Ipswich model information

Orwell Bridge

The model is a small section of an extensive regional model that covers 1600 km2. The full model extends to include Sudbury, Colchester and Harwich.

The model was built by BGS in 2004 using GSI3D and is calculated using a DEM with a 50 m grid spacing, it extends to about -300 m OD and is best viewed using a vertical exaggeration of between x10 and x15.

The model boundary follows the major orbital and ring roads. This includes the A14 along the western and southern sides as well as the Orwell Bridge crossing.

The model was constructed as part of an integrated remapping and modelling programme and takes full account of the BGS borehole database and the existing published scientific literature.

Superficial geology

The model straddles the alluvium of the River Gipping and its southerly transition into the intertidal flats of the Orwell Estuary. These valley floor deposits are locally flanked by river terrace deposits that accumulated during downcutting of the valley to its current level.

To either side of the main valley estuary the interfluves are underlain by a complex sequence up to 30 m thick of periglacial braided outwash deposits (Kesgrave sand and gravel) and the suite of sediments laid down by the Anglian ice-sheet including the Lowestoft Till (aka Chalk Boulder Clay) and fluvial and lake sediments. The Anglian ice sheet extended farther south in eastern England than younger ice-sheets so these deposits are well preserved with Ipswich marking the maximum limit of this ice-sheet in this area. Together these superficial deposits range in age from about 1.5–0.5 million years.

Bedrock geology

The bedrock geology comprises several distinct layers. The youngest late Pliocene deposits of the Red Crag comprise up to 20 m of heavily ironstained shelly shallow marine sands which are famous for their fossil content and phosphate pebbles (coprolites); formerly dug to manufacture fertilisers. The London Clay and Lower London Tertiaries are Palaeogene in age and comprise mainly clays, silts and sands with thin pebble beds. The Thames Group consists of the Harwich and the London Clay formations combined and is of marine origin. The Lower London Tertiaries are here made up of the mainly non-marine Lambeth Group with very thin Thanet Sands at the base that contain glauconite indicating their marine origin. The Palaeogene deposits are up to 30 m thick in total in this area and dip south-eastwards at less than 1°.

The entire area is underlain by the Upper Cretaceous Chalk which is up to 250 m thick comprising fine grained white and grey chalk with abundant flint nodules in its upper parts. The Chalk comprises a significant aquifer and most large farms around Ipswich are supplied by at least one deep well sunk into the Chalk. The basal layer of the model is the Lower Cretaceous marine Gault Clay up to 20 m thick; it is only encountered by the deepest boreholes in the region.

Reference

MATHERS, S J, WOODS, M A and SMITH N J P.  2006.  Geology of the Ipswich district — a brief explanation of the geological map.  Sheet Explanation of the British Geological Survey. 1:50 000 Sheets 207 Ipswich (England and Wales) 34pp plus 50K map.