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Geological structure of Great Britain

From Wikipedia, the free encyclopedia

The geological structure of Great Britain is complex, resulting as it does from a long and varied geological history spanning more than two billion years. This piece of the Earth's crust has experienced several episodes of mountain building or 'orogenies', each of which has added further complexity to the picture.

A wide range of geological structures occur across Britain and include examples at a variety of scales of:

Our understanding of Britain's large-scale structure has been gained over many decades by simple geological field survey together with an increasing range of technological methods including gravity surveys, seismic surveys, aeromagnetic surveys and other forms of remote sensing.

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Transcription

Welcome to the United Kingdom (and a whole lot more) explained by me, C. G. P. Grey The United Kingdom, England, Great Britain? Are these three the same place? Are they different places? Do British people secretly laugh those who use the terms wrongly? Who knows the answers to these questions? I do and I'm going to tell you right now. For the lost: this is the world, this is the European continent and this is the place we have to untangle. The area shown in purple is the United Kingdom. Part of the confusion is that the United Kingdom is not a single country but is instead a country of countries. It contains inside of it four co-equal and sovereign nations The first of these is England — shown here in red. England is often confused with the United Kingdom as a whole because it's the largest and most populous of the nations and contains the de facto capital city, London. To the north is Scotland, shown in blue and to the west is wales, shown in white. And, often forgotten even by those who live in the United Kingdom, is Northern Ireland shown in orange. Each country has a local term for the population. While you can call them all 'British' it's not recommended as the four countries generally don't like each other. The Northern Irish, Scottish and Welsh regard the English as slave-driving colonial masters — no matter that all three have their own devolved Parliaments and are allowed to vote on English laws despite the reverse not being true — and the English generally regard the rest as rural yokels who spend too much time with their sheep. However, as the four constituent countries don't have their own passports, they are all British Citizens, like it or not.They are British Citizens of the United Kingdom — whose full name by the way is the United Kingdom of Great Britain and Northern Ireland. So where's Great Britain hiding? Right here: the area covered in black is Great Britain. Unlike England, Scotland, Wales and Northern Ireland, Great Britain is a geographical rather than a political term. Great Britain is the largest island among the British Isles. Within the United Kingdom, the term 'Great Britain' is often used to refer to England, Scotland and Wales alone with the intentional exclusion of Northern Ireland. This is mostly, but not completely true, as all three constituent countries have islands that are not part of Great Britain such as The Isle of Wight, part of England, the Welsh Isle of Anglesey and the Scottish Hebrides, The Shetland Islands, Orkney Islands, Islands of the Clyde. The second biggest island in the British Isles is Ireland. It is worth noting that Ireland is not a country. Like Great Britain, it is a geographical, not political, term. The Island of Ireland contain on it two countries, Northern Ireland — which we have already discussed — and the Republic of Ireland. When people say they are 'Irish' they are referring to the Republic of Ireland which is a separate country from the United Kingdom. However, both the Republic of Ireland and the United Kingdom are members of the European Union even though England often likes to pretend that it's an Island in the mid-atlantic rather than 50km off the cost of France. But that's a story for another time. To review: The two largest islands in the British Isles are Ireland and Great Britain. Ireland has on it two countries — the republic of ireland and northern ireland, while Great Britain (mostly) contains three: England, Scotland and Wales. These last three, when combined with northern Ireland form the United Kingdom. There are still many unanswered questions. Such as, why, when you travel to Canada is there British Royalty on the money? To answer this, we need to talk about Empire. You can't have gone to school in the English-speaking world without having learned that the British Empire once spanned a 1/4th the worlds land and governed nearly a 1/4th its people. While it is easy to remember the part of the empire that broke away violently... We often forget how many nations gained independence through diplomacy, not bloodshed. These want-to-be nations struck a deal with the empire where they continued to recognize the monarchy as the head of state in exchange for a local, autonomous parliament. To understand how they are connected, we need to talk about the crown. Not the physical crown that sits behind glass in the tower of London and earns millions of tourist pounds for the UK but the crown as a complicated legal entity best thought of a a one-man corporation. Who created this corporation? God Did. According to British Tradition all power is vested in God and the monarch is crowned in a Christian ceremony. God however — not wanted to be bothered with micromanagement — conveniently delegates his power to an entity called the crown. While this used to be the physical crown in the tower of london — it evolved over time into a legal corporation sole able to be controlled only by the ruling monarch. It's a useful reminder that the United Kingdom is still technically a theocracy with the reigning monarch acting as both the head of state and the supreme governor of the official state religion: Anglicanism. Such are the oddities that arise when dealing with a 1,000 year-old Monarchy. Back to Canada and the rest. The former colonies that gained their independence through diplomacy and continue to recognize that authority of the crown are known as the Commonwealth Realm. They are, in decreasing order of population: Canada, Australia, Papua New Guinea, New Zealand, Jamaica, The Solomon Islands, Belize, The Bahamas, Barbados, Saint Lucia, Saint Vincent and the Grenadines, Grenada, Antigua and Barbuda, Saint Kitts and Nevis, and Tuvalu. All are independent nations but still recognize the monarchy as the head of state even though it has little real power within their borders. There are three further entities that belong to the crown and these are the Crown Dependencies: he Isle of Man, Jersey, Guernsey. Unlike the Commonwealth Realm, they are not considered independent nations, but are granted local autonomy by the crown and British Citizenship by the United Kingdom — though the UK does reserve the right to over-rule the laws of there local assemblies. Are we all done "now"? Almost, but not quite. There are still a couple of loose threads, such as this place: The tiny city of Gibraltar on the Southern Cost of Spain famous for its rock, its monkeys and for causing diplomatic tension between the United Kingdom and Spain. Or what about the Falkland Islands? Which caused so much tension between the United Kingdom and Argentina that they went to war over them. These places belong in the last group of crown properties know as: British Overseas Territories. But their former name — crown colonies — gives away their origins. They are the last vestiges of the British Empire. Unlike the Commonwealth Realm, they have not become independent nations and continue to rely on the United Kingdom for military and (sometimes) economic assistance. Like the Crown Dependencies, everyone born in their borders is a British Citizen. The Crown colonies are, in decreasing order of population: Bermuda, Cayman Islands,Turks and Caicos Islands, Gibraltar, The British Virgin Islands, Akrotiri and Dhekelia, Anguilla, Saint Helena, Ascension Islands, Tristan da Cunha, Montserrat, British Indian Ocean Territory, South Georgia and the South Sandwich Islands, Falkland Islands, British Antarctic Territory, Pitcairn Islands. For our final Venn diagram, the United Kingdom is a country situated on the British Isles and is part of The Crown which is controlled by the monarchy. Also part of the crown and the British Isles are the crown dependencies. The independent nations of the former empire that still recognize the crown are the Commonwealth Realm and the non-independent remnants of the former empire are the British Overseas Territories. Thank you very much for watching.

Contents

Terranes

A useful approach to considering Britain's geological structure is to examine the various terranes from which it is composed. These are essentially continental fragments whose boundaries are generally defined by faults. Individual terranes typically contain suites of structures, the histories and form of which differ from those of neighbouring terranes.

Terranes of Scotland

The Hebridean Terrane is defined to the east by the Moine Thrust, beyond which lies the Northern Highlands Terrane. This area in turn abuts against the Central Highlands (or Grampian) Terrane along the Great Glen Fault. Similarly the Highland Boundary Fault separates the Central Highlands Terrane from the Midland Valley Terrane, which is itself separated from the Central - Southern Uplands Terrane by the Southern Uplands Fault. Each of these terranes form a part of the former continent of Laurentia whose southeastern margin is defined by the Iapetus Suture Zone, which runs parallel to the English-Scottish border, though some miles to its south.

Terranes of England and Wales

The Central - Southern Uplands Terrane extends across the northernmost part of England. Its southern margin is the Iapetus Suture, south of which lies the Leinster - Lakesman Terrane which includes the Lake District and North Pennines. The majority of the rest of England and Wales north of the Variscan Front are considered to constitute the Avalon Composite Terrane. Central to this composite terrane is the triangular-shaped Midlands Microcraton; within it, the north-south aligned Malvern Line (or 'Malvern Lineament') divides the Wrekin Terrane in the west from the Charnwood Terrane in the east. The Isle of Anglesey and parts of the Lleyn peninsula are deemed to be composed of numerous micro-terranes, known collectively as the Rosslare - Monian Terranes.[1]

Pennine Block & Basin Province

Northern England is characterised by a series of fault-bound blocks separated by sedimentary basins whose origin can be traced back to the Carboniferous period. The North Pennines are formed on the Alston Block which is defined to the west by the Pennine Fault and to the north by the Stublick and Ninety Fathom Faults. It is separated from the Askrigg Block to the south by the Stainmore Trough. This latter block, coincident with the Yorkshire Dales, is defined to the west by the Dent Fault and to the south by the Craven Fault System. The Northumberland and Gainsborough Troughs lie to the north and south of these two blocks.

Structural legacy of mountain-building episodes

Another approach to the study of the geological structure of the area is through consideration of the variety of structures resulting from each of several orogenies (or 'mountain-building' episodes) which have taken place over geological history. Structures originating in one event may play a part in subsequent orogenic events and be modified by them. Thus lines of crustal weakness commonly associated with, for example, the Caledonian Orogeny will often predate this particular mountain-building period, much as some of those created during this phase were reactivated during later events.

The Caledonian Orogeny

The Caledonian Orogeny took place between about 490 and 390 million years ago as the former micro-continent of Avalonia collided obliquely with the former continent of Laurentia along a line approximating to the modern English-Scottish border. This long drawn-out, multi-phase event resulted in innumerable geological structures, many of which have persisted to the present day and help to shape the landscapes of much of Britain, from South Wales northwards to the Shetland Islands. Key structures include:

Each of these structures is aligned northeast - southwest, albeit with the more northerly of them trending closer to NNE - SSW. A map or satellite photo readily reveals these major trends. There are hundreds of other lesser faults and folds which follow a similar alignment - a trend known as the Caledonoid trend.

The Variscan Orogeny

The Variscan Orogeny was a complex affair whereby the former micro-continent of Armorica collided with Laurussia (otherwise known as Euramerica or the Old Red Continent), followed by a further collision between Gondwana and the enlarged Laurussia. In Britain it resulted in a variety of geological structures across the southwest from Pembrokeshire and South Glamorgan in Wales to Devon and Cornwall. Structures include:

  • Dodman-Start Thrust
  • Lizard Thrust
  • Carrick Thrust

The Alpine Orogeny

North-south cross-section through southern England, showing the Weald Basin, which was inverted during the Alpine Orogeny forming the Wealden Anticline. Vertical exaggeration 1:5.
North-south cross-section through southern England, showing the Weald Basin, which was inverted during the Alpine Orogeny forming the Wealden Anticline. Vertical exaggeration 1:5.

The Alpine Orogeny began 200 million years ago and continues to the present day. It comprises a series of collisions involving various micro-continents between northern Europe and Africa. Its effects are most evident in the Alps, Pyrenees, Carpathians and other mountain ranges of southern Europe, but the northernmost 'ripples' of this event have affected the structure of southern England. Structures include;

See also

References

  1. ^ British Geological Survey 1996 Tectonic map of Britain, Ireland and adjacent areas Pharaoh, T.C. et al 1:1,500,000 (Keyworth, Notts, BGS)
  2. ^ British Geological Survey. 1996, Tectonic Map of Britain, Ireland & adjacent areas, Pharaoh et al. 1:1500 000 (Keyworth, BGS)
  3. ^ Trewin N.H. (ed) 2002, The Geology of Scotland. The Geological Society, London
  4. ^ Brenchley P.J & Rawson P.F. (eds) 2006, The Geology of England and Wales. The Geological Society, London
This page was last edited on 3 June 2018, at 17:02
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