Northern Ireland - Geology: Difference between revisions

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==Recent surface deposits==
==Recent surface deposits==
Northern Ireland has widespread geological deposits of relatively recent origin, known as superficial deposits, which formed during the last 2-3 million years of the Earths’ history spanning the Ice Ages and Interglacial periods. By far the most abundant of these are glacial sediments, made of mixtures of clay, silt, sand and gravel that were laid down by the repeated growth and decay of former ice-sheets. Other sediments continue to form in lakes, rivers, estuaries and coastlines, whilst on high ground raised bogs of peat have steadily accumulated. Some of the most dramatic glacial landscapes are found in the Sperrin Mountains of counties Londonderry and Tyrone where the mountain ridges are separated by deeply eroded steep-sided valleys. In Antrim, Down and Armagh glacial deposits have been shaped into ridges and swarms of whale-back hills known as drumlins. Most of the superficial deposits are soft and easily eroded, as they have not been deeply buried and consolidated to form strong rocks.
Northern Ireland has widespread geological deposits of relatively recent origin, known as superficial deposits, which formed during the last 2 to 3 million years of the Earths’ history spanning the ice ages and interglacial periods. By far the most abundant of these are glacial sediments, made of mixtures of clay, silt, sand and gravel that were laid down by the repeated growth and decay of former ice sheets. Other sediments continue to form in lakes, rivers, estuaries and coastlines, whilst on high ground, raised bogs of peat have steadily accumulated. Some of the most dramatic glacial landscapes are found in the Sperrin Mountains of counties Londonderry and Tyrone where the mountain ridges are separated by deeply eroded steep-sided valleys. In Antrim, Down and Armagh glacial deposits have been shaped into ridges and swarms of whale-back hills known as drumlins. Most of the superficial deposits are soft and easily eroded, as they have not been deeply buried and consolidated to form strong rocks.


==Geology at depth==
==Geology at depth==
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In the course of the past 650 million years there have been periods when the area of Northern Ireland formed a landmass and was being eroded, and other periods when it was sinking and new layers of sediment were being deposited. The history of erosion and deposition has not been the same in all parts of Northern Ireland. The oldest sedimentary bedrocks are sandstones, siltstones and mudstones. These rocks were buried and deformed by the forces of continental plates moving against each other.
In the course of the past 650 million years there have been periods when the area of Northern Ireland formed a landmass and was being eroded, and other periods when it was sinking and new layers of sediment were being deposited. The history of erosion and deposition has not been the same in all parts of Northern Ireland. The oldest sedimentary bedrocks are sandstones, siltstones and mudstones. These rocks were buried and deformed by the forces of continental plates moving against each other.


[[Image:P902263.jpg|thumb|300px|Schematic cross-section through the geology of Northern Ireland. The alignment of the section and key are shown in Figure P902264. P902263.]]
[[Image:P902263.jpg|thumb|300px|Schematic cross-section through the geology of Northern Ireland. The alignment of the section and key are shown in '''Figure P902264'''. P902263.]]
Subsequently, younger sequences of sedimentary rocks, including limestones, sandstones and clays, were laid down. Although in some parts of Northern Ireland, sediments continued to be deposited relatively constantly, in most areas deposition stopped and instead uplift and erosion took place for tens of millions of years. As a result when deposition restarted the next layer was laid down on a variety of different older sediments. This situation where younger rocks rest directly on rocks of different older ages because of uplift and erosion is referred to by geologists as an unconformity.
Subsequently, younger sequences of sedimentary rocks, including limestones, sandstones and clays, were laid down. Although in some parts of Northern Ireland, sediments continued to be deposited relatively constantly, in most areas deposition stopped and instead uplift and erosion took place for tens of millions of years. As a result, when deposition restarted the next layer was laid down on a variety of different older sediments. This situation, where younger rocks rest directly on rocks of different older ages because of uplift and erosion, is referred to by geologists as an unconformity.


The geology of Northern Ireland is also affected by geological faults where the rocks on each side of the fracture have moved relative to one another. The relative movement of the rocks on either side of some of these faults can be very significant resulting in dramatic changes in geology over short distances. These include the Omagh Fault which runs in a northeast to southwest direction from Lower Lough Erne through Omagh to the coast near Ballycastle and the Southern Uplands Fault which runs northeastwards from Armagh to the coast near Belfast. These two major faults continue through Scotland to the edge of the North Sea.
The geology of Northern Ireland is also affected by geological faults where the rocks on each side of the fracture have moved relative to one another. The relative movement of the rocks on either side of some of these faults can be very significant resulting in dramatic changes in geology over short distances. These include the Omagh Fault which runs in a north-east to south-west direction from Lower Lough Erne through Omagh to the coast near Ballycastle and the Southern Uplands Fault which runs north-eastwards from Armagh to the coast near Belfast. These two major faults continue through Scotland to the edge of the North Sea.


[[Category:20. N. Ireland | 01]]
[[Category:20. N. Ireland | 01]]

Revision as of 16:01, 6 November 2014

Recent surface deposits

Northern Ireland has widespread geological deposits of relatively recent origin, known as superficial deposits, which formed during the last 2 to 3 million years of the Earths’ history spanning the ice ages and interglacial periods. By far the most abundant of these are glacial sediments, made of mixtures of clay, silt, sand and gravel that were laid down by the repeated growth and decay of former ice sheets. Other sediments continue to form in lakes, rivers, estuaries and coastlines, whilst on high ground, raised bogs of peat have steadily accumulated. Some of the most dramatic glacial landscapes are found in the Sperrin Mountains of counties Londonderry and Tyrone where the mountain ridges are separated by deeply eroded steep-sided valleys. In Antrim, Down and Armagh glacial deposits have been shaped into ridges and swarms of whale-back hills known as drumlins. Most of the superficial deposits are soft and easily eroded, as they have not been deeply buried and consolidated to form strong rocks.

Geology at depth

Below the superficial deposits, or with just a cover of soil where such deposits are absent, are older rocks which geologists broadly split into two distinct types:

  • The sedimentary bedrock geology is composed of quite hard rocks which were deposited a few hundred to tens of millions of years ago as layers of sediments in shallow seas, coastal plains or by the action of ancient river systems in times when Northern Ireland lay closer to the Equator and the climate and landscape were very different from those of today.
  • The basement geology, which underlies the sedimentary bedrock, is over 430 million years old and mainly comprises harder, denser rocks which have been strongly compacted and folded. The basement rocks comprise strongly folded and faulted sedimentary rocks, rocks that are products of volcanic activity (volcanic rocks) or formed from the solidification of molten rock below ancient volcanoes (igneous rocks) and rocks which may have started as either sediments or intrusions but have subsequently been changed into a different form by the high temperatures and pressures which they have been subjected to since (metamorphic rocks).

In the course of the past 650 million years there have been periods when the area of Northern Ireland formed a landmass and was being eroded, and other periods when it was sinking and new layers of sediment were being deposited. The history of erosion and deposition has not been the same in all parts of Northern Ireland. The oldest sedimentary bedrocks are sandstones, siltstones and mudstones. These rocks were buried and deformed by the forces of continental plates moving against each other.

Schematic cross-section through the geology of Northern Ireland. The alignment of the section and key are shown in Figure P902264. P902263.

Subsequently, younger sequences of sedimentary rocks, including limestones, sandstones and clays, were laid down. Although in some parts of Northern Ireland, sediments continued to be deposited relatively constantly, in most areas deposition stopped and instead uplift and erosion took place for tens of millions of years. As a result, when deposition restarted the next layer was laid down on a variety of different older sediments. This situation, where younger rocks rest directly on rocks of different older ages because of uplift and erosion, is referred to by geologists as an unconformity.

The geology of Northern Ireland is also affected by geological faults where the rocks on each side of the fracture have moved relative to one another. The relative movement of the rocks on either side of some of these faults can be very significant resulting in dramatic changes in geology over short distances. These include the Omagh Fault which runs in a north-east to south-west direction from Lower Lough Erne through Omagh to the coast near Ballycastle and the Southern Uplands Fault which runs north-eastwards from Armagh to the coast near Belfast. These two major faults continue through Scotland to the edge of the North Sea.