Citadel Bastion () is a rocky, flat-topped elevation at the south side of the terminus of Saturn Glacier, facing towards

George VI Sound George VI Sound or Canal Jorge VI or Canal Presidente Sarmiento or Canal Seaver or King George VI Sound or King George the Sixth Sound is a major bay/ fault depression, 300 miles (483 km) long and mainly covered by a permanent ice shelf. I ...

and the

Rymill Coast Rymill Coast is that portion of the west coast of Antarctic Peninsula between Cape Jeremy and Buttress Nunataks. It runs northward from English Coast and east of Alexander Island across George VI Sound, encompassing the Batterbee Mountains. It ...

, situated on the east side of

Alexander Island Alexander Island, which is also known as Alexander I Island, Alexander I Land, Alexander Land, Alexander I Archipelago, and Zemlja Alexandra I, is the largest island of Antarctica. It lies in the Bellingshausen Sea west of Palmer Land, Antarcti ...

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...

. Its maximum elevation is about 645 m. Citadel Bastion lies next to

Hodgson Lake Hodgson Lake is a perennially ice-covered freshwater lake, which is about 2 km (1.2 mi) long by about 1.5 km (0.93 mi) wide. It is located within the southern part of Alexander Island, west of Palmer Land in Antarctica, at app ...

. This mountain was mapped from trimetrogon air photography taken by the

Ronne Antarctic Research Expedition The Ronne Antarctic Research Expedition (RARE) was an expedition from 1947–1948 which researched the area surrounding the head of the Weddell Sea in Antarctica. Background Finn Ronne led the RARE which was the final privately sponsored exp ...

, 1947–48, and from survey by the

Falkland Islands Dependencies Survey The Falkland Islands and Dependencies Aerial Survey Expedition (FIDASE) was an aerial survey of the Falkland Islands Dependencies The Falkland Islands Dependencies was the constitutional arrangement from 1843 until 1985 for administering the v ...

, 1948–50. The name applied by the

UK Antarctic Place-Names Committee The UK Antarctic Place-Names Committee (or UK-APC) is a United Kingdom government committee, part of the Foreign and Commonwealth Office, responsible for recommending names of geographical locations within the British Antarctic Territory (BAT) an ...

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. .

Geology

Citadel Bastion is one of a series of

nunatak A nunatak (from Inuit language, Inuit ) is the summit or ridge of a mountain that protrudes from an ice field or glacier that otherwise covers most of the mountain or ridge. They often form natural pyramidal peaks. Isolated nunataks are also cal ...

s that provide outcrops of

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

(

Aptian The Aptian is an age (geology), age in the geologic timescale or a stage (stratigraphy), stage in the stratigraphic column. It is a subdivision of the Early Cretaceous, Early or Lower Cretaceous epoch (geology), Epoch or series (stratigraphy), S ...

–

Albian The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...

)

sedimentary Sedimentary rocks are types of rock formed by the cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or deposited at Earth's surface. Sedime ...

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

gravel Gravel () is a loose aggregation of rock fragments. Gravel occurs naturally on Earth as a result of sedimentation, sedimentary and erosion, erosive geological processes; it is also produced in large quantities commercially as crushed stone. Gr ...

ly interbedded with occasional beds of

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

(volcanic lithic

arenite Arenite (from the Latin ''arena'', "sand") is a sedimentary clastic rock with sand grain size between 0.0625 mm (0.00245 in) and 2 mm (0.08 in) and containing less than 15% matrix. The related adjective is ''arenaceous''. The e ...

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

, and conglomerate. At two horizons within these strata, beds of lithic and vitric

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock co ...

occur. well developed

paleosol In Earth science, geoscience, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The definition of the term in geology and paleontology is slightly different from its use in soil science. In geo ...

s, leaf

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

s, fossil wood, and fossil forests are widespread within this unit. The plant fossils consists mainly of

conifer Conifers () are a group of conifer cone, cone-bearing Spermatophyte, seed plants, a subset of gymnosperms. Scientifically, they make up the phylum, division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a sin ...

ous plants (mainly

podocarp Podocarpaceae is a large family of mainly southern hemisphere conifers, known in English as podocarps, comprising about 156 species of evergreen trees and shrubs.James E. Eckenwalder. 2009. ''Conifers of the World''. Portland, Oregon: Timber Pres ...

s). Fossils of

Bennettitales Bennettitales (also known as cycadeoids) is an extinct order of seed plants that first appeared in the Permian period and became extinct in most areas toward the end of the Cretaceous. Bennettitales were amongst the most common seed plants of th ...

pteridophytes A pteridophyte is a vascular plant (with xylem and phloem) that reproduces by means of spores. Because pteridophytes produce neither flowers nor seeds, they are sometimes referred to as " cryptogams", meaning that their means of reproduction is h ...

and

liverworts Liverworts are a group of non-vascular plant, non-vascular embryophyte, land plants forming the division Marchantiophyta (). They may also be referred to as hepatics. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in wh ...

, and to a lesser extent the

angiosperms Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (). The term angiosperm is derived from the Greek words (; 'container, vessel') and (; 'seed'), meaning that the seeds are enclosed within a fruit. T ...

, 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 A paleocurrent or paleocurrent indicator is a geological feature (typically a sedimentary structure) that helps one determine the direction of flowing water in the geologic past. This is an invaluable tool in the reconstruction of ancient deposi ...

data from and the

sedimentology Sedimentology encompasses the study of modern sediments such as sand, silt, and clay, and the processes that result in their formation (erosion and weathering), transport, deposition and diagenesis. Sedimentologists apply their understanding of m ...

of sedimentary rocks, which comprise Citadel Bastion and adjacent nunataks, found that these rocks accumulated as a large

alluvial fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to Semi-arid climate, semiar ...

about 15 km in radius. This alluvial fan built seaward along the edge of a Cretaceous

volcanic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc ...

into marine waters occupying a forearc basin. This alluvial fan was built by

braided river A braided river (also called braided channel or braided stream) consists of a network of river channel (geography), channels separated by small, often temporary, islands called ''braid bars'' or, in British English usage, ''aits'' or ''eyots''. ...

s flowing in a south to southwesterly direction. The thickness of

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

s that fill ancient channels indicate that the major channels of these rivers may have been ten meters or more deep. The

floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river. Floodplains stretch from the banks of a river channel to the base of the enclosing valley, and experience flooding during periods of high Discharge (hydrolog ...

s 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 Pyroclast, Pyroclastic or Pyroclastics may refer to: Geology * Pyroclast, or airborne volcanic tephra fragments * Pyroclastic rock, rock fragments produced and ejected by explosive volcanic eruptions * Pyroclastic cone, landform of ejecta fro ...

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.

Quaternary Geology

Evidence shows that a thick ice sheet covered Citadel Bastion during the

Last Glacial Maximum The Last Glacial Maximum (LGM), also referred to as the Last Glacial Coldest Period, was the most recent time during the Last Glacial Period where ice sheets were at their greatest extent between 26,000 and 20,000 years ago. Ice sheets covered m ...

. Striated bedrock surfaces are found in the

col A col is the lowest point on a mountain ridge between two peaks; a mountain pass or saddle. COL, CoL or col may also refer to: Computers * Caldera OpenLinux, a defunct Linux distribution * , an HTML element specifying a column * A collision sig ...

on the south side of Citadel Bastion. The striations are best preserved at the edge of freshly exposed glacial

till image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...

s. Striated bedrock has also been found on its slopes high as its summit. Numerous

glacial erratic A glacial erratic is a glacially deposited rock (geology), rock differing from the type of country rock (geology), rock native to the area in which it rests. Erratics, which take their name from the Latin word ' ("to wander"), are carried by gla ...

s are also found in the col and on its slope all of the way up to and on its summit. These erratics are commonly striated and faceted. Some of them are bullet-shaped and aligned according to bedrock striations and ice flow within the former ice sheet. They are typically composed of sandstone. Other erratics are composed of conglomerate containing cobble-sized

granitic A granitoid is a broad term referring to a diverse group of coarse-grained igneous rocks that are widely distributed across the globe, covering a significant portion of the Earth's exposed surface and constituting a large part of the continental ...

clasts Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks b ...

.Hodgson D.A., S.J. Roberts, M.J. Bentley, J.A. Smith, J.S. Johnson, E. Verleyen, W. Vyverman, A.J. Hodson, M.J. Leng, A. Cziferszky, A.J. Fox, and D.C.W. Sanderson (2009) ''Exploring former subglacial Hodgson Lake, Antarctica Paper I.'' Quaternary Science Reviews. 28(23-24):2295–2309.Johnson, J.S., J.D. Everest, P.T. Leat, N.R. Golledge, D.H. Rood, and F.M. Stuart, 2012, ''The deglacial history of NW Alexander Island, Antarctica, from surface exposure dating.'' Quaternary Research. 77(2):273–280. Cosmogenic

beryllium-10 Beryllium-10 (10Be) is a radioactive isotope of beryllium. It is formed in the Earth's atmosphere mainly by cosmic ray spallation of nitrogen and oxygen. Beryllium-10 has a half-life of 1.39 million years, and decays by beta decay to stable bor ...

isotope dating of a few of the erratics indicate that they were exposed by the thinning of the ice sheet after the Last Glacial Maximum. The erratics within the col were exposed by thinning ice sheet about 13,500 years ago. Those at the summit of Citadel Bastion were exposed by thinning ice sheet about 10,200 years ago. These and other cosmogenic beryllium-10 isotope dates demonstrate a significant and progressive thinning and shrinkage of the ice sheet covering Alexandria Island from at least 22,000 years ago to about 10,000 years ago.

References

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Geology

Quaternary Geology

References

See also