The Geology of the Isle of Wight
Figure 1. a) Simplified geological map of the Isle of Wight b) Main structural elements (Based on Insole et al. 1998).
Geologically, the Isle of Wight is rather special. Only sedimentary rocks are exposed at the surface on the Island, and they belong to only three periods: the Cretaceous, the Palaeogene and the Quaternary (Fig. 1).
The northern part of the Isle of Wight consists largely of clays and limestones deposited during the Palaeogene period. The oldest of these, called the Reading Beds, are Palaeocene in age and can be seen in Alum and Whitecliff Bays where they rest unconformably on the Cretaceous Chalk formations. Although they were deposited in lakes and rivers that surely must have teemed with life, fossils are not common in these deposits. Lying on top of the Reading Beds are Eocene sediments represented mainly by clays and sands deposited in shallow seas and lagoons. They are richly fossiliferous, with marine molluscs being especially abundant in the lower part of the sequence. The upper part of the sequence was deposited in a large freshwater lake that was periodically inundated by the sea, turning it into a brackish water lagoon. Freshwater periods produced limestones that are locally rich in the freshwater gastropods Limnaea and Planorbis. Fossils are very common in the limestones that crop out at Bembridge, Thorness and elsewhere on the north coast of the Island. Non-marine snails are always the most common fossils, but bones of turtles, crocodiles and fishes can be found in the clays and marls that lie above and below the limestones. Mammal remains also occur occasionally. These freshwater sediments were formerly thought to be of Oligocene age, but now only a small part at the very top of the succession is considered to be Oligocene (Insole et al. 1998). Younger Miocene and Pliocene strata have been eroded away, or perhaps were never deposited.
Lying unconformably on top of both Palaeogene and Cretaceous strata are rocks of Pleistocene age. These are not well exposed except on the tops of some cliffs on the southwest of the Island, and at very low tides for only a few minutes on the northern coast at Newtown Hay. They consist mostly of unconsolidated sands and gravels and are most widespread in the floors of some of the larger valleys. From these deposits an Ipswichian age level at Newtown yields occasional remains of large mammals, particularly bison, deer and elephant. Sometimes hand axes and other pieces of worked flints are found in these gravels, attesting to the presence of early man in the region.
Underlying the Palaeogene strata are rocks of Cretaceous age: the last period of the Mesozoic, and a time when the Earth was dominated by giant dinosaurs on land, equally giant marine reptiles in the oceans and spectacular flying reptiles in the skies. The Cretaceous period began about 144 million years ago and ended 65 million years ago when a massive meteorite impacted on the Earth causing global climatic change with devastating consequences for life. This event is not recorded in the rocks on the Island: they had been removed by erosion before the first sediments of the Palaeogene period were deposited.
The oldest rocks exposed on the Isle of Wight were deposited just a few million years after the Cretaceous period began (to examine the very earliest rocks of the Cretaceous period in England you must visit the nearby Isle of Purbeck in Dorset). The earliest Cretaceous rocks on the Isle of Wight are exposed between Compton and Brighstone Bays on the southwest coast and near Sandown on the east coast. Called the Wessex Formation, these strata belong to the Wealden Group and consist mainly of clays and sandstones. In all, approximately 250-260 metres thickness of the Wealden Group is exposed (180-190 metres of Wessex Formation and approximately 70 metres of Vectis Formation). The clays are usually varicoloured in hues of pink, purple, green, ochre and grey. Sometimes orange-brown patches and veining can be seen in them, and these varicoloured clays are thought to represent areas where ar~cient soil-generating processes altered the clays (Allen 1998). Most of these clays were deposited on river floodplains and in shallow lakes that often contracted during long periods of drought, the evidence of which is found in the form of mudcracks, produced when the wet clays dried out and shrank. The shrinkage produced cracking and fissuring of the ground in irregular polygonal shapes. When the next wet period came and flooded the parched ground the cracks were filled with mud, silt or sand. These now show up as irregular patterns on exposed rock faces.
The Wealden Group rocks are the source of virtually all of the dinosaur fossils found on the Isle of Wight. Exceptions are some rare and fragmentary dinosaur bones from the rocks that overlie the Wealden Group, collectively known as the Greensands. The Wealden Group is divided into two formations: the lower Wessex Formation and the younger Vectis Formation. The Wealden Group belongs to the Wealden Supergroup that, on the mainland, comprises the Wealden Group and the underlying Hastings Group (Ruffell et al. 1996). This latter group has not been detected on the Isle of Wight.
Within the mostly clay sequence of the Wessex Formation are two other rock types. The sandstones of the Wessex Formation are texturally variable. Most are very fine-grained and well-sorted, but some are very coarse, even gritty or pebbly. These were deposited in meandering river channels, by periodically flooding rivers draining from higher ground to the west and southwest. Sometimes the rivers flooded their banks and deposited their finer sand load on the river floodplain. When the floodplain had been trampled by dinosaurs the sand filled in the deep holes left by the biggest of the dinosaurs. When the sand infilling the faotprints became cemented to produce sandstone, the impressions of the dinosaurs' feet became preserved. These can be found as isolated boulders on the beach at such places as Hanover Point (SZ 378838) and Yaverland (SZ 617853).
The second rock type is a chaotic mixture of sand, silt and mud, with the remains of plant fragments, ranging from small twigs and pieces of plant debris to large logs several metres in length. These deposits are known as plant debris beds and are rich in vertebrate fossils.
The highest beds of the Wealden Group are called the Vectis Formation. They have been studied in detail recently by Ruffell (1988), Stewart et al. (1991), Radley and Barker (1998) and Radley et al. (1998a, b) who all consider that they were deposited largely in a series of very shallow low salinity lagoons. Occasionally the lagoon margins dried up to produce mudcracks. These surfaces occasionally yield fossil footprints, showing that dinosaurs walked over the mudflats. Remains of dinosaur skeletons are rare in these deposits, though pterosaurs occur infrequently.
Above the Wealden Group are a series of thicker sandstone units with intervening beds of clays. These rocks, called the Upper and Lower Greepsands and Gault Clay, are rich in marine fossils. They indicate a period when the Cretaceous sea levels began to rise around the world resulting in the drowning of vast areas of coastal plain and of river valleys. For the dinosaurs of the coastal plains this meant a considerabIe contraction in their available living space. Forests blanketing the coastal swamplands were destroyed and the sea began to erode some of the earlier Jurassic rocks exposed as slightly higher ground to the north and west of the region. Jurassic fossils were washed out to sea during storms and redeposited as derived fossils. They are abundant in sediments at the very top of the Wealden Group and at several levels of the Lower Greensand. These derived fossils are the oldest from the Island with some having been derived from the Lower Jurassic (Sinemurian, ex Lias Group (Radley 1993a)). Several types have been found, including ammonites and bivalves, and isolated plesiosaur bones.
Continued rising of global sea level during the Cretaceous eventually submerged the low ground to the north. One effect of this marine transgression was to cut off the supply of land-derived sediment to the basin. Consequently the only material to accumulate on the sea floor over southern England and indeed much of Europe during the latest Cretaceous were the remains of microscopic calcareous algae called coccolithophoroids. Their limy remains form the beautiful white chalk cliffs that makes the coastline of southern England so distinctive, and are the very reason why this Island is called the Isle af Wight. The chalk cliffs on the Isle of Wight are rich in fossils, mainly of shelly invertebrates, but the teeth of sharks and other fishes, and the bones of marine reptiles can also be found.
