a few of the island’s cave sites including some of the most beautiful ones, those surviving are waiting to be explored and have attracted the attention of nation-wide caving clubs and groups. Please respect ongoing digs and underground formations. Consult local tide tables before venturing into any sea-caves as exits and entries may be difficult.

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The Wessex
Cave Club

Reading up on Portland Caves: The Caves of the Isle of Portland, 104 pp, Mike O’Conner and Nigel Graham, Wessex Cave Club Occasional Publications Series 3 No.3, 1996, ISBN 0950043346. This publication and others can be obtained through the The Wessex Cave Club link.
Portland Geology: An excellent source of information to the caves and geology of Portland with a regional breakdown, charts and formation information can be found at the web site of Southampton University .