Adapted from Daley and Balson, 1999.
Charlesworth first identified the Red Crag as a separate part of the Crag Formation in 1835, naming it after its characteristic red-brown colour caused by iron oxides in surface exposures. However, at depth in boreholes and below the water table the sediments are grey.
The outcrop of the Red Crag Formation is restricted to southeast Suffolk and northeast Essex. The formation is up to 20 m thick and is separated from the underlying Coralline Crag by a marked unconformity (as seen, for example, at Ramsholt Cliff and Rockhall Wood), and from the overlying Norwich Crag probably by a disconformity.
Over most of its outcrop the Red Crag unconformably overlies the London Clay Formation but it oversteps Palaeocene formations and rests on the Late Cretaceous Chalk in the west. A discontinuous conglomerate up to 30 cm thick occurs at the base of the Red Crag. This lag deposit, episodically exposed in the cliff section at Bawdsey, is composed mainly of phosphatic mudstone pebbles in some places but is elsewhere dominated by flint pebbles and cobbles.
In former times the Red Crag was placed at the base of the Pleistocene. Based on the presence of the planktonic foram Neogloboquadrina atlantica, the formation has been dated at 3.2-2.4 Ma. Until recently, this date placed it in the Pliocene but a change in the position of the Pliocene–Pleistocene boundary to 2.59 Ma means that at least some of the Red Crag may now be of early Pleistocene age.
Harmer (1900) subdivided the Red Crag into three ‘stages’ on the basis of the molluscan faunas: Waltonian, Newbournian and Butleyan. These stages were thought to show the progressive retreat and cooling of the Red Crag Sea northwards, with ever-greater proportions of extant and ‘boreal’ mollusc species. The southernmost exposures of Red Crag, seen particularly well at Walton-on-the-Naze in Essex, were assigned to the Waltonian and considered to represent the oldest Red Crag containing the warmest water fauna, including many species that are rare or absent elsewhere in the Red Crag. In contrast the Newbournian and Butleyan, outcropping further to the north, probably indicated cool temperate conditions.
The Red Crag Formation accumulated in a shallow marine environment, with sediments deposited under a regime of tidal dominance and high energy. The lower part of the Red Crag contains large-scale, high-angle, approximately planar tabular cross-sets (e.g. Walton-on-the-Naze, Bawdsey, and the lowermost part of the sequence at Vale Farm, Broom Covert and Orford Lodge). The middle part shows complex trough-cross-bedding, low-angled bedding and common mud-drapes (e.g. Vale Farm, Broom Covert and Orford Lodge). The more heterogeneous upper part is dominated by horizontal or near-horizontal bedding (e.g. Vale Farm, Broom Covert). This sequence indicates shallowing, probably the result of increasing constriction of a tide-dominated estuary.
Below is a list of Red Crag localities, shown on the map below. Please follow the links for more information.
- Hascot Hill Pit
- Buckanay Farm (images)
- Bawdsey Cliff (images)
- Vale Farm (images)
- Broom Covert
- Orford Lodge (images)
- Waldringfield Heath (images)
- Neutral Farm Pit (images)
- Walton-on-the-Naze (images)
Figure. Map of the Crag outcrop in southern East Anglia.