The area of Christchurch and Poole Bays has been cut by the sea in the period since the breaching of the Chalk ridge between the Isles of Wight and Purbeck late in the Flandrian transgression. Erosion of the soft Tertiary rocks must have been fast and is still active. In the Barton area the cliffs have been receding at about 1 m/year since 1895 (May, unpublished). There the cliffs are composed of badly cemented sands and sandy clays, capped with plateau gravels about 3 m thick, and erosion of them has released large quantities of sand and gravel, sufficient to maintain features such as Hurst Spit. This Spit approaches within 1.3 km of the Isle of Wight, but is separated from it by the tidal channel of the West Solent, in this area up to 60 m deep. In these narrows the south west flowing ebb current reaches 2.25 m/sec, whereas the flood current only reaches 2 m/sec.