June 2024 – updated 6/22/2024 (Dorset)

Southwest England (General)

[Mostly sourced from BGS Regional Summaries: SW England; some synthesized from across document]Southwest England is characterized by  Devonian and Carboniferous (420 – 299 Ma) sedimentary rocks like shales, sandstones, and limestones that were folded and faulted during the Variscan orogeny (which contributed to the formation of Pangea) during the same period. The sedimentary rocks result from a series of shallow, largely marine environments; thus fossils are abundant. There are also intrusions of granitic rocks (~ 280 Ma), and some very recent sedimentary beds resulting from glacial melting over the last 2 Ma. 

Rock Types: Sedimentary, Metamorphic, Igneous

There are three predominant types of rock in southwest England: ‘recent’ sedimentary rocks (25-300Ma ); older sedimentary rocks (315-410 Ma); and plutons connected to the Cornubian batholith.

• The recent sedimentary rocks were laid down during more tropical climatic periods when the area was close to the equator. They represent environments that were marine (primarily shallow), fluvial, or desert. Many of them have abundant fossils of marine and terrestrial life.
• The older sedimentary rocks represent predominantly marine environments (shallow and deep). They were subject to tectonic forces about 310 Ma, and are consequently folded and subjected to high temperature (and sometimes high pressure) metamorphism. 
• The igneous rocks are granite, and the plutons, including those at Dartmoor and St Austell) are all connected to the Cornubian batholith. The granite intruded into the overlaying sedimentary rocks around 280 Ma. See red areas of the figure below; also see the section on “The Granite of Dartmoor.” This lead to regional and contact metamorphism, as well as subsequent hydrothermal alteration.

These sedimentary rocks have been subject to low grade regional metamorphism and, over extensive areas, and have also experienced high grade contact metamorphism, metasomatism and mineralisation associated with intrusion of the Cornubian batholith. 

The majority of the meta-sediments are pelites[metamorphosed mudstones] or greywackes[metamorphosed sandstones] and are of Devonian age, although in North Devon a large synclinal structure preserves Carboniferous sediments of similar facies. 

[Summary] Paleozoic Timeline with focus on Southwest England

• 541 to 485 Ma (Cambrian): Cool climate and shallow seas. Trilobites, Graptolites, and Molluscs appear.
• 485 to 444  Ma (Ordovician): Still cool; Sea separating what is now North America/Scotland & England/Wales begins to close.
• 444 to 419 Ma (Silurian): Climate warms to tropical; Britain covered by shallow sea. Corals, brachiopods, trilobites, graptolites.
• 419 to 359 Ma (Devonian): Semi-arid, with shallow seas and land. “Old Red Sandstone” deposition. Variscian orogeny begins.
• 359-299 Ma (Carboniferous): Tropical: shallow seas/swamps. Orogeny continues; basins form. Lizard cmplx uplift. Pangea complete.
• 299 to 252 Ma (Permian): (1) Orogeny ends, (2) Extension (3) Cornubian batholith intrudes. (4) Basin formation + deposition.
• 252-201 Ma (Triassic): Hot, dry: deserts, fluvial environments. Deposition of mudstones, sandstones, halites. First dinosaurs.
• 201-145 Ma (Jurassic): Shallow seas; uplift, basin formation & deposition; Ammonites and Dinosaurs; Rifting as Pangea breaks up.
• 145-66 Ma (Cretaceous): Shallow seas; Uplift and basin formation; deposition; chalk! KT events extinguish Ammonites/Dinosaurs
• 66-23 Ma (Paleogene): Local uplift and inversion; warm climate gradually cools
• 2.5 Ma on (Quartenary): Continued cooling; Glaciation in the north, periglacial environment in SW England; first humans arrive

Paleozoic Timeline with focus on Southwest England

[Sources: various; much text from https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/geological-timechart/]

The rock types mentioned in the previous section were subject to intense deformation during the Variscan Orogeny (late Carboniferous to early Permian) and the coastal sections (particularly in the vicinity of Bude [northern Cornwall]) provide many of the classical examples of fold and thrust geometries that are seen in structural geology textbooks. 

The deformation has a single fold phase providing the dominant structural style. The intrusion of granites occurred towards the end of the Variscan events, and cooling and alteration of the granites and their country rock and the associated mineralisation continued for a very considerable time after emplacement of the granites.

541 to 485 Ma (Cambrian): Cool climate and shallow seas. Trilobites, Graptolites, and Molluscs appear.

In the Cambrian Period, England and Wales lay near the south pole and experienced a cold climate. They were separate from Scotland, which was joined to North America. A shallow sea covered much of the area and animals such as trilobites, graptolites and molluscs first appeared.

Apparently there are Cambrian rocks in Wales – as Cambria is another name for Wales. 

485 to 444  Ma (Ordovician): Still cool; Sea separating what is now North America/Scotland & England/Wales begins to close.

In the Ordovician Period, Britain lay south of the equator and had a cool climate. Seas covered Britain and there was dramatic volcanic activity as the ocean separating England and Wales from Scotland started to close.

444 to 419 Ma (Silurian): Climate warms to tropical; Britain covered by shallow sea. Corals, brachiopods, trilobites, graptolites.

In the Silurian Period, Britain lay south of the equator with a tropical to subtropical climate. A sea covered Britain that was shallower in the south (limestones) and deeper in the north (sandstones and mudstones). Silurian fossils include corals, brachiopods, trilobites and graptolites.

~ 435 Ma: SW England is located north of Rheic Ocean

419 to 359 Ma (Devonian): Semi-arid, with shallow seas and land. “Old Red Sandstone” deposition. Variscian orogeny begins.

In the Devonian Period, Britain lay south of the equator and had a semi-arid climate. Marine limestones, sandstone and mudstones were developed in the south-west. Elsewhere sandstones were deposited by rivers on the coastal plain (sometimes known as the ‘Old Red Sandstone’). Fossils include fish and the first higher plants. 

The Variscan orogeny resulted from the collision of several continental plates, including the major landmasses of Gondwana (to the south) and Laurussia (to the north) and smaller plates such as Armorica. This culminated in the formation of the supercontinent Pangea. 

359-299 Ma (Carboniferous): Tropical: shallow seas/swamps. Orogeny continues; basins form. Lizard cmplx uplift. Pangea complete.

In the early Carboniferous Period, Britain lay near the equator. Limestones containing corals, brachiopods and trilobites were deposited in shallow seas. Later, sandstones, mudstones and coals were deposited in coastal swamps dominated by forests of giant ferns and horsetails.

~ 360 – 300 Ma: Variscian orogeny 
The main phase of the orogeny, characterized by intense folding, faulting, and metamorphism of the affected regions. Peak deformation and mountain-building occurred around 330 to 300 Ma.

Rheic ocean closes; another small ocean basin opens which is where the material that comprises older rocks of southwest England was deposited.

• Passive margin to the North: Over time, sediment from the continent and an unsubducted oceanic plate create a passive margin. This led to the creation of rift basins, six E-W trending basins containing sedimentary rocks : sandstones, mudstones; limestone; cherts
• Active margin to the South: Collision and accretion is happening along a southern active margin. 

By ~ 300 Ma the plift of the Lizard Peninsula ophiolite has occurred

299 to 252 Ma (Permian): (1) Orogeny ends, (2) Extension (3) Cornubian batholith intrudes. (4) Basin formation + deposition.

During the Permian Period, Britain lay on the supercontinent of Pangaea (fully assembled by 300 Ma) and experienced hot, dry conditions. Sandstones formed from desert sand dunes. Limestones and salts were deposited in a nearby inland sea (the Zechstein Sea). 

~ 300:  Collision and inversion of the sedimentary basins, followed by extension causing further deformation

~ 300 – 280 Ma: Final stages of Variscan orogeny
The later stages of the orogeny where tectonic activity decreased, and the region begins  to stabilize. Subsequent post-orogenic collapse and associated magmatic activity, including the intrusion of granitic bodies like the Cornubian Batholith.

~ 295-270: Intrusion of Cornelian batholith, with ensuing contact metamorphism

~ 280 Ma: Orogeny ends
The Variscan orogeny ceased during the Early Permian and left a rugged, arid landscape with intermontane basins containing volcanic rocks. 

~ 260 Ma: Rifting, basin formation and sediment deposition

By Late Permian to Early Triassic time, crustal rifting led to the formation of fault-bounded basins in which very thick red bed sequences accumulated; 

252-201 Ma (Triassic): Hot, dry: deserts, fluvial environments. Deposition of mudstones, sandstones, halites. First dinosaurs.

During the Triassic Period, a hot and dry environment led to the deposition of sandstones, salts and mudstones in deserts, rivers and shallow lakes. Reptiles were common and the first dinosaurs evolved. 

This was followed during the Jurassic by more widespread deposition of marine sequences. 

201-145 Ma (Jurassic): Shallow seas; uplift, basin formation & deposition; Ammonites and Dinosaurs; Rifting as Pangea breaks up.

Parts of Britain were covered by shallow tropical seas in which mudstones, limestones and sandstones were deposited. The abundant life in the Jurassic included ammonites and dinosaurs. More globally, at this time Pangea begins to break apart, one aspect of this being the opening of the North Atlantic.

~ 170 Ma: uplift, erosion, more basin formation and sediment deposition

By the Middle Jurassic, renewed crustal stress produced local uplift, followed by more extensive doming and erosion during Late Jurassic to earliest Cretaceous time. Simultaneously, crustal stresses related to the opening of the North Atlantic led to the formation of basins and sedimentary deposits in the  Western Approaches Trough. 

145-66 Ma (Cretaceous): Shallow seas; Uplift and basin formation; deposition; chalk! KT events extinguish Ammonites/Dinosaurs

In the Early Cretaceous, Britain experienced a warm climate with lagoonal, lake and fluvial environments. Rocks of this age contain dinosaur remains. Higher sea levels led to chalk deposition in the Late Cretaceous. 

~ 120 MA: Rifting and subsidance

By the Aptian, oceanic crust was being created in the newly formed ocean at the base of the Western Approaches continental margin. During this rifting phase, the extended crust under the present continental slope suffered very rapid subsidence which then slowed at an exponential rate during the Late Cretaceous and early Tertiary. 

~ 66 Ma: KT Extinction Events (Chixlub; Siberian Traps)

Ammonites, dinosaurs, et al., become extinct

66-23 Ma (Paleogene): Local uplift and inversion; warm climate gradually cools

Tectonism was renewed during the Palaeogene as a result of collision between Europe and Africa; these Alpine stresses produced local uplift during the Paleocene, and a subsequent series of inversion pulses that reached a climax during the late Eocene to Oligocene. These movements continued sporadically at a reduced scale into the Miocene and helped fashion the present topography of the region.

In the Palaeogene (66-23 Ma), Britain had a very warm climate, but it gradually cooled as Britain drifted northwards.

2.5 Ma on (Quartenary): Continued cooling; Glaciation in the north, periglacial environment in SW England; first humans arrive

By the Quaternary, glaciers covered the middle and north of Britain, shaping the landscape we see today. The first humans occupied Britain during the Quaternary.

Glaciation

[Sources: Various]

The most recent glacial period in UK stretched from 100-12 Ka. I’ve found various descriptions of its southerly extent, but am not sure which, if any, are definitive. All agree, however, that the region of southwest England where we are walking was not glaciated, but that its geomorphology is shaped by periglacial influences, including intense freeze-thaw weathering (e.g. Dartmoor tors), and sediment deposits due to glacial meltwater flooding. 

Major Faults

I believe these were mostly formed during the Variscan orogeny; transform faults (not shown) were formed later

History vis a vis Rock Types

[Extracted from material produced  by Perplexity (ChatGPT); Other Sources]

Precambrian to Paleozoic Eras 

Tectonic, orogenic and magmatic events; some erosion and deposition

• >540 Ma (Precambrian): Formation of mafic and ultramafic rocks of  Lizard Peninsula ophiolite
• ~540 – 420 Missing rocks / unconformity () – but present in Wales
• 420-359 Ma (Devonian ): The Old Red Sandstone, found in areas like Portishead, North Somerset, and the Morte Slates in North Devon
• 430-300 Ma: Variscan Orogeny, and uplift of the Lizard Peninsula ophiolite
• (359-299 Ma (Carboniferous): Carboniferous Limestone (Mendip Hills, Somerset feature; Burrington Oolite)
• ~289 Ma: Intrusion of Cornubian batholith, and metamorphism of older sedimentary rocks

Mesozoic Era 

Often covered by shallow seas and fluvial environments leading to sedimentary deposition

• Triassic (252-201): Red and grey mudstones ( Mercia Mudstone and Penarth Groups), visible at Aust Cliff along the Severn Estuary.
• Jurassic (202-145): Jurassic Coast features a sequence of rock formations from the Triassic, Jurassic, and Cretaceous periods.
• Cretaceous  (145-66): The chalk formations (e.g. Old Harry Rocks near Swanage).

Cenozoic Era

Uplift and erosion, and also various forms of marine and fluvial environments.

• Paleogene and Neogene (66-2.5 Ma): Sediments (Branksome Sand Formation in Poole Harbour) laid down in swamp, estuary, or delta settings

Quaternary Period 

Erosion, periglacial-weathering

• The Quaternary period, (2 Ma to present), has had significant climatic fluctuations (glaciation; periglacial effects; sea level change) that have shaped the landscape of Southwest England as it is today

Dorset

[From Wikipedia, Dorset]

Geography of Dorset

Dorset has a varied landscape of chalk downs, steep limestone ridges, and low-lying clay valleys. The majority of its coastline is part of the Jurassic Coast World Heritage Site due to its geological and palaeontologic significance, and features notable landforms such as Lulworth Cove, the Isle of Portland, Chesil Beach and Durdle Door. The north of the county contains part of Cranbourne Chase, a chalk downland. [Wikipedia, Dorset]

Geology of Dorset

Dorset contains considerable variety in its underlying geology, which is partly responsible for the diversity of landscape. A large percentage (66%) of the county comprises either chalk, clay or mixed sand and gravels. The remainder includes Portland and Purbeck stone, other limestones, calcareous clays and shales.

Dorset has a number of limestone ridges which are mostly covered in either arable fields or calcareous grassland supporting sheep. These limestone areas include a wide NE-SW band of Cretaceous chalk which crosses the county as a range of hills f incorporating Cranborne Chase and the Dorset Downs, and a narrow band running from south-west to south-east, incorporating the Purbeck Hills. Between the chalk hills are large, wide vales and wide flood plains.

West. In the far west of the county and along the coast there are frequent changes in rock strata, which appear in a less obviously sequential way compared to the landscapes of the chalk and the heath. In the west this results in a hilly landscape of diverse character that resembles that of neighboring county Devon. 

East (Jurassic) Coast. All of the principal rock types of the Mesozoic can be seen on the Jurassic Coast of Dorset and East Devon, where they document the entire era from west to east.

More on Dorset

[Extracted from  Physical Influences on the Dorset Landscape

The central parts of Dorset are dominated by an extensive swathe of chalk running North East to South West and through the South of the County round Weymouth to Purbeck. It extends North East towards Salisbury Plain in Wiltshire. It is an elevated, spacious landscape with a prominent escarpment which dominates and defines the undulating farmlands to the North, West and South of the county. 

The shallow dip slope of the chalk borders the heathland landscapes of the Poole Basin. This undulating lowland contains the remnants of a once extensive area of heathland; Hardy’s Egdon Heath and is based on thin and impoverished sands and gravels. It is drained by the alluvial basins of the Frome, Piddle, Stour and Avon, four of Dorset’s main rivers which flow out to sea through extensive stretches of pasture, marsh and mudflats associated with Poole and Christchurch harbours.

[…]

To the West of the county the landscape is more varied reflecting the complex geology, with steep distinctive greensand ridges separating clusters of deeply incised valleys and broad rolling farmland. In the middle of West Dorset, the Marshwood Vale, formed on the marls of the Lower Lias, is a secluded, bowl shaped depression, almost hidden by the surrounding complex landforms. These landforms include the broad arc of heathy greensand ridges and some of the highest points in the county…

The lowlands to the West and east of Weymouth are different again to the West Dorset landscapes. A series of broad, hogback shaped limestone ridges alternate with spacious shallow clay valleys. The landforms all follow an east-West alignment and the landscape has a steady consistent rhythm. This ‘ridge and vale’ landscape flows down to the quiet edges of the brackish Fleet lagoon contrasting with the more exposed seaward side of this distinctive feature which is separated from the sea by the sweep of Chesil Beach. This distinctive shingle bank links the rocky limestone peninsula of Portland to the mainland.

The far South eastern corner of the county has a unique and remarkable geological context. The Isle of Purbeck is renowned for the variety and structural clarity of its rocks and landforms. A high chalk ridge, which once linked Purbeck to the Isle of Wight, separates the heathlands of the Poole Basin from the secluded clay valley of the River Corfe to the South. A further ridge and distinctive plateau, this time of limestone, separates this valley from the sea.

…When the sea rose following the ice age leading to the severance of the chalk ridge joining the Isle of Purbeck to the Isle of Wight and the flooding of the upper reaches of the Solent River to form Poole harbour. The chalk stacks of Old Harry Rocks to the East of Ballard Point are the remnants of former chalk ridge which linked the Isle of Wight.

Visible Features in Dorset

@Chesil Beach

Further south at Chickerell Hive Point there are exposures of Oxford Clay. Here the clay splits into thin layers along the bedding planes and form what is known as a shale. This clay was exploited in the past at Crookhill brickworks. The manufacture of bricks ceased in 1969. This part of the rock succession is organic rich and this helped in the firing of the bricks. Fossils found include the ammonite Kosmoceras (Picture 11) as well as small bivalves. The shape seen is an impression of the shell, the shell itself has been lost through geological processes.

Another feature of the Oxford Clay south of Chickerell Hive Point is large concretions called septarian nodules (Picture 12). These formed in the sediment below the sea floor and are made of calcium carbonate. When pure it forms white crystals of calcite, a polymorph of calcium carbonate. The concretions can be up to a metre across and are left weathered out on the beach because they are stronger than the clay. Examples of septarian nodules can be seen in Portland Museum.

At Tidmoor Point south of Chickerell Hive there are more exposures of Oxford Clay where pyritised ammonites, especially Quenstedtoceras (Picture 13) can be found as well as small belemnites and crinoid fragments (ossicles). The clay slumps onto the beach and wave action washes out the fossils.

@Abbotsbury 

These ancient landslide slopes can be seen around the edge of the Marshwood Vale (Sliding Hill at Bettiscombe is a good example), on the western side of Quarr Hill, Chideock below the Bellstone on Eggerdon and both sides of Abbotsbury Hill Fort.

@Quantock Hills

The Quantock Hills are largely formed by rocks of the Devonian period, which consist of sediments originally laid down under a shallow sea and slowly compressed into solid rock. In the higher north-western areas older Early Devonian rocks known as Hangman Grits (or, more formally, the Hangman Sandstone Formation).

Hangman Sandstones (upper to lower):

• sandstone and conglomerate (Hodders Combe Beds )
• green sandstone and mudstones (Triscombe Beds) .  
• siltstones and slates ( Little Quantock Beds)

Devon

 [from Wikipedia, Devon]

Devon Geography

Devon has a varied geography. It contains Dartmoor and part of Exmoor, two upland moors which are the source of most of the county’s rivers, including the Taw, Dart, and Exe. The longest river in the county is the Tamar, which forms most of the border with Cornwall and rises in the Devon’s northwest hills. The southeast coast is part of the Jurassic Coast World Heritage Site, and characterized by tall cliffs which reveal the Triassic, Jurassic and Cretaceous geology of the region. The county gives its name to the Devonian geologic period, which includes the slates and sandstones of the north coast. 

Devon Geology

[ Possibly from “The Geology of Dartmoor.”] and [From Wikipedia, South West England]

Geologically the region is divided into the largely igneous and metamorphic west and sedimentary east, the dividing line slightly to the west of the River Exe.

 West. Cornwall and West Devon’s landscape is of rocky coastline and high moorland, notably at Bodmin Moor and Dartmoor. These are due to the granite and slate that underlie the area. In North Devon the slates of the west and limestones of the east meet at Exmoor National Park. 

East. The east of the region is characterized by wide, flat clay vales, and chalk and limestone downland. The vales, with good irrigation, are home to the region’s dairy agriculture. The Blackmore Vale was Thomas Hardy’s “Vale of the Little Dairies.” The Southern England Chalk Formation extends into the region, creating a series of high, sparsely populated and archaeologically rich downs, most famously Salisbury Plain, but also Cranborne Chase, the Dorset Downs and the Purbeck Hills. These downs are the principal area of arable agriculture in the region. Limestone is also found in the region, at the Cotswolds, Quantock Hills and Mendip Hills, where they support sheep farming.The sedimentary rocks of Devon are primarily of shallow-water marine origin, and include mudstones, sandstones and limestones. There are also metamorphic rocks resulting from high temperature/high pressure tectonics, and, around Dartmoor, high temperature/low pressure contact metamorphism.

“The rocks of the Devonian period in Devon are all of marine origin, but are not necessarily the deposits of a deep sea. Thus, the Devonianlimestones of South Devon, Plymouth, Yealmpton, Ashburton, Newton Abbot and Torquay, are fossil coral reefs, and there is no sound reason for the supposition that the species of coral which built those reefs were capable of living and growing at any greater depth below the surface than can the species now living. At a generous estimate this would give a depth of forty fathoms to the waters in which the reefs originated. No similar approximation can be given to the Emit of depth at which the rocks known as ‘ grits’ have been formed, but sandstones of any sort are not consistent with deep-sea beds.”

Surface Geology of Devon

Dartmoor National Park

Dartmoor National Park is centered around a 625 K² area of granitic pluton intruded into older Devonian and Carboniferous sedimentary rock (shales, sandstones, and limestones) about 280 Ma; the pluton is part of the Cornubian batholith that underlies much of southwest England. Around the margins of the granite, these sedimentary rocks were altered by thermal metamorphism into hornfels (hard, fine-equigranular, HT/LP rock that preserves some of the textural characteristics of the protoliths, but lacks foliation due to lack of pressure during metamorphism). 

Dartmoor Tors

[Source: Unsure… partly Wikipedia] 

Dartmoor is noted for its tors (Welsch for cluster or heap), low granitic rock formations resulting from freeze-thaw weathering. 

The tors of Dartmoor have a castellated form, controlled by closely spaced vertical jointing, as well as (sometimes) horizontal jointing. Most Dartmoor tors are less than 100,000 years old, having emerged at the start of the last major glaciation, no doubt as a result of freeze-thaw weathering.

Most tors are composed of megacrystic granite with orthoclase tablets up to 5 cm in length. At lower elevations, the granite is more finely-grained and has few or no megacrysts.

Also see this lovely map of the tors of Dartmoor that Charlie found. It appears to be a work of obsessive enthusiasm, though, oddly it does not give any information about the rock itself. See: https://www.torsofdartmoor.co.uk/map.php. Be sure to follow the link under the name of each tor.

Dartmoor Granite

Dartmoor Granite

[Some of this is from https://wessexcoastgeology.soton.ac.uk/Dartmoor.htm and some is from [The Geology of Cornwall]

The megacrystic granite found at higher elevations contains large orthoclase feldspar phenocrysts 7 cm long, which indicates very slow cooling of the granite magma in the roof zone of the pluton.  (But I don’t understand why that would allow the formation of megacrysta. Perhaps, if it were very deep, it may be that orthoclase crystals are less dense than the magma, and floated towards the top of the chamber (that happens with plagioclase, but I don’t know if it happens with orthoclase). The orthoclase concentration may indicate assimilation of country rock which increased the potassium content and thus the orthoclase (relative to the plagioclase). This granite is also dominated by biotite, in contrast to the finer-grained lower elevation (‘blue’) granite which has more muscovite.

In general, Dartmoor granite is characterized by a relatively high proportion of tourmaline (a boron-bearing mineral) which suggests the granite magma had a mantle component.

“Tourmaline is commonly recognised in the field in Devon and Cornwall, as though it was a single mineral. In fact “tourmaline” is a group of 11 minerals. The common black (in field appearance) mineral is southwest England is probably mostly schorl, but this is generally referred to in the local literature as tourmaline, and is not often analysed. Microscopically the colour of the tourmaline may be blue or brown but this may not be seen in the field; it is just the very dark colour that is obvious. In composition it is a complex boron and aluminium silicate with some iron (or magnesium etc). The interesting aspect is the presence of boron. The origin of the boron is a topic of special interest. 

“Tourmaline belongs to the trigonal crystal system and has a hardness of 7 to 7.5 (i.e. it is hard like quartz and cannot be scratched with a knife). Its very dark colour and vitreous lustre is distinctive in the area of the granites of southwest England. It often shows crystal faces. It is extremely common, particularly in faults and fissures and often found with quartz, producing black and white veins. It is generally one of the most common late vein minerals associated with the granites. It is frequently seen in the field and is not valuable (valuable, coloured varieties do exist, but not usually here). 

“In the Castle Drogo area pebbles or cobbles consisting of quartz and tourmaline veins are easily seen in the banks of the River Teign. The dark vitreous mineral should not be mistaken for the tin ore – cassiterite, which is much more difficult to find. 

“Because tourmaline is a constituent of veins in the Dartmoor granite it is usually a late product. The granite emplaced about 298 million years ago, had cooled sufficiently to fracture in a brittle manner. Perhaps the granite was down to only 250 or 300 degrees C. 

“However, it certain cases the tourmaline is found more dispersed within the granite, rather than just in veins. A rock with dispersed acicular “suns” of black tourmaline is know as Luxullianite. It takes its name from Luxulyan near St. Austell. This rock type is apparently the result of metasomatic replacement of granite in situ, according to Lister (1978) (and not from the emplacement within the granite batholith of a late stage boron-rich magma).”

Cornwall

Cornwall Geography

[From Wikipedia, Cornwall: quoted verbatim or lightly paraphrased]

Cornwall is characterized by steep cliffs and, to the south, several rias [drowned river valleys], including those at the mouths of the rivers Fal and Fowey. It includes the southernmost point on Great Britain, Lizard Point, and forms a large part of the Cornwall National Landscape. The national landscape also includes Bodmin Moor, an upland outcrop of the Cornubian batholith granite formation. The county contains many short rivers; the longest is the Tamar, which forms the border with Devon.

The north and south coasts have different characteristics. 

• The north coast is more exposed and therefore has a wilder nature. The High Cliff, between Boscastle and St Gennys, is the highest sheer-drop cliff in Cornwall at 732 feet.
• The south coast is more sheltered and has several broad estuaries offering safe anchorages, such as at Falmouth and Fowey. Beaches on the south coast usually consist of coarser sand and shingle, interspersed with rocky sections of wave-cut platform.
• The interior of the county consists of a roughly east–west spine of infertile and exposed upland, with a series of granite intrusions, such as Bodmin Moor, which contains the highest land within Cornwall. From east to west, and with approximately descending altitude, these are Bodmin Moor, Hensbarrow north of St Austell, Carnmenellis to the south of Camborne, and the Penwith or Land’s End peninsula. 

Cornwall Geology

[From Wikipedia, Cornwall: quoted verbatim or lightly paraphrased]

The granitic intrusions in the interior are the central part of the granite outcrops that form the exposed parts of the Cornubian batholith of south-west Britain, which also includes Dartmoor to the east in Devon and the Isles of Scilly to the west, the latter now being partially submerged.

The intrusion of the granite into the surrounding sedimentary rocks gave rise to extensive metamorphism and mineralisation, and this led to Cornwall being one of the most important mining areas in Europe until the early 20th century. It is thought tin was mined here as early as the Bronze Age, and copper, lead, zinc and silver have all been mined in Cornwall. Alteration of the granite also gave rise to extensive deposits of China Clay, especially in the area to the north of St Austell, and the extraction of this remains an important industry.

The uplands are surrounded by more fertile, mainly pastoral farmland that lies mainly on Devonian sandstone and slate. The north east of Cornwall lies on Carboniferous rocks known as the Culm Measures. In places these have been subjected to severe folding, as can be seen on the north coast near Crackington Haven and in several other locations.

The geology of the Lizard peninsula is unusual, in that it is mainland Britain’s only example of an ophiolite. Much of the peninsula consists of the dark green and red Precambrian serpentinite, which forms spectacular cliffs, notably at Kynance Cove, and carved and polished serpentine ornaments are sold in local gift shops. This ultramafic rock also forms a very infertile soil [similar to Ring Mountain Preserve in Marin County] which covers the flat and marshy heaths of the interior of the peninsula. This is home to rare plants, such as the Cornish Heath, which has been adopted as the county flower.

On the Variscan Orogeny

[From “The Geology of Cornwall,” whose source I have misplaced]

The peninsula of Cornwall has a geological character distinct from t the rest of the UK. It represents the eroded basement of a Palaeozoic Mountain Belt formed by the Variscan Orogeny (300 – 275 Ma, late Carboniferous – Permian)., with the S-type granites so indicative of mountain building processes, and shows many features associated with mountain building processes: compressional structures, thrust faults and folds, the remnants of a closed ocean (the Lizard Ultramafic Complex), a low grade regional metamorphism. and the intrusion of the aforementioned granites which produced contact metamorphic aureoles and abundant mineralisation.

Mineralization and Alteration of Cornubian Granites

General References

South-west England area – Geology  (British Geological Society) – does not appear to really be in official Wikipedia

Geology of the Wessex coast of SW England, Ian West. https://wessexcoastgeology.soton.ac.uk/index.htm

The Granite of Dartmoor.pdf URL NEEDED—see perplexity query

The Devon Landscape – pdf at  https://www.devonassoc.org.uk/f16/f011.pdf

British Geological Survey: Regional Geological Summaries – South West England:
 https://www.bgs.ac.uk/download/regional-geological-summaries-south-west-england/

The Variscan Orogeny. https://variscancoast.co.uk/variscan-orogeny

Wednesday, 7 December 2022

Summary

• 200 Ma, on: Subduction of the Farallon plate resulted in arc volcanism that produced a range of mountains along the west coast of North America. (These will eventually erode, and their batholiths will be exhumed and form today’s mountains and the eroded sediment will accumulate in basins forming soil and lithifying into sedimentary rock.)
• 20 Ma. As the Farallon plate completed its subduction, it broke into microplates, and
  (1) SAFS. the East Pacific Rise subducted initating the transformation of a convergent margin into a transform margin and the San Andreas Fault system, and
  (2) Cabo volcanoes. continued subduction of the microplates produced volcanism that can be seen around Cabl
• ~17 Ma. Complex tectonics, due to the SAF system, that I don’t really understand, generated the basin and range system farther north, and this in turn is thought to have channelized the stress that eventually opened the gulf of California.
• ~15 – 6 Ma. Tectonics lead to capture of Baja by Pacific plate, transitional shearing in the proto-Gulf of California, and at 6 Ma the beginning of the ‘unzipping’ of Baja.
• ~3.5 Ma. Development of spreading centers in the Gulf of California, and extrusion of mostly basaltic oceanic crust.

Continue reading Notes on the Geology of Baja