The Crag Group is a
geological group
In geology, a group is a lithostratigraphy, lithostratigraphic Stratigraphic unit, unit consisting of a series of related Geological formation, formations that have been classified together to form a group. Formations are the fundamental unit of ...
outcropping in
East Anglia
East Anglia is an area of the East of England, often defined as including the counties of Norfolk, Suffolk and Cambridgeshire, with parts of Essex sometimes also included.
The name derives from the Anglo-Saxon kingdom of the East Angles, ...
, England and adjacent areas of the
North Sea
The North Sea lies between Great Britain, Denmark, Norway, Germany, the Netherlands, Belgium, and France. A sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian Se ...
. Its age ranges from approximately 4.4 to 0.478 million years
BP, spanning the late
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58[Pleistocene
The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...](_blank)
epochs. It comprises a range of marine and estuarine sands, gravels, silts and clays deposited in a relatively shallow-water, tidally-dominated marine embayment on the western margins of the North Sea basin. The sands are characteristically dark green from
glauconite
Glauconite is an iron potassium phyllosilicate ( mica group) mineral of characteristic green color which is very friable and has very low weathering resistance.
It crystallizes with a monoclinic geometry. Its name is derived from the Greek ...
but weather bright orange, with
haematite
Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...
'iron pans' forming. The
lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...
of the lower part of the Group is almost entirely
flint
Flint, occasionally flintstone, is a sedimentary cryptocrystalline form of the mineral quartz, categorized as the variety of chert that occurs in chalk or marly limestone. Historically, flint was widely used to make stone tools and start ...
. The highest formation in the Group, the
Wroxham Crag, contains over 10% of far-travelled lithologies, notably
quartzite
Quartzite is a hard, non- foliated metamorphic rock that was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tecton ...
and
vein quartz from the Midlands,
igneous rock
Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava.
The magma can be derived from partial ...
s from Wales, and
chert
Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...
from the
Upper Greensand
Greensand or green sand is a sand or sandstone which has a greenish color. This term is specifically applied to shallow marine sediment that contains noticeable quantities of rounded greenish grains. These grains are called ''glauconies'' and co ...
of southeastern England. This exotic rock component was introduced by rivers such as the
Bytham River
The Bytham River is said to have been one of the great Pleistocene rivers of central and eastern England until it was destroyed by the advancing ice sheets of the Anglian Glaciation around 450,000 years ago. The river is named after Castle By ...
and
Proto-Thames.
The constituent formations of the Crag Group are the
Coralline Crag (mid to late Pliocene); the
Red Crag (late Pliocene / early Pleistocene); the
Norwich Crag (early Pleistocene) and the
Wroxham Crag (early to Middle Pleistocene). The sedimentary record is incomplete, leading to difficulties in correlating and dating sequences
The term Crag was first used in a geological sense by R.C. Taylor in 1823, a word commonly used in Suffolk to designate any shelly sand or gravel.
[Woodward, HB (1881): The Geology of the Country around Norwich. Memoirs of the Geological Survey of the United Kingdom; p.32]
See also
*
List of fossiliferous stratigraphic units in England
See also
* Lists of fossiliferous stratigraphic units in Europe
* Lists of fossiliferous stratigraphic units in the United Kingdom
References
*
{{DEFAULTSORT:Fossiliferous stratigraphic units in England
England
England is a Countr ...
References
Geological groups of the United Kingdom
Geologic formations of England
Neogene England
East Anglia
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