The Red Crag Formation is a series of marine deposits at the base of the Pleistocene (which started 2.58 million years ago) in Suffolk and Essex. This material rests on an erosion surface of Cretaceous to Palaeogene rocks. It is a shore deposit, medium to coarse-grained and locally shelly.
The most extensive exposure of Red Crag is found at Bawdsey Cliff, the site of which is designated a Site of Special Scientific Interest (SSSI). Around 2 kilometres (1.2 mi) of Crag from the Gelasian are exposed. Red Crag deposits have been dated from Gelasian age at The Naze SSSI to Preludhamian age at Sizewell on the basis of microfossils. The Naze is designated the type site for the British Waltonian stage, the first phase of the Red Crag exposure and of the Pleistocene.
Both the Red Crag and the Waltonian are dated by some authorities to the preceding Piacenzian, the last stage of the Pliocene.
Crag is a local word for a shelly sand. The Red Crag is generally stained red by ferruginous compounds.