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Citadel Bastion
Citadel Bastion () is a rocky, flat-topped, rocky elevation at the south side of the terminus of Saturn Glacier, facing towards George VI Sound and the Rymill Coast, situated on the east side of Alexander Island, Antarctica. Its maximum elevation is about 645 m. Citadel Bastion lies next to Hodgson Lake. This mountain was mapped from trimetrogon air photography taken by the Ronne Antarctic Research Expedition, 1947–48, and from survey by the Falkland Islands Dependencies Survey, 1948–50. The name applied by the UK Antarctic Place-Names Committee because it resembles a fortified structure with a watchtower at the end of a wall.〔United States Geological Survey (nd) (''Citadel Bastion'' ). (Geographic Names Information System ), U.S. Department of the Interior, U.S. Geological Survey, Reston, Virginia.〕〔Stewart, J. (2011) ''Antarctic An Encyclopedia'' McFarland & Company Inc, New York. 1776 pp. ISBN 9780786435906.〕
==Geology==
Citadel Bastion is one of a series of nunataks that provide outcrops of Cretaceous (Aptian–Albian) sedimentary rocks that comprise part of the 6.8 kilometer-thick Fossil Bluff Group, which underlies easternmost Alexander Island. These sedimentary rocks accumulated in shallow marine and terrestrial environments. The sedimentary rocks, which are exposed in Citadel Bastion and adjacent nunataks, consist of a 700 meter-thick sequence of laterally persistent beds of very fine to very coarse, rarely gravelly interbedded with occasional beds of sandstone (volcanic lithic arenite), mudstone, and conglomerate. At two horizons within these strata, beds of lithic and vitric tuff occur. well developed paleosols, leaf fossils, fossil wood, and fossil forests are widespread within this unit. The plant fossils consists mainly of coniferous plants (mainly podocarps). Fossils of Bennettitales, pteridophytes and liverworts, and to a lesser extent the angiosperms, are also common.〔Cantrill, D J. (1996) Fern thickets from the Cretaceous of Alexander Island, Antarctica containing Alamatus bifarius Douglas and Aculea acicularis sp. nov. Cretaceous Research. 17(2):169–82.〕〔Nichols, G.J., and D.J. Cantrill, (2002) Tectonic and climatic controls on a Mesozoic forearc basin succession, Alexander Island, Antarctica. Geological Magazine 139(3):313–330.〕 Citadel Bastion is notable for bedrock outcrops that expose multiple fossil forests, which consist of upright standing trunks buried in place, and their associated paleosols.〔Howe, J., and J.E. Francis (2005) Metamorphosed palaeosols associated with Cretaceous fossil forests, Alexander Island, Antarctica. Journal of the Geological Society. 162(6):951–957.〕
The analysis of paleocurrent data from and the sedimentology of sedimentary rocks, which comprise Citadel Bastion and adjacent nunataks, found that these rocks accumulated as a large alluvial fan about 15 km in radius. This alluvial fan built seaward along the edge of a Cretaceous volcanic arc into marine waters occupying a forearc basin. This alluvial fan was built by braided rivers flowing in a south to southwesterly direction. The thickness of sediments that fill ancient channels indicate that the major channels of these rivers may have been ten meters or more deep. The floodplains between channels were stable enough, despite periodic deposition of coarse and fine sediment during floods, to allow the formation of soils and development of forests. The source area for the fluvial detritus was a volcanic arc, which now forms the Antarctic Peninsula, to the east. Within this volcanic arc, volcanic activity provided pyroclastic ash fall deposits to the area. Fossil plants found in these sedimentary rocks indicate that the palaeoclimate was warm and humid throughout the period of deposition despite their polar position.〔〔
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