SUSSEX MARINE SITES OF NATURE CONSERVATION IMPORTANCE

Site: The Mixon Hole						Ref. No. 2
Location: 2 km SE of Selsey Bill					Other conservation designation?: Within proposed Solent SAC
Lat./Long. position of centre of site: 50° 42' 24" N  0° 46' 28" W			OS grid ref. of centre of site: SZ 867 901			Author: Robert Irving
Sea bed type: Limestone cap rock overlying cliff of 'soft grey' and 'stiff blue' clay; boulders, pebbles and shells at base.			Depth range (below chart datum): 2-25 m			Date identified: November 1995

Summary

The Mixon Hole consists of an almost vertical, 20 m high, clay cliff on its north side, with a similar, though less impressive and smaller, cliff on its southern side. To the east and west the slope of the sea bed is much more gradual. The exposed soft clay of the main cliff has numerous ledges, crevices and fissures, and is continually being eroded by currents sweeping across its face. This is one of the best known dive sites in Sussex, described by McDonald (1985) rather lavishly as "one of the top diving sites in the south of England, if not the whole country". This is in part due to the great depth it offers close inshore. The Hole is believed to be a segment of an ancient river gorge swept clear by tidal current. It is a remarkable feature, of particular interest to coastal geomorphologists. It is also of interest to marine archaeologists because of the remains of worked stone at its base (large cuboidal blocks and spherical catapult balls), known to date from Roman times.

Biological Description

The soft light grey clay which forms the cliff is overlain by an exposure of stiff blue clay (from 1-6 m depth), which itself has a cap of limestone bedrock, breaking the surface at low water spring tides. This shallowest part is covered by occasional kelp plants and other algae. There is a ledge at 4 m with loose limestone slabs covered by foliose red algae. Much of the cliff proper is bare and uncolonised. It is extensively bored by piddocks Pholas dactylus, but, as with chalk cliffs further to the east, many of the holes are empty. There are many species of crustacea including squat lobster Galathea squamifera, edible crab Cancer pagurus and velvet swimming crab Necora puber. Tompot blennies Parablennius gattorugine and leopard-spotted gobies Thorogobius ephippiatus are frequently seen on cliff ledges. Other fish observed swimming at or near the top of the cliff face include pollack Pollachius pollachius, ballan wrasse Labrus bergylta, corkwing wrasse Crenilabrus melops, goldsinny wrasse Ctenolabrus rupestris and shoals of the small two-spotted goby Gobiusculus flavescens.

Towards the base of the cliff, the steep slope is covered by cobbles and boulders that have fallen from the cliff. Attached to these are hydroids Plumularia setacea, Hydrallmania falcata and Amphisbetia operculata, the keel worm Pomatoceros triqueter and the sea squirt Dendrodoa grossularia. At the base of the cliff (@ 25 m) is a level area of tide-swept pebbles and shells with lumps of muddy clay. Found frequently here are chains of slipper limpets Crepidula fornicata, hermit crabs Pagurus bernhardus and occasional lobsters Homarus gammarus.

References: Ackers, G. 1977. Mixon Hole Project Dolphins Underwater Club Project. Underwater Conservation Year 'Adopt a Site' Scheme Irving, R. A. 1994. Report of the West Sussex Seasearch Project, 1992-1993: Chichester Harbour to Littlehampton Unpublished Report, Coldwaltham, West Sussex McDonald, K. 1985. Dive Sussex - A Diver Guide Underwater World Publications, London McDonald, K. 1995. Marvellous, mysterious Mixon Diver magazine April 1995, 54-56

Sussex SEASEARCH dive nos.: 95/97 & 101

Sussex Sublittoral Survey site no.: 2

Site Location

Diagrammatic representation of site: The Mixon Hole