THE GEOLOGY OF TORBAY
Torbay is noted for its fine scenery, especially along the coast. This is mainly due to the great mixture of different types of rocks which can be found in the area, and the things that have happened to them in the past.
Many of the types of rocks in the area were formed by the laying down or deposition of small pieces of material or sediment. This sediment was carried in water or by the wind. When it was deposited it hardened and formed layers of rock, one on top of another.
The first, and oldest, group in the area is made up of rocks originally laid down underneath the sea. They are collectively known as the 'Devonian' group. The group includes Slates, Limestones and Tuffs. Each of these rock types was made from a different kind of sediment; slate is mud which has been turned to stone, limestone is the compressed material from old coral reefs and tuffs are the cooled and compressed ash from a volcano.Each of these rock types was laid down during a separate period and the layers form a sequence in time.
The second, younger Permian group also include rocks formed from sediments, but these were built up on a land surface. At the time of their formation Torbay was part of a large desert area. Erosion of the bedrock due to weathering produced a wide range of sediment. The sandstones that were formed are commonly red in appearance and give the soil and beaches of the Bay their rich colour today. Unlike the first group, the different rock types were not formed at different times. There are two main types; 'Breccia', which was formed by large, angular fragments held in a mixture of sand, silt and clay, and 'Conglomerates', which were formed from a similar mixture carrying rounded fragments.
One very important difference between the two groups is that the Devonian rocks, which were formed much earlier than the Permian rocks, were affected by violent earth movements, which lasted for several hundred thousand, or even million, years, 300 million years ago. The original rigid layers of rock were subjected to great forces from beneath and from the side.
Sometimes this caused them to bend, or "fold" over, and even to be completely overturned. On other occasions the pressure merely produced cracks and fractures, or "faults", within the layers. The rock-mass on one side of a fault has often been moved, or displaced, relative to the rock-mass on the other side of the fault. This displacement may have been caused by one mass being pushed over another, or by the two masses being pulled apart. This formative period in Earth's history is known as the Variscan or Hercynian orogenic cycle, and led to the formation of mountain chains in North Africa, Europe and North America. The Permian rocks did not form until after these great disturbances and consequently show little breakage and only slight tilting.
The Devonian group was given its name because these types of rock were first described in Devon, in 1839. Most of the different types can be found 'outcropping' in Torbay today.
On top of this 'bedrock' (Devonian + Permian) other sediments have been laid down in recent times. Some of these materials were deposited by the sea, but much more was produced by the breaking-up of the bedrock under very cold climates, when vast icesheets periodically covered much of Britain during the last 500,000 years. The sea has left behind 'raised beach' deposits which consist of layers of rounded pebbles, sand and silts often with shells, as for example at Hope's Nose, providing evidence of former, higher sea-levels.
The cold climate, or periglacial, processes produced extensive layers of material over slopes, both along the coast and inland. The type depends on the bedrock from which the material was formed. If it formed over an area of slate for example, it will be made up of broken fragments of slate in a mixture of clay, silt and sand. If however it developed in an area of sandstone it might be difficult to distinguish from weathered sandstone or breccia.
Other superficial deposits include those produced during warmer phases between the glacial periods, and found today mostly in caves such as Kents Cavern. These sediments contain many fascinating fossil remains of the animals living during these times.
The melting of icesheets, beginning some 15,000 years ago and marking the end of the last glaciation, caused quite a spectacular rise in sea-level, flooding the floor of the English Channel and Torbay. This drowned and eroded forests, so that many fragments of fossil vegetable matter and animal bones are still being washed up around the Bay.
In considering the cave bearing rocks of north Torbay, two types are of overwhelming importance: the Middle Devonian limestones and the limestone rich Permian conglomerates which overlie the Devonian sequence.
Middle Devonian limestone deposition took place on shallow shelf areas just offshore. These shallows had been formed when subterranean forces caused the separate parts of the earths surface ("plates") to be shifted and raised (Tectonic movements); away from these shelves deepwater slate deposition continued. The lowest formation, the Daddyhole limestone, consists of a dark grey well-bedded limestone with locally abundant corals which can be clearly seen in the cliffs below Daddyhole Plain. At its top the appearance of the fossilized remains of stromatopores (tiny marine animals whose strong calcareous skeletons often formed the basis for the development of coral reefs) and unconformities within the limestone sequence mark a period of shoaling. In the Devonian period this resulted in the development of stromatoporoid reefs and the deposition of the thick overlying Walls Hill Limestone. This is a massive pale grey limestone, which contain abundant fossils of tiny marine animals.
Toward the end of the Middle Devonian, further deepening of the sea resulted from the subsidence of the reefs, and the uppermost limestones are rocks composed of the broken up remains of corals and other animals (Barton Limestone). Finally the sinking reefs at the end of the Middle Devonian were covered by the deep water Gurrington Slate.
The Devonian rocks of the Torbay area have been very extensively affected by Hercynian deformation at the end of the Carboniferous period. Folding and thrust faulting has produced an outcrop pattern characterised by many small discrete blocks of limestone, slates and other rocks. Thrust faults are those fractures which occur in an almost horizontal plane ( any subsequent displacement is usually in a near horizontal direction also. ) The main blocks of limestone are centred around Walls Hill and Torquay Harbour and a series of limestone outcrops stretches north to where they disappear under a cover of Permian conglomerates around Watcombe. Within these blocks the limestones may be further folded and thrust faulted, with well developed cleavage planes. Cleavage planes are surfaces related to the molecular structure of the mineral or rock along which it may break when under stress. Such planes are independent of bedding planes, being formed during deformation of the rock. They may be important in cave development, as in the Ore Stone cave, which is formed on a thrust fault.
The overlying Permian ("New Red") conglomerates were deposited after 'post-Hercynian' uplift raised the area many hundreds of metres to produce an upland region bordering a basin to the east. In a semi desert environment, huge amounts of material were eroded from the resistant Devonian limestone ridges.
Where the conglomerates contain a very high proportion of limestone, they are effectively impure limestones, similar to the Dolomitic Conglomerate in Somerset, where the River Axe has carved out Wookey Hole. The Oddicombe Breccia, which is composed of limestone with a small proportion of Devonian Slate, is a hard, very massive well-jointed rock composed of cobbles cemented by a fine matrix and calcite (a form of calcium carbonate). The limestone conglomerates outcropping further south at Livermead Head are similar, but also contain a substantial amount of porphyry. This is an 'igneous1 rock, formed by the cooling of volcanic products from the Dartmoor area. It is characterised by the presence of large crystals (phenocrysts) which formed initially during a period of very slow cooling. These are held in a groundmass of much finer crystals which formed during the following period, when more rapid cooling occurred. These limestone rich conglomerates outcrop in a sequence also containing thick deposits of slate conglomerates (the result of rapid early erosion of the soft Devonian slates).
The Devonian slate and limestone outcrops to the north of Torbay represent a Permian landform, a ridge exhumed from beneath the conglomerates: its western continuation has been displaced to the north by extensive faulting. To the north of this outcrop the limestone appearing at Petit Tor and Barton represent the tops of limestone hills sticking out from beneath the conglomerates. The Limestone conglomerates have extensive outcrops to either side of the ridge. To the north the Oddicombe Breccia, which is over 100 metres thick, outcrops within a sequence of northward sloping layers of conglomerates: it is exposed on the coast between Maidencombe and Babbacombe. To the south limestone conglomerates are exposed on the coast at Hollicombe and Livermead Head. Conglomerates composed of other rocks are generally too soft for caves to form in them, but an extensive complex of sea caves has formed in well cemented porphory-limestone conglomerates at Corbyns Head.