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
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 Earths 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)
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.
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!
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).
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 Annes 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.
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