In comparison to Poole Bay, the shoreface is wider and shallower, promoting wave shoaling and refraction. The landward margin is pre-dominantly a currently or previously eroding cliffed coast of m in height e. Photo 3, and includes Christchurch Harbour confined by low spit beaches near its western extremity Photo 1. The study area extends several kilometres offshore to depths of between -6 and m CD to include the submerged rock platform of Christchurch Ledge and the sediment accumulations of Dolphin and Shingles Banks.
The bay is of relatively recent origin, formed by coastline retreat during the mid to late Holocene transgression, although its contemporary geomorphology has been influenced by earlier events. Pleistocene evolution was dominated by the Solent River which flowed across the floors of Christchurch and Poole Bays and eastwards through the Solent Everard , Dyer , Velgrakis et al During climatically-controlled fluctuating sea-levels of this period, superimposed upon possible neotectonic uplift Maddy, et al, , the river and its tributaries deposited a sequence of progressively wider gravel-floored channels and terraces which mantled the Tertiary deposits of Christchurch Bay Nicholls , Allen and Gibbard, A critical factor in the evolution of Christchurch Bay was the breaching of the Chalk ridge which previously extended between the Needles and Handfast Point, Purbeck.
Opinion is divided as to the date of this event. Until recently, the prevailing view favoured breaching in the early Holocene eg Everard Recent studies of the rates of Chalk erosion; remnant buried and infilled palaeo- channels located in a west to east sequence offshore; the chronology of the lower terrace sequence of the Stour and Avon; early human occupation sites around Christchurch Harbour, and the depth, relief and inclination of the planation surface which truncates the Purbeck-Wight Chalk ridge indicate an early to mid-Devensian breach West , Wright , Nicholls , Nicholls , Allen and Gibbard, , Brampton et al , Velegrakis et al , Maddy, et al An extension of the River Frome may have cut a gap through the western part of the ridge by the mid-Devensian, but later Holocene denudation of this feature appears to have utilised cols created by the headward erosion of rivers originally flowing southwards from its crest.
It is suggested that much of Poole Bay was eroded during the mid to late Devensian, but Christchurch Bay was largely protected by the resistant strata including ironstone concretionary seams of a previously much more extensive Hengistbury Head.
Velegrakis and Brampton et al describe a 2 m thick gravel ridge approximately 12 kms south of the Needles that both tentatively interpret as a relict barrier structure. This might define the position of the ancestral coastline in mid-Devensian times, when sea-level was between and m OD, prior to the final breakthrough of the chalk ridge. It was not until the early to mid-Holocene sea-level transgression years BP that the Chalk ridge was removed, and the barrier between the two bays now forming Christchurch Ledge was cut back, and rapid erosion of Christchurch Bay proceeded Wright , Bray and Hooke, b.
