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The Chalk Group (often just called the Chalk) is the lithostratigraphic unit (a certain number of rock strata) which contains the late Cretaceous limestone succession in southern and eastern England. The same or similar rock sequences occur across the wider northwest European chalk 'province'. It is characterised by thick deposits of chalk, a soft porous white limestone, deposited in a marine environment. Chalk is a limestone that consists of coccolith biomicrite.〔As discussed in (Chalk Facts ) by C. S. Harris and Scholle ''et al.'' (1983)〕 A biomicrite is a limestone composed of fossil debris ("bio") and calcium carbonate mud ("micrite"). The majority of the fossil debris comprising chalk consists of the microscopic plates, which are called coccoliths, of microscopic green algae known as coccolithophores. In addition to the coccoliths, the fossil debris includes a variable, but minor, percentage of the fragments of foraminifera, ostracods and mollusks. The coccolithophores lived in the upper part of the water column. When they died, the microscopic calcium carbonate plates, which formed their shells settled downward through the ocean water and accumulated on the ocean bottom to form a thick layer of calcareous ooze, which eventually became the Chalk Group. The Chalk Group usually shows few signs of bedding, other than lines of flint nodules which become common in the upper part. Nodules of the mineral pyrite also occur and are usually oxidized to brown iron oxide on exposed surfaces. Well-known outcrops include the White Cliffs of Dover, the southern coastal cliffs of the Isle of Wight and the quarries and motorway cuttings at Blue Bell Hill, Kent, (which has been classified as a ''Site of Special Scientific Interest'') and at the Stokenchurch Gap on the Oxfordshire/Buckinghamshire border where the M40 motorway cuts through the Aston Rowant NNR. ==Subdivisions== The Chalk Group is now divided into a ''White Chalk Subgroup'' and a ''Grey Chalk Subgroup'', both of which are further subdivided into formations. These modern divisions replace numerous earlier divisions, references to which occur widely on geological maps and in other geological literature. Previously no subgroups were defined but three formations were identified; the Upper Chalk, Middle Chalk and Lower Chalk. Different formations are defined within the 'northern' and 'southern' provinces, from Norfolk northwards and south of the Thames valley respectively. A 'transitional province' between the two and covering much of East Anglia and the Chiltern Hills is also recognised. A different approach again is taken as regards the succession beneath the North Sea.〔Hopson, P.M., 2005 ''A Stratigraphical Framework for the Upper Cretaceous Chalk of England and Scotland with Statements on the Chalk of Northern Ireland and the UK Offshore Sector'', British Geological Survey Research Report RR/05/01 (downloaded from www.bgs.ac.uk)〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Chalk Group」の詳細全文を読む スポンサード リンク
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