Plymouth Sound, Shores and Cliffs
Encyclopedia
Plymouth Sound, Shores and Cliffs is a Site of Special Scientific Interest
(SSSI) around the Plymouth Sound
, a large area of water where the River Plym
and Tamar
meet. It stretches across the two ceremonial counties of Devon
and Cornwall
and the unitary authority area of Plymouth
. It contains fossils of plants and sea creatures and its cliffs show a timeline of the Middle to Early Devonian
period hundreds of millions of years ago.
Lower to early Middle Devonian
units of rock are visible from sections through the coastal cliffs and intertidal zone
s, which were laid down 417–354 million years ago. There are fossil remains of coral
s, brachiopod
s, bryozoa
, gastropods, rare orthoconic
nautiloid
s, ostracod
s, trilobite
s, phacopids), crinoids, ostracoderm
and acanthodian fish.
Jennycliff Bay has steeply inclined folds, which flexure into open recumbent folds — some with chevron form. The folds have developed cleavage
in the slate
and in the siltstone
s and sandstone
s, although it is more widely spaced throughout the silt and sandstones. To the south, the cleavage dips at moderate low angles and the strata
, which become overturned in places, dip steeply. The original direction of the strata can be established using the graded and cross-bedding of sedimentary structures. It demonstrates that the section is situated on the overturned, steep or northern limb of the Dartmouth Antiform
, which can be traced from Newquay
to Dartmouth
. This is evidence a period of tectonic plate collision between 330 and 300 million years ago, which exerted pressure on the rock. Folds of three generations with cleavages are visible through the section in Bovisand Bay. Generally, the first generation folds have gently southern dipping limbs which are overturned or steep. The cleavage consistently dips about 40 degrees south. This indicates that this section has the Dartmouth Antiform passing through it. The far south consists of red and green slates with sandstones that were deposited in rivers and lakes.
On the western flank (Cornwall
), there are harder rocks such as Staddon Grit
, which form a headland from Staddon Point. There are also deposits of sandstones in the shape of marine sand bars. Along most of the coast dramatic faults and folds in the rocks can be seen and, in some places, a chevron pattern is visible.
Site of Special Scientific Interest
A Site of Special Scientific Interest is a conservation designation denoting a protected area in the United Kingdom. SSSIs are the basic building block of site-based nature conservation legislation and most other legal nature/geological conservation designations in Great Britain are based upon...
(SSSI) around the Plymouth Sound
Plymouth Sound
Plymouth Sound, or locally just The Sound, is a bay at Plymouth in England.Its southwest and southeast corners are Penlee Point in Cornwall and Wembury Point on Devon, a distance of about 3 nautical miles . Its northern limit is Plymouth Hoe giving a north-south distance of nearly 3 nautical miles...
, a large area of water where the River Plym
River Plym
The River Plym is a river in Devon, England. Its source is some 450m above sea level on Dartmoor, in an upland marshy area called Plym Head. From the upper reaches which contain antiquities and mining remains the river flows roughly southwest and enters the sea near to the city of Plymouth, where...
and Tamar
River Tamar
The Tamar is a river in South West England, that forms most of the border between Devon and Cornwall . It is one of several British rivers whose ancient name is assumed to be derived from a prehistoric river word apparently meaning "dark flowing" and which it shares with the River Thames.The...
meet. It stretches across the two ceremonial counties of Devon
Devon
Devon is a large county in southwestern England. The county is sometimes referred to as Devonshire, although the term is rarely used inside the county itself as the county has never been officially "shired", it often indicates a traditional or historical context.The county shares borders with...
and Cornwall
Cornwall
Cornwall is a unitary authority and ceremonial county of England, within the United Kingdom. It is bordered to the north and west by the Celtic Sea, to the south by the English Channel, and to the east by the county of Devon, over the River Tamar. Cornwall has a population of , and covers an area of...
and the unitary authority area of Plymouth
Plymouth
Plymouth is a city and unitary authority area on the coast of Devon, England, about south-west of London. It is built between the mouths of the rivers Plym to the east and Tamar to the west, where they join Plymouth Sound...
. It contains fossils of plants and sea creatures and its cliffs show a timeline of the Middle to Early Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
period hundreds of millions of years ago.
Geology
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
units of rock are visible from sections through the coastal cliffs and intertidal zone
Intertidal zone
The intertidal zone is the area that is above water at low tide and under water at high tide . This area can include many different types of habitats, with many types of animals like starfish, sea urchins, and some species of coral...
s, which were laid down 417–354 million years ago. There are fossil remains of coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s, brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
s, bryozoa
Bryozoa
The Bryozoa, also known as Ectoprocta or commonly as moss animals, are a phylum of aquatic invertebrate animals. Typically about long, they are filter feeders that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia...
, gastropods, rare orthoconic
Orthocone
An orthocone is a usually long straight shell of a nautiloid cephalopod. During the 18th and 19th centuries, all shells of this type were named Orthoceras, but it is now known that many groups of nautiloids developed or retained this type of shell....
nautiloid
Nautiloid
Nautiloids are a large and diverse group of marine cephalopods belonging to the subclass Nautiloidea that began in the Late Cambrian and are represented today by the living Nautilus. Nautiloids flourished during the early Paleozoic era, where they constituted the main predatory animals, and...
s, ostracod
Ostracod
Ostracoda is a class of the Crustacea, sometimes known as the seed shrimp because of their appearance. Some 65,000 species have been identified, grouped into several orders....
s, trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
s, phacopids), crinoids, ostracoderm
Ostracoderm
Ostracoderms are any of several groups of extinct, primitive, jawless fishes that were covered in an armor of bony plates. They belong to the taxon Ostracodermi, and their fossils are found in the Ordovician and Devonian Period strata of North America and Europe...
and acanthodian fish.
Jennycliff Bay has steeply inclined folds, which flexure into open recumbent folds — some with chevron form. The folds have developed cleavage
Cleavage (geology)
This article is about rock cleavage, for cleavage in minerals see Cleavage Cleavage, in structural geology and petrology, describes a type of planar rock feature that develops as a result of deformation and metamorphism. The degree of deformation and metamorphism along with rock type determines the...
in the slate
Slate
Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...
and in the siltstone
Siltstone
Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...
s and sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
s, although it is more widely spaced throughout the silt and sandstones. To the south, the cleavage dips at moderate low angles and the strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
, which become overturned in places, dip steeply. The original direction of the strata can be established using the graded and cross-bedding of sedimentary structures. It demonstrates that the section is situated on the overturned, steep or northern limb of the Dartmouth Antiform
Anticline
In structural geology, an anticline is a fold that is convex up and has its oldest beds at its core. The term is not to be confused with antiform, which is a purely descriptive term for any fold that is convex up. Therefore if age relationships In structural geology, an anticline is a fold that is...
, which can be traced from Newquay
Newquay
Newquay is a town, civil parish, seaside resort and fishing port in Cornwall, England. It is situated on the North Atlantic coast of Cornwall approximately west of Bodmin and north of Truro....
to Dartmouth
Dartmouth, Devon
Dartmouth is a town and civil parish in the English county of Devon. It is a tourist destination set on the banks of the estuary of the River Dart, which is a long narrow tidal ria that runs inland as far as Totnes...
. This is evidence a period of tectonic plate collision between 330 and 300 million years ago, which exerted pressure on the rock. Folds of three generations with cleavages are visible through the section in Bovisand Bay. Generally, the first generation folds have gently southern dipping limbs which are overturned or steep. The cleavage consistently dips about 40 degrees south. This indicates that this section has the Dartmouth Antiform passing through it. The far south consists of red and green slates with sandstones that were deposited in rivers and lakes.
On the western flank (Cornwall
Cornwall
Cornwall is a unitary authority and ceremonial county of England, within the United Kingdom. It is bordered to the north and west by the Celtic Sea, to the south by the English Channel, and to the east by the county of Devon, over the River Tamar. Cornwall has a population of , and covers an area of...
), there are harder rocks such as Staddon Grit
Gritstone
Gritstone or Grit is a hard, coarse-grained, siliceous sandstone. This term is especially applied to such sandstones that are quarried for building material. British gritstone was used for millstones to mill flour, to grind wood into pulp for paper and for grindstones to sharpen blades. "Grit" is...
, which form a headland from Staddon Point. There are also deposits of sandstones in the shape of marine sand bars. Along most of the coast dramatic faults and folds in the rocks can be seen and, in some places, a chevron pattern is visible.