| Crag Group | |
| Period: | Pliocene |
| Age: | Plio-Pleistocene ~ |
| Type: | Geologic group |
| Prilithology: | Sand |
| Otherlithology: | Gravel, clay, silt, calcarenite |
| Subunits: | Coralline, Red, Norwich & Wroxham Crag Formations |
| Underlies: | Dunwich, Albion Glacigenic, Britannia Catchments & British Coastal Deposits Groups |
| Overlies: | Unconformity with Chalk Group and London Clay Formation |
| Thickness: | Up to 70m (230feet) offshore |
| Namedby: | R.C. Taylor |
| Year Ts: | 1823 |
| Region: | East Anglia |
| Extent: | East Anglia & North Sea |
The Crag Group is a geological group outcropping in East Anglia, UK and adjacent areas of the North Sea. Its age ranges from approximately 4.4 to 0.478 million years BP, spanning the late Pliocene and early to middle Pleistocene epochs.[1] [2] 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 but weather bright orange, with haematite 'iron pans' forming. The lithology of the lower part of the Group is almost entirely flint. The highest formation in the Group, the Wroxham Crag, contains over 10% of far-travelled lithologies, notably quartzite and vein quartz from the Midlands, igneous rocks from Wales, and chert from the Upper Greensand of southeastern England. This exotic rock component was introduced by rivers such as the Bytham River and Proto-Thames.[3] [4]
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).[5] The sedimentary record is incomplete, leading to difficulties in correlating and dating sequences[6]
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.[7]