The parish of Sudbourne is situated on a low northeast-southwest ridge of CRAG covered in places by glacially derived sand and gravel. The ridge is about 18 kilometres long and 2-3 kilometres wide. The southeast facing edge of the ridge is seen as a low (10 metres) but distinct feature when viewed from the river Alde/Ore, but there are no landscape features that clearly reveal the northwest limit.
The ridge is of considerable geological interest. The main core of the ridge consists of CORALLINE CRAG, a formation that exists nowhere else in Britain. Surrounding it and to a large extent surmounting it are deposits of the later RED CRAG. The two formations are distinguishable by the different fossils found in the crag. The term crag is used throughout East Anglia for any shelly, pebbly sand. The numerous fossils indicate a warm water origin for the Coralline Crag, estimated to be more than 2.5 million years old and a cold-water origin for the younger Red Crag. Geologists have used this distinction to take the junction of the top of the Coralline Crag and the base of the Red Crag as the boundary between the older PLIOCENE and the younger PLEISTOCENE epochs. The term Coralline is misleading, as there are no corals present. Various bryozoa were mistaken for corals when this crag was first studied in the early 19th century. The crags were formed as offshore shoals in strong tidal currents, conditions similar to those that exist off the east coast at the present time.
In much of the Sudbourne area the Coralline Crag is about 15-20 metres thick, becoming thinner at the edges where it is overlapped by the Red Crag. Below the crags lies the London Clay (Eocene age). There are numerous pits throughout the area where the crag and overlying sand were extracted for many years, though very little extraction goes on at present. The high shell content of the crag rendered it useful as a substitute for agricultural lime on the acidic sandy soils of the area. Its porosity continues to make it a suitable surface material on farm tracks today.
Most of the Coralline Crag is too friable for it to be used as a building stone, but some layers in the upper part of the formation are sufficiently well cemented for it to have been used for building in the past. The best examples of this can be seen in the towers of Chillesford and Wantisden churches to the west of Sudbourne and in the interior of Orford castle, which stands on the prominent southeastern escarpment of the crag ridge. Fragments of walls of Coralline Crag, some built in medieval times can be seen in a few places.
Most of the old crag pits have been disused for many years and are now completely overgrown, but good exposures of Coralline Crag can be seen in crag pits at Richmond Farm in Gedgrave (Grid Ref. TM412492) and at Crag Farm in Sudbourne (TM 429523), where also the farmyard is cut out of the crag. Both pits are on the southeastern slope of the ridge.
1. British Regional Geology. East Anglia and adjoining areas. HMSO 1968.
2. The Lithostratigraphy of the Coralline Crag (Pliocene) of Suffolk. Proc. Geologists' Assoc. 1993, Vol. 104, 59-70.