The Denstone Expedition to Inaccessible Island

Abstract

Inaccessible Island, the second largest in the Tristan da Cunha group, is a volcanic remnant which, owing to its steep cliffs on all sides, remained unmapped and largely unexplored until 1982, when the Denstone Expedition to Inaccessible Island, a party of 16 in all, carried out the first shore-based survey and scientific exploration there. The main party was put ashore from a South African ship and established a base on the only part of the Island from where there is access to the interior. This paper gives a brief description of Inaccessible Island, a summary of the work undertaken in the major disciplines of biology and geology, and includes the resulting map. INACCESSIBLE ISLAND is situated 40 km south west of Tristan da Cunha in the South Atlantic Ocean and is British territory, under the Protectorate of St Helena. It is roughly rhomboidal in shape, approximately 4x5 km in area, of volcanic origin and characterized by steep cliffs on all sides, with narrow beaches. Prior to the Denstone Expedition, there had been only six previous scientific visits, all of short duration and limited, in all but two cases, by failure to gain access to the interior of the Island. Thus, no previous shore-based map of Inaccessible Island existed and both biological and geological collections were limited. No detailed study had been attempted of any endemic species, or of the Island's geomorphology from ashore. The object of the Denstone Expedition was to make good these deficiencies. This was successfully achieved between October 1982 and February 1983 by a team of past and present pupils of Denstone College, Staffordshire,with three Tristan Island guides. Figure 1 is the resulting initial map. Topography The Island is surrounded by near-vertical sea cliffs of between 155 m at East Point and 490 m in the south west. The western part ofthe Island contains the highest ground from where a number of ridges run south-eastwards, forming the major catchment area of three drainage basins emptying immediately south of Joe's Hill. The extreme north-west part of the Island is drained to the north by two streams with outlets over the cliffs at North Point. This area is separated from the north-central and north-eastern parts of the Island by the prominent Long Ridge running north-south. To the east of the ridge lies the third drainage basin containing a stream which flows over the cliffs at the western end of Waterfall Beach. The eastern side of the Island is dominated by Round Hill, a breached parasitic cone. Inland of East Point is a low, flat plateau with no drainage system. The Island is subject to severe weathering and erosion as evidenced by numerous and often large landslides along the cliffs. Blenden Hall is part of a large low-lying area at sea level which would appear to constitute landslide material, possibly instigated during the formation of a large parasitic cone just to the north of the area. On its surface is Skua Bog. Pebble and boulder beaches occur intermittently around the Island, totalling approximately two-thirds of the coastal perimeter, and a well developed storm beach occurs at West Point.