The Crag Group is a

geological group In geology, a group is a lithostratigraphic unit consisting of a series of related formations that have been classified together to form a group. Formations are the fundamental unit of stratigraphy. Groups may sometimes be combined into superg ...

outcropping in East Anglia, UK and adjacent areas of the

North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian S ...

. Its age ranges from approximately 4.4 to 0.478 million years BP, spanning the late

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

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 () m ...

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. Flint was widely used historically to make stone tools and start fir ...

. 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 which 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 tect ...

and vein quartz from the Midlands,

igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock ...

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 Byth ...

and

Proto-Thames The Ancestral Thames is the geologically ancient precursor to the present day River Thames. Relationships with other rivers During the Early and Middle Pleistocene, central and southern Britain had two main rivers of more than : the Bytham and ...

. The constituent formations of the Crag Group are the Coralline Crag (mid to late Pliocene); the

Red Crag ''Red Crag'' or ''Red Rock'' () was a 1961 novel based partly on fact by Chinese authors Luo Guangbin and Yang Yiyan, who were former inmates in a Kuomintang prison in Sichuan. It was set in Chongqing during the Chinese Civil War in 1949, and fe ...

(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

References

Geological groups of the United Kingdom Geologic formations of England Neogene England East Anglia {{England-stub

See also

References