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

Saturn Glacier Saturn Glacier () is a glacier lying in southeast Alexander Island, Antarctica. The glacier is 15 nautical miles (28 km) long and 6 nautical miles (11 km) wide, flowing southeast into the George VI Ice Shelf of George VI Sound north of ...

, 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. It ...

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 is ...

, 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, Antarc ...

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

. 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 a ...

. 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 British Antarctic Survey (BAS) is the United Kingdom's national polar research institute. It has a dual purpose, to conduct polar science, enabling better understanding of global issues, and to provide an active presence in the Antarctic on ...

, 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) and ...

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 ''nunataq'') 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 are also called glacial islands. Examples are natural pyramidal peaks. ...

s that provide outcrops of

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

(

Aptian The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ago), a ...

–

Albian The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± 0.9 M ...

)

sedimentary Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed by cementation (geology), cementation. Sedimentati ...

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 throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...

ly interbedded with occasional beds of

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

(volcanic lithic

arenite Arenite (Latin: ''arena'', "sand") is a sedimentary clastic rock with sand grain size between 0.0625 mm (0.00246 in) and 2 mm (0.08 in) and contain less than 15% matrix. The related adjective is ''arenaceous''. The equivalent ...

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 (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...

, and

conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** Co ...

. 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 cont ...

occur. well developed

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

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 preserved ...

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 single ...

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 Pre ...

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 are among the most common Mesozoic seed plants ...

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

and

liverworts The Marchantiophyta () are a division of non-vascular land plants commonly referred to as hepatics or liverworts. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of ge ...

, and to a lesser extent the

angiosperms Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...

, 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 depositio ...

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 mo ...

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 semiarid climates, but a ...

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 lo ...

into marine waters occupying a

forearc basin Forearc is a plate tectonic term referring to a region between an oceanic trench, also known as a subduction zone, and the associated volcanic arc. Forearc regions are present along a convergent margins and eponymously form 'in front of' the vol ...

. This alluvial fan was built by

braided river A braided river, or braided channel, consists of a network of river channels separated by small, often temporary, islands called braid bars or, in English usage, ''aits'' or ''eyots''. Braided streams tend to occur in rivers with high sediment l ...

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

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...

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 which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...

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 Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...

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 Late Glacial Maximum, was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent. Ice sheets covered much of Northern North America, Northern Eur ...

. Striated bedrock surfaces are found in the col 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 glacially deposited rock differing from the type of rock native to the area in which it rests. Erratics, which take their name from the Latin word ' ("to wander"), are carried by glacial ice, often over distances of hundred ...

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 generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quart ...

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 ...

.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 × 106 years, and decays by beta decay to stable boron- ...

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