Solid Geology
300 million years ago, the land around Great Yarmouth sat on the North-Eastern edge of a large mudstone and limestone deposit: the London – Brabant Massif. North of Yarmouth, the North Sea Basin was a large desert (Anderton et al., 1987, 182). The remains of this junction survive as a wedge of Permian shales and Triassic sandstones overlying Silurian mudstones and carboniferous limestone that appear at Gorleston, and continue northwards under the North Sea Basin. Later Triassic and Jurassic deposits have been eroded away, so the Triassic rocks are overlain unconformably by Cretaceous Gault and Upper Chalk (Arthurton et al., 1994, 11). The chalk is overlain unconformably by Tertiary deposits: Ormesby clay (Thanet clay) and the Hales, Harwich and Walton members of the London Clay (Arthurton et al. 1994, 25ff). The whole of East Anglia was lifted up out of the sea during the Miocene epoch, culminating in the development of a large river system and delta running northeast from west of London. The delta of this river system left up to 30m of crag (deposits of shelly sands and interbedded clays), dating from 5 million years ago to about 1.5 million years ago (the Pliocene) (Arthurton et al., 1994, 30).
The crag, along with the remains of the proto-Thames delta are covered throughout most of East Anglia with Anglian deposits of Lowestoft Till. Under Great Yarmouth itself, however, the crag is covered unconformably by deposits of Yare Valley sands and gravels. These are thought to have derived from fluvial deposits laid down by the East Anglian river system (Arthurton et al., 1994, 70).
Drift Geology
The valley formed by the river that deposited the Yare Valley Formation is buried by a succession of peats and clays known as the Breydon formation. The lower peat, dated to between 9000 and 7500BP is attributed to the freshwater river system that formed the river valley. Around 7500BP, conditions in the estuary become far more saline, leading to the deposition of the lower clay. Coles and Funnell (1981) attribute the increase in salinity to a rise in sea levels (the Flandrian transgression). During the period of the lower clay, coastal conditions led to the formation of a coastal barrier, which all but closed off the estuary around 4500BP. The closed estuary accumulated thick peat deposits (the Middle Peat), until about 2000BP, when the breaching of the coastal barrier led to more estuarine environments (Coles and Funnell, 1981). The cause of the breach in the coastal barrier is unknown, but studies of the main river channel through the estuary show a larger freshwater discharge than the present day. Alderton suggests that this may be connected with vegetation clearance and hence higher run-off greater upstream in the Waveney Valley (and by implication in the Bure and Yare valleys also) (Alderton, 1983, 307). The barrier reformed at some point in the first millennium AD (Coles and Funnell’s dates suggest around 500AD (1981, p128), while Arthurton et al. (1994, 87) and Alderton (1983, 308) suggest later, towards the end of the first millennium AD), creating the modern estuary.
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