Vectis Formation
The full thickness of the Vectis Formation (Fig 5) can be seen at three places on the Isle of Wight, where it reaches a maximum thickness of around 66 m (Insole et al. 1998). Radley and Barker (1998) record 55 m at Atherfield and 34 metres at Compton Bay. The base of the formation can easily be examined in the foot of the cliffs and on the foreshore at Compton Hay, and in the base of the cliffs between Barnes High and Cowleaze Chine. It is also easily accessible at Yaverland. Its base is marked by a change in colour from the predominantly reddish clays of the Wessex Formation to light and dark grey clays and silty clays of the Vectis Formation. Between Barnes High and Cowleaze Chine the base is a sandstone known as the 'White Rock' which serves as a convenient marker bed. Within the Vectis Formation is a 3 to 6 m thick yellow/orange sandstone body that conveniently divides the formation into basal, middIe and upper units (Stewart 1981a). Each of these units has now been formally named: in ascending order, the Cowleaze Chine Member, the Barnes High Sandstone Member and the Shepherd's Chine Member. Thin, fine- grained sandstones and finely laminated muds in the upper part of the Shepherd's Chine Member are known to yield pterosaur remains, often of articulated specimens (Hooley 1913, M. Green pers. com.). This is also the horizon containing rare plesiosaur remains (Langhan Turner pers. com.). Isolated vertebrate material comprising mainly fish bones is common in the base of silt or sand-filled gutter casts in the Shepherd's Chine Member. .
Invertebrates are common, and include several species of mollusc, with the bivalve Filosina comprising the main component of some shelly limestones in higher parts of the Shepherd's Chine Member (Radley and Barker 1998). Other abundant molluscs include the bivalves Cunecorbula, Nemocardium, Praeexogyra and the gastropods Viviparus, Paraglauconia and Procerithium. Insects are abundant in some of the mudstone concretions and gutter casts in the Shepherd's Chine Member where they are usually associated with comminuted plant material (Twitchett 1994). Ostracods occur in super-abundance in some of the shales of the Shepherd's Chine Member. .
Trace fossils occur throughout the section, and include those of invertebrates as well as dinosaur footprints (Radley et al. 1998). Invertebrate traces are particularly common on the undersides of some of the thin, fine-grained sandstones. .
In general the environments represented by the Vectis Formation are of very shallow lacustrine or lagoonal settings with fluctuating salinities (Stewart et al.l991). Much of the succession represents freshwater to oligohaline conditions with well-marked mesohaline to brachyhaline intercalations occurring in the highest part. .
Derived Jurassic fossils can be abundant in the upper parts of the Vectis Formation, and include the phosphatised steinkerns of Kimmeridgian perisphinctid ammonites and calcitic oysters and echinoid spines of Late Jurassic age (Radley et al. 1998).
Figure 5. Stratigraphic Logs for the Vectis Formation.
