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Cross bedding, rip up fragments and small pebbles in sandstone. Liverpool Anglican Cathedral wall, right side, near rock outcrop.

BRITISH ROCKS and

the GEOLOGIC TIME

SCALE

We are lucky in Britain in that we have representatives of almost all rock types, and we have rocks of almost all ages. This ensures that we have a diverse landscape. It was also important in giving us a wide range of mineral resources to support our developing industry at all stages from the Stone Age to modern times. The Earth’s origin was about 4.5 billion years ago or 4,500 Million Years Ago (often written as 4,500 Ma or as “mya”). The names of Geological Periods etc have been adopted and developed over time, and as geological knowledge of more of the Earth became available. This has resulted in a unique set of names – a number distinctly British in origin.

Rocks are divided into 3 types, based on how they form.

1. Igneous – rocks that solidify from a molten mass (magma).

2. Sedimentary – rocks formed on land or in water, from sediment produced by the breakdown of earlier rocks.

3. Metamorphic – rocks which have been altered by heat and pressure.

CONSIDER THE GEOLOGICAL TIME SCALE

(If you go to the BGS website you can get an outline Geological Timescale and very

detailed breakdowns. There are also many other

sources, especially USA universities)

Carboniferous Carboniferous

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Holocene Hot ) Pleistocene Plates ) Pliocene Pink ) Miocene My ) Oligocene Off ) TERTIARY Eocene Eat ) Palaeocene Please ) Cretaceous Cooled ) Jurassic Juice ) MESOZOIC Triassic Tomato ) Permian Proper ) Carboniferous Cook ) UPPER PALEOZOIC Devonian Do ) Silurian Swedes ) Ordovician Or ) LOWER PALEOZOIC Cambrian Carrots ) ) PRECAMBRIAN

QUATERNARY Geological Periods – with Mnemonics.

Learn from the bottom up – as older rocks are at the bottom.

Geological Time Scale.

Hadian is used more than “Priscoan” for the oldest division.

Hadian

NOTE: Geologists normally read tables of rock succession FROM THE BOTTOM – because they are normally the first formed rocks. So if a list of geological beds are numbered, the numbers will start at the bottom and get larger upwards. This is perfectly sensible if you consider the order in which the beds formed.

The Geological Time Scale supplied today has been agreed by the International Commission on Stratigraphy. Hadean ( =Pre Archean) – solar system forming, including Earth, but the Earth surface was molten. Minerals (zircon) 4.4 billion years old have been dated in the Jack Hills, Australia.. The Archean originally meant the first rocks (Latin), but older rocks are now known! The Archean contains primitive life – stromatolites – cyanobacteria (blue green algae). Proterozoic means Earlier Life in Greek. An oxygen rich atmosphere developed due to photosynthesis by bacteria in mid Proterozoic. The end of the Proterozoic is usually taken as the development of the first hard shelled animals , especially Trilobites. This was at about 550 million years ago – at the beginning of the Cambrian when we first see abundant and varied fossils.

Modern stromatolites in Shark Bay, Western Australia. Sediment is trapped

in films of cyanobacteria (blue green algae). Stromatolites have been found

up to 3.5 billion years old.

The Proterozoic and Archean together are commonly called the “Pre Cambrian” and represent 87% of geologic time. Phanerozoic means “visible” or “evident” life – fossils in many rocks when animals developed hard parts in their bodies. The main divisions of the Phanerozoic are Palaeozoic ("old life“), Mesozoic ("middle life" ) and Cenozoic ("recent life“). Cambrian was from the Roman name for Wales. Ordovician and Silurian are from ancient Welsh tribes. Devonian is from the county name Devon. Carboniferous comes from the Coal Measures (which are a part). Permian is from Perm in Russia. Triassic is from the 3 parts of the rocks of this age in Germany. Jurassic comes from the Jura Mountains in SE France. Cretaceous is from Creta – the Latin for chalk - named by a Belgian geologist working in the Paris basin!

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Cenozoic Epochs Palaeocene – “early” + “new” (or “recent”) especially mammal species. (from Greek) Eocene – “dawn” of “new/recent” Oligocene – “few” modern/new/recent species (especially) mammals Miocene – “few new” (less recent that Pliocene) Pliocene – “more new/recent” mammals Pleistocene – from Greek for “most” and “new” Holocene - “entirely” “recent/new”

Precambrian in Britain Note that on the BGS map key on the map given above the sedimentary later/upper part of the Proterozoic is called the Neoproterozoic when it is sedimentary, but it is called the Upper Proterozoic when it is metamorphic. This is because the time boundaries have some differences. If you go to the Canadian Shield – you find rocks that were right alongside the PreCambrian rocks of the Scottish Highlands at one time – and much bigger areas of them. Now the Atlantic has opened! There are some small areas of PreCambrian in England and Wales – on the map above - see Anglesea & the Welsh Borders (there are some others in England too small to show at that scale).

Mudcracks in the PreCambrian Torridon Sandstone. NW tip of Scotland. Some unchanged sediments survive.

The metamorphic rocks of the Precambrian, as seen widely north of the Highland Boundary Fault, include many very mangled rocks – they have been subjected to great pressure and often great heat as well (PRESSURE and HEAT = Regional Metamorphism). E.g. The Lewisian Gneiss of the Hebrides and NW Scottish Highlands. These rocks have probably been pushed down to quite a few kilometres depth to allow the necessary temperature to develop – and later the rock burying them has been eroded off. Some of these rocks are Proterozoic and some are Lower Palaeozoic.

Cambrian, Ordovician, Silurian = The Lower Palaeozoic. Mainly sediments – sands and muds (silt & clay). They are now mudstones, shales and sandstones. If mudstones are lightly metamorphosed by pressure (e.g. often in North Wales) then slates are formed. Cambrian is especially found in NW Wales. Ordovician and Silurian in Wales and the Southern Uplands.

Trilobites were a widespread characteristic fossil in the Lower

Palaeozoic (and persisted into the Upper Palaeozoic).

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Graptolites (“writing on rock”) were widespread floating planktonic animals in

the Lower Palaeozoic, and are important zone fossils. Biologically they

belong to a now very rare phyllum, the hemichordates (e.g. see Wikipedia).

Both the Snowdon Volcanic Group and the

Borrowdale Volcanic Group are Ordovician in age.

There are lavas, volcanic ashes and sometimes

slates formed from metamorphosed volcanic ash.

The Borrowdale Volcanic Group has been

interpreted as island arc volcanics. There was an

ocean (the Iapetus Ocean) between what is now

most of Scotland and most of England. The

oceanic crust was subducted beneath the English

continental crust and an island arc was formed.

Many of the granite masses in the Scottish

highlands were emplaced around the Lower

Palaeozoic (between 600mya & 390 mya).

UPPER PALAEOZOIC = Devonian, Carboniferous and Permian Consider what processes are going on now and where? Sedimentation, igneous activity and metamorphism. Sediments mainly in the oceans, generally thicker near land and especially near big rivers. Also coral reefs and other more specialised sediments e.g. salt deposits and peats. Igneous rocks are being formed where we find active volcanoes. Metamorphism in the Himalayas, the Andes & Rockies and the Alps. These processes vary from place to place, and through time. In the Devonian Period in Britain, in Devon there were marine sedimentary rocks being formed. These are usually referred to as the Devonian. However there are also desert sediments (terrestrial), which are usually referred to as the Old Red Sandstone. They are seen in South Wales, Hereford and Scotland. They are usually darker red in colour, often sandstones (wind or water deposited) but may be mudstones. Typical Old Red Sandstone, St Anne’s Head, Pembrokeshire.

In the Carboniferous Period Coal Measures were deposited. However in Yorkshire & Lancashire, Derbyshire, Somerset, North and South Wales and Scotland Carboniferous Limestone was deposited first [Carboniferous Limestone is a formal formation name, so both words get capital letters. Any limestone not part of a formal name does not e.g. PreCambrian limestones]. Usually there are mudstones (shales) along with the limestones.

Carboniferous Limestone is usually 95% or more calcium

carbonate. Many limestones form in relatively shallow water

where there is little input of land derived sediment e.g. the

Bahamas banks today.

Carboniferous Limestone – old

Quarry, near Ingleton.

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The Coal Measures are most commonly the upper part of the Carboniferous with the coal as a small % of the total thickness and the rest being shales and sandstones. In North America the limestone and associated shales are the Missippian, and the Coal Measures are the Pennsylvanian. In the USA they refer to Mississippian and Pennsylvanian, NOT Carboniferous!

Loading Coal East Chevington, Northumberland.

Opencast coal is better quality because under better controlled conditions it has less waste rock added than underground coal.

Old underground workings exposed in opencast.

From the viewpoint of

industrial development the

Irish were unlucky. They

have very widespread

Carboniferous Limestone

but no Coal Measures.

Permian and Triassic During the Permian Period and the Triassic Period Britain had a desert climate again. The Permian and Triassic are often referred to together as the New Red Sandstone There are some other rocks other than sandstone, especially the Magnesian Limestone (a named formation that is part of the Permian), and also mudstones. However much of the Permian and Triassic are light red sandstones – often a pale orange-red colour (like light coloured new red and orange bricks. Old Red Sandstone is like darker red bricks). New Red Sandstone is widespread in Cheshire, Lancashire, and nearby areas. Mainly Triassic but some Permian.

New Red Sandstone Pebble

New Red Sandstone on Hilbre Island (N.W.Wirral)

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Cross bedding in sandstone. Liverpool Anglican Cathedral

wall, right side near main door (often seen in many buildings)

Geol. N. Wales, 1961 Interpretation

Clwydian

Hills Vale of

Clywd Chester

plain

The Vale of Clwyd Fault causes a repetition of the succession

Silurian to Triassic along the NE Wales coast.

(for Triassic read “New Red Sandstone – now interpreted as

including Permian as well as Triassic)

Geology of North Wales – post 2000 interpretation

KEY TR Triassic PE Permian CM Coal

Measures CLst Carboniferous

Limestone Sil Silurian See BGS Viewer for Yourself!

(Find via Google)

TR

TR

TR

TR

PE

PE

PE

PE & CM

PE

Sil Sil

Sil Sil

Sil

CM

CM

CM

CM

CM

PE & CM

Superficial Deposits

Around Ruthin rather less than 50 % is covered by superficial deposits (a bit more till over some hills to the west).

ALLUVIUM along river valleys GLACIAL TILL - deposited directly from the ice (old “boulder clay”). GLACIAL SAND & GRAVEL – fluvioglacial deposits.

The MESOZOIC ERA begins with the Triassic Period, followed by the Jurassic Period and then the Cretaceous Period.

The Jurassic has been made popular with “Jurassic Park” and it is the time of widespread dinosaurs on the planet. The British Jurassic rocks consist of a variety of mainly marine sediments. This includes clays, marls, sandstones and quite a lot of limestones. The limestones give some of Britain’s best known building stones – limestones which can be readily carved into decorative parts of churches and cathedrals – Bath Stone, Portland Stone and Purbeck stone – all limestones, oolitic and fossiliferous. Ooliths are small concretions 1-2mm diameter, deposited in shallow seas like the present day Bahamas Banks.

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Oolitic Limestone

The full succession of British Jurassic rocks is: 14 Purbeck Rocks Limestones and clays 13 Portlandian Rocks Limestones and sandstones 12 Kimmeridge Clay 11 Corallian Rocks Limestones and grits 10’ Oxford Clay 10 Kellaways Rock 9 Cornbrash Thin limestones 8 Forest Marble Thin limestones 7 Great Oolite Limestone and clay 6 Fullers Earth Clay 5 Inferior Oolite Limestone 4 Upper Lias Clay 3 Middle Lias Marlstone 2 Lower Lias Clay and Limestone 1 Rhaetic beds – transitional from Triassic

Upper ( Oolites (

Middle ( Oolites (

( Lower ( Oolites ( ( (

Lias ( (

The zones of the Jurassic are largely distinguished by the widespread, varied, fast evolving AMMONITES. Though other fossil groups are also used e.g. corals, brachiopods and sponges.

Folded Jurassic Limestones – Durdle Door, Dorset

Ammonites

The CRETACEOUS Rocks of Britain are also marine deposited sediments: 6. The Chalk Soft Limestone 5. The Upper Greensand. Green sandy beds 4. The Gault Clay 3. The Lower Greensand Green and iron-stained and white sands 2. The Weald Clay Thick clay 1. The Hastings or Wealden Sands Various sands and clays.

Ammonites persist, but some strange shapes evolve. Sea urchins are common, dinosaurs and other reptiles are relatively common and full bony fish appear for the first time. The Cretaceous Period is the final period of the Mezozoic.

The Chalk formation is extremely widespread in England from

Yorkshire to East Anglia, Chilterns, Kent, Salisbury Plain,

Hampshire and Devon.

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The TERTIARY sedimentary Rocks of Britain are mainly in South East England. Large areas of Tertiary volcanic rocks are found in NW Scotland, especially the islands, and NE Ireland (Giant’s Causeway). In the Palaeocene Period England was largely land with erosion over most of the area. There are some Palaeocene sediments in south east England and rarely elsewhere, but not large areas. Eocene rocks are found in the Hampshire and London basins. In the London basin there are the Thanet Sands, Woolwich and Reading Beds (fluvial sediments), the London Clay and the Bagshot Sands. Similar beds are found in the Hampshire basin, with the Bracklesham and Barton beds on top. Oligocene fluvial sediments are also found in the Hampshire Basin, Devon and offshore in northern Cardigan Bay.

Shark’s teeth Crab ?Turtle ? Fish Book: £25. Published by the Medway Mineral and Lapidiary Society (£5 only to members)

Landslide in Barton sands and clay. Barton Court, Barton

Shark’s tooth just found washed out of Barton Clay, part of the Eocene of the Hampshire Basin. Must be marine sediments!

In the Miocene Period much of Britain was land being

eroded so there is little or no Miocene rock in Britain. A gap

in the British Succession. This is the time of mountain

building in the Alps (still ongoing) so there is much erosion

and few new rocks of Miocene age there.

The Pliocene Period deposits are of shelly sediments at

limited sites across East Anglia, especially in the East. They

are called “crags” which occur as exposures of small size.

The fossils in the crags indicate a cooling of climate. If the

crags were lithified they would be shelly limestones.

Coralline and Red Crag, Ramsholt (a few Km N of Felixstowe) This is deposited on top of London Clay.

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The final geological period is the Quaternary Period, which covers from about 2 million years ago (just over 2 million years – ignore the 1.6million on the table), until the present. This is the period when there were many ice ages with great ice sheets growing and shrinking, along with huge changes of climate. Old school geologists like to distinguish the Holocene – the last 10,000 years or so (referred to by Quaternary scientists as “The Post Glacial”). Then the Quaternary without the Holocene (= Post Glacial) is termed the Pleistocene.

Oxygen isotope record for the past 2,6

million years. Peaks represent warm

Earth, troughs a cold Earth

The fluctuations reflect the amount of water held as ice on

land - H2O with lighter oxygen isotopes evaporates more

easily, so the proportion of heavier isotopes in ocean water

increases as more water is locked up on land.

Merseyside, Cheshire & the Welsh mountains were like

this 18,000 years ago!

Trans Antarctic Mountains NOW

There have been many plate tectonic movements that have

affected the rocks and structure of the British Isles.

One major orogeny affecting the British Isles was the

Caledonian Orogeny, mainly early Ordovician to Mid

Devonian (490-390 mya), though it affected the

Scandinavian mountains and runs to Svalbard, Greenland

and the Eastern USA and over this wider area the orogeny

spread over a longer period.

It produced structures running NE-SW, like the Cairngorm

mountains and also features in Northern Ireland. This

orogeny produced much of the regional metamorphism of

the main Scottish Highlands.

The Variscan Orogeny (alternatively known as the

Hercynian Orogeny) was over the period 380 to 280

mya, affecting large parts of Germany, parts

of the Alps, NW Spain, the Apalachian

mountains in the USA and

Labrador/Newfoundland in Canada.

In the British Isles the Variscan orogeny

affected SW Ireland, the Gower Peninsula

and the Mendips (E-W trend), and Devon and

Cornwall (producing the granite masses).

The Alpine Orogeny is associated with the

collision of the African Plate with the Eurasian

Plate. The Alpine Orogeny is dated 65mya to

2 mya, or to the present. The Alps, Pyrenees

and Cantabrian Mountains are all part of the

effects of this orogeny.

In Britain most of the folding of the Jurassic,

Cretaceous and Tertiary rocks is the result of

the Alpine Orogeny – the folds of the North

and South Downs, the Isle of Wight, Dorset

(Lulworth, Purbeck etc) are all the result of

the Alpine Orogeny.

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Alum Bay sands of Eocene

age have been folded to a

vertical position by Alpine

folding (like the nearby chalk

of The Needles),

Quaternary Mammals in Britain

Woolly Rhinoceros

That’s all for today Folks. Does this remind you of anyone you know?