Early commentators were puzzled by the similarity in appearance of the cliff faces at Steamer Point, on the north side of the harbour, and Hengistbury Head, on the south side of the harbour. The sequence of Boscombe Sand, pebble bed(s) and Barton Clay appeared in both cliff faces but the Hengistbury Head face stood several tens of metres lower than the projected dip of about one degree across the harbour would have placed it. This led to a confusing series of interpretations as commentators struggled to place the 'Hengistbury Beds' into some sort of orderly sequence in the local succession.

Prestwich (1848) put the height difference down to the fact that Hengistbury Head stood further to the south than the general line of the coast thereby placing it lower in the trough of the Hampshire Basin. Curry (1976) thought that a downfold in the strata below Christchurch Harbour was the cause, possibly combined with faulting of some description. He described the height deficiency as 40–50 metres. Bristow, Freshney and Williams (BGS sheet 329, NERC 1991) show a North West to South East trending fault down thrown to the South West running down the Stour Valley, the Stour Valley Fault. This brief study does not purport to add fresh evidence to these previous assertions except to suggest that the disturbance to the strata probably takes place to the SW of Mudeford Quay as is indicated on the BGS sheet.

In 1999 Christchurch Borough Council (CBC) undertook an engineering project at the entrance to Christchurch Harbour which involved sinking 14 metre Mudeford Quay shaft (PDF, 331kb) (opens in a new window)  on both Mudeford Quay and Mudeford Sandbank for the purpose of boring a narrow gauge tunnel beneath the harbour entrance channel. This tunnel, at -12 ma OD, was to carry mains services, sewage and electricity, across to the beach hut colony on the sandspit. In all a total of 20 small bore drilling investigations had been undertaken, mainly on Mudeford Quay, prior to various engineering schemes so that by 1999 a reasonably sound knowledge of the subterranean conditions existed. The proven existence of Branksome Sand bedrock was theoretically confirmed by a simple calculation involving the distance, heights and dip angle from the cliff at Steamer Point which placed the tunnel in Branksome Sand 'E' horizon. The Branksome Sand bedrock was everywhere overlain by fluvial sands and gravels.

When contours were plotted on the Branksome Sand erosion surface a trough was noted which suggested that an ancient incised channel had once existed at a depth of about -10 ma OD, running from west to east beneath the present day car park and promenade at Mudeford Quay. Research since the 1980s has identified several incised channel complexes in Christchurch Bay and the 'Mudeford palaeochannel' can be tentatively linked in with these.

Additionally, gravely/irony clumps of about 0.3 metre diameter were discovered during the excavation of the Mudeford Quay shaft, down to a depth of about -8 ma OD, reinforcing the idea of fluvial incision. These were felt possibly to be disrupted fragments from higher gravel terrace levels and may have come from the terraces visible in the cliff face between Hengistbury Head and Southbourne (BGS Terraces 3 & 5).

Of course, underground information gleaned from these unrefined engineering sources cannot be compared to the sort of detailed information yielded by prolonged scientific investigation. However, any information is better than none and it is believed that CBCs 1999 engineering project provided some pioneering glimpses into the geological situation that links the cliff sections of Poole and Christchurch Bays.

The writer is indebted to Ray Chapman for posing the question of, "What happens in the gap between Highcliffe and Hengistbury Head ?". This question sparked off the research into CBC's engineering records and the piecing together of the various bits of information making up this brief report. Also worthy of credit is Frank Tyhurst who was CBC's coastal engineer at the time of the tunnel boring. It was Frank who encouraged the writer to study and record the geological situation surrounding the project. The main contractor involved in the works was Quegan Civil Engineering Ltd who were tunnelling specialists. (M.H. Apr. '08)

• Beaches and Harbours (Christchurch)
  Name: Gary Foyle
  Email: beachesandharbour@christchurch.gov.uk
  Tel: 01202 495070
  Full details for Gary Foyle