South-west England area - Geology - Revision history

Dbk at 09:05, 19 April 2016

2016-04-19T09:05:01Z

← Older revision		Revision as of 10:05, 19 April 2016
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	Older rocks which have been faulted against the main area of basement rocks are located on the Lizard peninsula and at Start Point ('''Plate P902610'''). The Lizard is made up of a complex range of crystalline igneous rocks, many formed at and beneath an ancient ocean floor. They are darker and denser than the younger granite. These fragments of ancient ocean floor, of similar age to some of the older sediments to the north, have been thrust up from depth by the compression and earth movement described above. In the process, still older Lower Palaeozoic sedimentary and igneous rocks have been caught up in the faults.		Older rocks which have been faulted against the main area of basement rocks are located on the Lizard peninsula and at Start Point ('''Plate P902610'''). The Lizard is made up of a complex range of crystalline igneous rocks, many formed at and beneath an ancient ocean floor. They are darker and denser than the younger granite. These fragments of ancient ocean floor, of similar age to some of the older sediments to the north, have been thrust up from depth by the compression and earth movement described above. In the process, still older Lower Palaeozoic sedimentary and igneous rocks have been caught up in the faults.

	[[Category:~~17.~~ South-west England area \| 01]]		[[Category:South-west England area - summary \| 01]]

Jeth1 at 16:15, 3 November 2014

2014-11-03T16:15:17Z

Show changes

Dbk: /* Granite intrusions */

2014-10-10T14:00:12Z

Granite intrusions

← Older revision		Revision as of 15:00, 10 October 2014
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	We can infer the shape and extent of the main granite body (geologists call it a batholith) at depth from the distribution of the granite pinnacles at surface and from regional magnetic and gravity surveys. It seems the main granite body has an elongate shape oriented NE-SW and a very irregular top surface, which locally extends to depths greater than 1 km. It is uncertain exactly how thick the granite mass is but it is likely to be several kilometres.		We can infer the shape and extent of the main granite body (geologists call it a batholith) at depth from the distribution of the granite pinnacles at surface and from regional magnetic and gravity surveys. It seems the main granite body has an elongate shape oriented NE-SW and a very irregular top surface, which locally extends to depths greater than 1 km. It is uncertain exactly how thick the granite mass is but it is likely to be several kilometres.

			[[Image:P902610.jpg\|thumb\|300px\|Highly deformed metamorphic rock of the Start Point Complex. P902610.]]
	The granite pinnacles that rise to the present-day surface have many associated mineral veins which were formed by the cooling of mineral rich liquids associated with the magma as it was emplaced. These have been mined for tin, copper, tungsten, lead and small amounts of gold and silver. The main areas of mining were around St Just, Tregonning and Gwinear, Wendron, Camborne and Redruth, Gwennap, St Agnes, Caradon and the Tamar Valley. Some of the mines in these areas reach depths of over 500 m and exceptionally the Dolcoath Mine reached a depth of 1000 m.		The granite pinnacles that rise to the present-day surface have many associated mineral veins which were formed by the cooling of mineral rich liquids associated with the magma as it was emplaced. These have been mined for tin, copper, tungsten, lead and small amounts of gold and silver. The main areas of mining were around St Just, Tregonning and Gwinear, Wendron, Camborne and Redruth, Gwennap, St Agnes, Caradon and the Tamar Valley. Some of the mines in these areas reach depths of over 500 m and exceptionally the Dolcoath Mine reached a depth of 1000 m.

Dbk: /* Older basement rocks */

2014-10-10T14:00:00Z

Older basement rocks

← Older revision		Revision as of 15:00, 10 October 2014
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	===Older basement rocks===		===Older basement rocks===
	~~[[Image:P902610.jpg\|thumb\|300px\|Highly deformed metamorphic rock of the Start Point Complex. P902610.]]~~
	Older rocks which have been faulted against the main area of basement rocks are located on the Lizard peninsula and at Start Point (Plate P902610). The Lizard is made up of a complex range of crystalline igneous rocks, many formed at and beneath an ancient ocean floor. They are darker and denser than the younger granite. These fragments of ancient ocean floor, of similar age to some of the older sediments to the north, have been thrust up from depth by the compression and earth movement described above. In the process, still older Lower Palaeozoic sedimentary and igneous rocks have been caught up in the faults.		Older rocks which have been faulted against the main area of basement rocks are located on the Lizard peninsula and at Start Point (Plate P902610). The Lizard is made up of a complex range of crystalline igneous rocks, many formed at and beneath an ancient ocean floor. They are darker and denser than the younger granite. These fragments of ancient ocean floor, of similar age to some of the older sediments to the north, have been thrust up from depth by the compression and earth movement described above. In the process, still older Lower Palaeozoic sedimentary and igneous rocks have been caught up in the faults.

	[[Category:17. South-west England area \| 01]]		[[Category:17. South-west England area \| 01]]

Dbk: /* Basement Rocks */

2014-10-10T13:59:47Z

Basement Rocks

← Older revision		Revision as of 14:59, 10 October 2014
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	[[Image:P211236.jpg\|thumb\|300px\|Highly folded and disturbed interbedded sandstones and mudstones of the Culm, Hartland Quay, west of Bideford. P211236.]]		[[Image:P211236.jpg\|thumb\|300px\|Highly folded and disturbed interbedded sandstones and mudstones of the Culm, Hartland Quay, west of Bideford. P211236.]]
	[[Image:P212763.jpg\|thumb\|~~300hpx~~\|Tightly folded and overturned Devonian mudstones near Ilfracombe, north Devon. P212763.]]		[[Image:P212763.jpg\|thumb\|300px\|Tightly folded and overturned Devonian mudstones near Ilfracombe, north Devon. P212763.]]
	The ridges are commonly formed by harder sandstones Towards the top of this sequence of sediment layers, the Carboniferous rocks are rich in sandstone, siltstone and mudstone. The youngest sediments accumulated around Westward Ho! near Bideford in a swampy environment, with river-borne sediments. This produced coal-bearing mudstones and sandstone with thin developments of lignite and coal that were worked in small pits until 1969. The underlying sandstone and mudstone sequence, known by geologists as the Culm (Plate P211236), now occupies the greater part of central Devon and the north Cornwall area around Bude. It lies in the heart of a large down fold or syncline, but as well as this large fold structure, the rocks are intensely deformed throughout and the muddy beds have acquired many of the characteristics of slate. The Culm sedimentary rocks are estimated to reach depths in excess of 1000 m through the centre of this large syncline in the area around Hatherleigh. It represents the deposits within a large lake or shallow sea. Beneath the Culm, the oldest of the Carboniferous rocks comprise mainly black mudstones with local development of hard chert, deposited in a deep sea, with localised eruptions from volcanoes represented by lavas and tuffs within the sediments.		The ridges are commonly formed by harder sandstones Towards the top of this sequence of sediment layers, the Carboniferous rocks are rich in sandstone, siltstone and mudstone. The youngest sediments accumulated around Westward Ho! near Bideford in a swampy environment, with river-borne sediments. This produced coal-bearing mudstones and sandstone with thin developments of lignite and coal that were worked in small pits until 1969. The underlying sandstone and mudstone sequence, known by geologists as the Culm (Plate P211236), now occupies the greater part of central Devon and the north Cornwall area around Bude. It lies in the heart of a large down fold or syncline, but as well as this large fold structure, the rocks are intensely deformed throughout and the muddy beds have acquired many of the characteristics of slate. The Culm sedimentary rocks are estimated to reach depths in excess of 1000 m through the centre of this large syncline in the area around Hatherleigh. It represents the deposits within a large lake or shallow sea. Beneath the Culm, the oldest of the Carboniferous rocks comprise mainly black mudstones with local development of hard chert, deposited in a deep sea, with localised eruptions from volcanoes represented by lavas and tuffs within the sediments.

Dbk at 13:42, 10 October 2014

2014-10-10T13:42:11Z

← Older revision		Revision as of 14:42, 10 October 2014
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			__NOTOC__
	==Recent surface deposits==		==Recent surface deposits==
	Most of the region has few to no recent surface deposits. Those that do occur comprise sands and gravels developed along the larger river systems, and in the east of the region flinty gravels overlie Chalk bedrock. Most rivers in the region flow south to the English Channel. The poorly drained and wet higher parts of the region, usually above 300 m, contain blankets of peat a few metres thick. This region lay beyond the extent of the ice-sheets that have dominated the evolution of the British landscape over the past 2-3 million years.		Most of the region has few to no recent surface deposits. Those that do occur comprise sands and gravels developed along the larger river systems, and in the east of the region flinty gravels overlie Chalk bedrock. Most rivers in the region flow south to the English Channel. The poorly drained and wet higher parts of the region, usually above 300 m, contain blankets of peat a few metres thick. This region lay beyond the extent of the ice-sheets that have dominated the evolution of the British landscape over the past 2-3 million years.

Dbk: /* Basement Rocks */

2014-10-10T13:41:33Z

Basement Rocks

← Older revision		Revision as of 14:41, 10 October 2014
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	During the Devonian the region formed a deep sea with high ground draining from the north bringing large quantities of sediments into the sea. These deposited pebble beds, sandstones and mudstones with some volcanic rocks, and where the sea was shallow, in central and eastern Devon and around Torquay, limestones were deposited. The Devonian Rocks that lie at surface in the north of this area typically comprise coarser grained and thicker sequences of sandstone that retain many of their sedimentary features and are much less deformed than rocks of the same age present to the south and west (Plate P212763). Farther south these rocks become mainly mudstones with some limestones that were deposited in a quiet deeper part of the sea which covered this area. Volcanic activity at the time also produced widespread flows of lava within this sequence of rocks, particularly further west. These are preserved from the north coast of Cornwall, where they form the towering cliffs of Pentire, Newquay; a second strip extends from the south of Bodmin Moor past Liskeard and Plymouth to Torquay.		During the Devonian the region formed a deep sea with high ground draining from the north bringing large quantities of sediments into the sea. These deposited pebble beds, sandstones and mudstones with some volcanic rocks, and where the sea was shallow, in central and eastern Devon and around Torquay, limestones were deposited. The Devonian Rocks that lie at surface in the north of this area typically comprise coarser grained and thicker sequences of sandstone that retain many of their sedimentary features and are much less deformed than rocks of the same age present to the south and west (Plate P212763). Farther south these rocks become mainly mudstones with some limestones that were deposited in a quiet deeper part of the sea which covered this area. Volcanic activity at the time also produced widespread flows of lava within this sequence of rocks, particularly further west. These are preserved from the north coast of Cornwall, where they form the towering cliffs of Pentire, Newquay; a second strip extends from the south of Bodmin Moor past Liskeard and Plymouth to Torquay.

	The basement rocks have been extensively folded and faulted in the millions of years since they were first formed; they are estimated to be over 4000 m in total thickness, with the sequence compressed by pressure from the south resulting in folds which are aligned with an east-west trend. The forces were sufficient to bend the strata over itself resulting in some layers now lying upside down. In the south, where the compression was the greatest it was strong enough to break the rocks creating faults and shunting slabs of the older rocks northwards over younger rocks. In some places the compression and folding resulted in temperatures and pressures high enough to ~~“bake”~~ the rocks so that they now have a slaty character allowing them to break easily in one direction leading to the use as roofing slates, as at Delabole, 4 km west of Camelford.		The basement rocks have been extensively folded and faulted in the millions of years since they were first formed; they are estimated to be over 4000 m in total thickness, with the sequence compressed by pressure from the south resulting in folds which are aligned with an east-west trend. The forces were sufficient to bend the strata over itself resulting in some layers now lying upside down. In the south, where the compression was the greatest it was strong enough to break the rocks creating faults and shunting slabs of the older rocks northwards over younger rocks. In some places the compression and folding resulted in temperatures and pressures high enough to 'bake' the rocks so that they now have a slaty character allowing them to break easily in one direction leading to the use as roofing slates, as at Delabole, 4 km west of Camelford.

	===Granite intrusions===		===Granite intrusions===

Dbk: /* Basement Rocks */

2014-10-10T13:40:32Z

Basement Rocks

← Older revision		Revision as of 14:40, 10 October 2014
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	==Basement Rocks==		==Basement Rocks==
	These rocks occur at surface in most of the area to the west and south of a line from Watchet to Torquay. They comprise rocks of sedimentary origin that have been intensely deformed, then intruded by granite bodies. They are marked by a landscape that is dominated in the north by long east-west trending valleys and ridges. Harder rock layers form the long ridges separated by valleys of softer rock (~~Figure 6~~). These Devonian and Carboniferous strata were deposited between about 410 to 315 million years ago.		[[Image:P211134.jpg\|thumb\|300px\|left\|Ridge and valley topography developed in Carboniferous rocks in the central part of the region at Lapford, east of Hatherleigh. P211134.]]
			These rocks occur at surface in most of the area to the west and south of a line from Watchet to Torquay. They comprise rocks of sedimentary origin that have been intensely deformed, then intruded by granite bodies. They are marked by a landscape that is dominated in the north by long east-west trending valleys and ridges. Harder rock layers form the long ridges separated by valleys of softer rock (Plate P211134). These Devonian and Carboniferous strata were deposited between about 410 to 315 million years ago.

	The ridges are commonly formed by harder sandstones Towards the top of this sequence of sediment layers, the Carboniferous rocks are rich in sandstone, siltstone and mudstone. The youngest sediments accumulated around Westward Ho! near Bideford in a swampy environment, with river-borne sediments. This produced coal-bearing mudstones and sandstone with thin developments of lignite and coal that were worked in small pits until 1969. The underlying sandstone and mudstone sequence, known by geologists as the Culm (~~Figure 7~~), now occupies the greater part of central Devon and the north Cornwall area around Bude. It lies in the heart of a large down fold or syncline, but as well as this large fold structure, the rocks are intensely deformed throughout and the muddy beds have acquired many of the characteristics of slate. The Culm sedimentary rocks are estimated to reach depths in excess of 1000 m through the centre of this large syncline in the area around Hatherleigh. It represents the deposits within a large lake or shallow sea. Beneath the Culm, the oldest of the Carboniferous rocks comprise mainly black mudstones with local development of hard chert, deposited in a deep sea, with localised eruptions from volcanoes represented by lavas and tuffs within the sediments.		[[Image:P211236.jpg\|thumb\|300px\|Highly folded and disturbed interbedded sandstones and mudstones of the Culm, Hartland Quay, west of Bideford. P211236.]]
			[[Image:P212763.jpg\|thumb\|300hpx\|Tightly folded and overturned Devonian mudstones near Ilfracombe, north Devon. P212763.]]
			The ridges are commonly formed by harder sandstones Towards the top of this sequence of sediment layers, the Carboniferous rocks are rich in sandstone, siltstone and mudstone. The youngest sediments accumulated around Westward Ho! near Bideford in a swampy environment, with river-borne sediments. This produced coal-bearing mudstones and sandstone with thin developments of lignite and coal that were worked in small pits until 1969. The underlying sandstone and mudstone sequence, known by geologists as the Culm (Plate P211236), now occupies the greater part of central Devon and the north Cornwall area around Bude. It lies in the heart of a large down fold or syncline, but as well as this large fold structure, the rocks are intensely deformed throughout and the muddy beds have acquired many of the characteristics of slate. The Culm sedimentary rocks are estimated to reach depths in excess of 1000 m through the centre of this large syncline in the area around Hatherleigh. It represents the deposits within a large lake or shallow sea. Beneath the Culm, the oldest of the Carboniferous rocks comprise mainly black mudstones with local development of hard chert, deposited in a deep sea, with localised eruptions from volcanoes represented by lavas and tuffs within the sediments.

	During the Devonian the region formed a deep sea with high ground draining from the north bringing large quantities of sediments into the sea. These deposited pebble beds, sandstones and mudstones with some volcanic rocks, and where the sea was shallow, in central and eastern Devon and around Torquay, limestones were deposited. The Devonian Rocks that lie at surface in the north of this area typically comprise coarser grained and thicker sequences of sandstone that retain many of their sedimentary features and are much less deformed than rocks of the same age present to the south and west (~~Figure 8~~). Farther south these rocks become mainly mudstones with some limestones that were deposited in a quiet deeper part of the sea which covered this area. Volcanic activity at the time also produced widespread flows of lava within this sequence of rocks, particularly further west. These are preserved from the north coast of Cornwall, where they form the towering cliffs of Pentire, Newquay; a second strip extends from the south of Bodmin Moor past Liskeard and Plymouth to Torquay.		During the Devonian the region formed a deep sea with high ground draining from the north bringing large quantities of sediments into the sea. These deposited pebble beds, sandstones and mudstones with some volcanic rocks, and where the sea was shallow, in central and eastern Devon and around Torquay, limestones were deposited. The Devonian Rocks that lie at surface in the north of this area typically comprise coarser grained and thicker sequences of sandstone that retain many of their sedimentary features and are much less deformed than rocks of the same age present to the south and west (Plate P212763). Farther south these rocks become mainly mudstones with some limestones that were deposited in a quiet deeper part of the sea which covered this area. Volcanic activity at the time also produced widespread flows of lava within this sequence of rocks, particularly further west. These are preserved from the north coast of Cornwall, where they form the towering cliffs of Pentire, Newquay; a second strip extends from the south of Bodmin Moor past Liskeard and Plymouth to Torquay.

	The basement rocks have been extensively folded and faulted in the millions of years since they were first formed; they are estimated to be over 4000 m in total thickness, with the sequence compressed by pressure from the south resulting in folds which are aligned with an east-west trend. The forces were sufficient to bend the strata over itself resulting in some layers now lying upside down. In the south, where the compression was the greatest it was strong enough to break the rocks creating faults and shunting slabs of the older rocks northwards over younger rocks. In some places the compression and folding resulted in temperatures and pressures high enough to “bake” the rocks so that they now have a slaty character allowing them to break easily in one direction leading to the use as roofing slates, as at Delabole, 4 km west of Camelford.		The basement rocks have been extensively folded and faulted in the millions of years since they were first formed; they are estimated to be over 4000 m in total thickness, with the sequence compressed by pressure from the south resulting in folds which are aligned with an east-west trend. The forces were sufficient to bend the strata over itself resulting in some layers now lying upside down. In the south, where the compression was the greatest it was strong enough to break the rocks creating faults and shunting slabs of the older rocks northwards over younger rocks. In some places the compression and folding resulted in temperatures and pressures high enough to “bake” the rocks so that they now have a slaty character allowing them to break easily in one direction leading to the use as roofing slates, as at Delabole, 4 km west of Camelford.

	===Granite intrusions===		===Granite intrusions===
	Granite igneous intrusions occur throughout the region, except in the north and east. As granite is a hard crystalline rock it is not easily eroded and forms rugged upland areas. At surface the rock weathers into the form of the distinctive tors (~~Figure 9~~), as seen on the top of areas like Dartmoor, forming hills over 300 m high, including the two highest peaks in southern England.		[[Image:P209965.jpg\|thumb\|300px\|left\|Typical granite tor at Blackingstone Rock, in the Dartmoor National Park. P209965.]]
			Granite igneous intrusions occur throughout the region, except in the north and east. As granite is a hard crystalline rock it is not easily eroded and forms rugged upland areas. At surface the rock weathers into the form of the distinctive tors (Plate P209965), as seen on the top of areas like Dartmoor, forming hills over 300 m high, including the two highest peaks in southern England.

	The granite intrusions formed towards the end of the phase of compression and earth movement that folded the surrounding basement rocks about 295 million years ago. A large molten mass of magma rose from within the Earth’s crust and ascended to near the surface where it spread out to form a long cylindrical body extending from Dartmoor to the Scillies. Extending upwards from this body, six smaller pinnacles rose still further and now form the granite moors and high ground of Dartmoor, Bodmin Moor, north of St Austell, west of Falmouth, Penzance and the Isles of Scilly. The rocks into which this hot magma intruded were locally baked by the intense heat, changing their nature to harder re-crystallised rocks. In some places they were also shot through with mineral veins by fluids released from the crystallising magma.		The granite intrusions formed towards the end of the phase of compression and earth movement that folded the surrounding basement rocks about 295 million years ago. A large molten mass of magma rose from within the Earth’s crust and ascended to near the surface where it spread out to form a long cylindrical body extending from Dartmoor to the Scillies. Extending upwards from this body, six smaller pinnacles rose still further and now form the granite moors and high ground of Dartmoor, Bodmin Moor, north of St Austell, west of Falmouth, Penzance and the Isles of Scilly. The rocks into which this hot magma intruded were locally baked by the intense heat, changing their nature to harder re-crystallised rocks. In some places they were also shot through with mineral veins by fluids released from the crystallising magma.
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	===Older basement rocks===		===Older basement rocks===
	Older rocks which have been faulted against the main area of basement rocks are located on the Lizard peninsula and at Start Point (~~Figure 10~~). The Lizard is made up of a complex range of crystalline igneous rocks, many formed at and beneath an ancient ocean floor. They are darker and denser than the younger granite. These fragments of ancient ocean floor, of similar age to some of the older sediments to the north, have been thrust up from depth by the compression and earth movement described above. In the process, still older Lower Palaeozoic sedimentary and igneous rocks have been caught up in the faults.		[[Image:P902610.jpg\|thumb\|300px\|Highly deformed metamorphic rock of the Start Point Complex. P902610.]]
			Older rocks which have been faulted against the main area of basement rocks are located on the Lizard peninsula and at Start Point (Plate P902610). The Lizard is made up of a complex range of crystalline igneous rocks, many formed at and beneath an ancient ocean floor. They are darker and denser than the younger granite. These fragments of ancient ocean floor, of similar age to some of the older sediments to the north, have been thrust up from depth by the compression and earth movement described above. In the process, still older Lower Palaeozoic sedimentary and igneous rocks have been caught up in the faults.

	[[Category:17. South-west England area \| 01]]		[[Category:17. South-west England area \| 01]]

Dbk: /* Sedimentary Bedrock (Eastern area) */

2014-10-10T13:33:06Z

Sedimentary Bedrock (Eastern area)

← Older revision		Revision as of 14:33, 10 October 2014
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	The youngest of these tilted deposits comprise Jurassic sediments, 200 to 185 million years old, that include mainly limestones and underlying mudstones of the Great and Inferior Oolite and Lias Group, respectively.		The youngest of these tilted deposits comprise Jurassic sediments, 200 to 185 million years old, that include mainly limestones and underlying mudstones of the Great and Inferior Oolite and Lias Group, respectively.

	These form an impressive part of the World Heritage Site along the Devon-Dorset coastline between Exmouth and Bridport, an area known as the Jurassic Coast. These layers extend northwards but are absent in the area between Taunton and Bridgwater. They are then present again on the north coast of the region between Watchet (~~Figure~~ P211333) and 3 km north of Bridgwater. These rocks reach a depth of 366 m below sea-level around Crewkerne in the east of the region. Oil shale was discovered in 1900 on the coast near Watchet, but commercial extraction was abandoned due to the high sulphur content.		These form an impressive part of the World Heritage Site along the Devon-Dorset coastline between Exmouth and Bridport, an area known as the Jurassic Coast. These layers extend northwards but are absent in the area between Taunton and Bridgwater. They are then present again on the north coast of the region between Watchet (Plate P211333) and 3 km north of Bridgwater. These rocks reach a depth of 366 m below sea-level around Crewkerne in the east of the region. Oil shale was discovered in 1900 on the coast near Watchet, but commercial extraction was abandoned due to the high sulphur content.

	To the west of the area occupied by the Lias, between Sidmouth and Seaton and extending northwards to Taunton and Bridgwater and then along the north Somerset coast to Minehead, is a thick succession of red mudstones which locally contain gypsum (~~Figure~~ P211333). These are known as the Mercia Mudstone, which is nearly 500 m thick and extends eastwards beneath the Lias to depths of about 870 m to the south of Crewkerne.		[[Image:P781352.jpg\|thumb\|300px\|Sandstones with pebble layers of the Sherwood Sandstone, Burlscombe, near Wellington. P781352.]]
			To the west of the area occupied by the Lias, between Sidmouth and Seaton and extending northwards to Taunton and Bridgwater and then along the north Somerset coast to Minehead, is a thick succession of red mudstones which locally contain gypsum (Plate P211333). These are known as the Mercia Mudstone, which is nearly 500 m thick and extends eastwards beneath the Lias to depths of about 870 m to the south of Crewkerne.

	The oldest strata in this sequence of sedimentary rocks were formed in part of an extensive desert in which rare storms washed rock debris into basins and huge scree slopes accumulated at the foot of mountains, forming a succession of red sandstones with pebble layers (~~Figure 5~~) and mudstones. This was a landscape with active volcanism and both lavas and the sites of small volcanoes are found around Exeter. These were probably formed at the end of the period of magma intrusion that gave rise to the granite masses found further west. The upper part of this sequence of rocks, known as the Sherwood Sandstone, is approximately 250 m thick. These rocks are present at the surface from east of Exmouth in the south to Minehead in the north. The pores or voids between the sand grains and pebbles allow water to flow easily through the rock as well as flowing through fractures within it, making this unit a locally important aquifer used for drinking water supply.		The oldest strata in this sequence of sedimentary rocks were formed in part of an extensive desert in which rare storms washed rock debris into basins and huge scree slopes accumulated at the foot of mountains, forming a succession of red sandstones with pebble layers (Plate P781352) and mudstones. This was a landscape with active volcanism and both lavas and the sites of small volcanoes are found around Exeter. These were probably formed at the end of the period of magma intrusion that gave rise to the granite masses found further west. The upper part of this sequence of rocks, known as the Sherwood Sandstone, is approximately 250 m thick. These rocks are present at the surface from east of Exmouth in the south to Minehead in the north. The pores or voids between the sand grains and pebbles allow water to flow easily through the rock as well as flowing through fractures within it, making this unit a locally important aquifer used for drinking water supply.

	==Basement Rocks==		==Basement Rocks==

Dbk: /* Sedimentary Bedrock (Eastern area) */

2014-10-10T13:31:18Z

Sedimentary Bedrock (Eastern area)

← Older revision		Revision as of 14:31, 10 October 2014
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	The youngest deposits are approximately 55 to 25 million years old (Palaeogene sediments) and are of very restricted extent. They comprise clay, sand and silt layers deposited in small lakes and rivers and occur between Bovey Tracey and Newton Abbot and also farther west about 5 km south of Great Torrington. These deposits are relatively soft and form low-lying ground. They have infilled rapidly subsiding hollows or basins in the older rocks that were formed by the movement of geological faults. Although these basin areas are relatively small, the deposits have been proved to extend to at least 600 m below sea-level in parts of these areas. Much of the sediment has been derived by erosion of the Dartmoor granite mass, and the clay deposits in the Bovey Tracey area have been an important source of clay for ceramics.		The youngest deposits are approximately 55 to 25 million years old (Palaeogene sediments) and are of very restricted extent. They comprise clay, sand and silt layers deposited in small lakes and rivers and occur between Bovey Tracey and Newton Abbot and also farther west about 5 km south of Great Torrington. These deposits are relatively soft and form low-lying ground. They have infilled rapidly subsiding hollows or basins in the older rocks that were formed by the movement of geological faults. Although these basin areas are relatively small, the deposits have been proved to extend to at least 600 m below sea-level in parts of these areas. Much of the sediment has been derived by erosion of the Dartmoor granite mass, and the clay deposits in the Bovey Tracey area have been an important source of clay for ceramics.

			[[Image:P211333.jpg\|thumb\|300px\|Grey Lias Group mudstones overlying red Mercia Mudstone with white gypsum in the foreground, near Watchet. P211333.]]
	The main Eastern part of the region includes a sequence of gently-dipping sedimentary layers (Cretaceous sediments). These are composed of sandstones and clays capped by Chalk, 110 to 90 million years old, occurring in the area south of Taunton. The Chalk is a fine grained white and grey rock composed of minute grains and fossils made of calcium carbonate, it caps the tops of the hills along the coast between Sidmouth and Lyme Regis and inland to the area around Chard reaching a maximum thickness of 80 m.		The main Eastern part of the region includes a sequence of gently-dipping sedimentary layers (Cretaceous sediments). These are composed of sandstones and clays capped by Chalk, 110 to 90 million years old, occurring in the area south of Taunton. The Chalk is a fine grained white and grey rock composed of minute grains and fossils made of calcium carbonate, it caps the tops of the hills along the coast between Sidmouth and Lyme Regis and inland to the area around Chard reaching a maximum thickness of 80 m.

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	The youngest of these tilted deposits comprise Jurassic sediments, 200 to 185 million years old, that include mainly limestones and underlying mudstones of the Great and Inferior Oolite and Lias Group, respectively.		The youngest of these tilted deposits comprise Jurassic sediments, 200 to 185 million years old, that include mainly limestones and underlying mudstones of the Great and Inferior Oolite and Lias Group, respectively.

	These form an impressive part of the World Heritage Site along the Devon-Dorset coastline between Exmouth and Bridport, an area known as the Jurassic Coast. These layers extend northwards but are absent in the area between Taunton and Bridgwater. They are then present again on the north coast of the region between Watchet (Figure 4) and 3 km north of Bridgwater. These rocks reach a depth of 366 m below sea-level around Crewkerne in the east of the region. Oil shale was discovered in 1900 on the coast near Watchet, but commercial extraction was abandoned due to the high sulphur content.		These form an impressive part of the World Heritage Site along the Devon-Dorset coastline between Exmouth and Bridport, an area known as the Jurassic Coast. These layers extend northwards but are absent in the area between Taunton and Bridgwater. They are then present again on the north coast of the region between Watchet (Figure P211333) and 3 km north of Bridgwater. These rocks reach a depth of 366 m below sea-level around Crewkerne in the east of the region. Oil shale was discovered in 1900 on the coast near Watchet, but commercial extraction was abandoned due to the high sulphur content.

	To the west of the area occupied by the Lias, between Sidmouth and Seaton and extending northwards to Taunton and Bridgwater and then along the north Somerset coast to Minehead, is a thick succession of red mudstones which locally contain gypsum (Figure 4). These are known as the Mercia Mudstone, which is nearly 500 m thick and extends eastwards beneath the Lias to depths of about 870 m to the south of Crewkerne.		To the west of the area occupied by the Lias, between Sidmouth and Seaton and extending northwards to Taunton and Bridgwater and then along the north Somerset coast to Minehead, is a thick succession of red mudstones which locally contain gypsum (Figure P211333). These are known as the Mercia Mudstone, which is nearly 500 m thick and extends eastwards beneath the Lias to depths of about 870 m to the south of Crewkerne.

	The oldest strata in this sequence of sedimentary rocks were formed in part of an extensive desert in which rare storms washed rock debris into basins and huge scree slopes accumulated at the foot of mountains, forming a succession of red sandstones with pebble layers (Figure 5) and mudstones. This was a landscape with active volcanism and both lavas and the sites of small volcanoes are found around Exeter. These were probably formed at the end of the period of magma intrusion that gave rise to the granite masses found further west. The upper part of this sequence of rocks, known as the Sherwood Sandstone, is approximately 250 m thick. These rocks are present at the surface from east of Exmouth in the south to Minehead in the north. The pores or voids between the sand grains and pebbles allow water to flow easily through the rock as well as flowing through fractures within it, making this unit a locally important aquifer used for drinking water supply.		The oldest strata in this sequence of sedimentary rocks were formed in part of an extensive desert in which rare storms washed rock debris into basins and huge scree slopes accumulated at the foot of mountains, forming a succession of red sandstones with pebble layers (Figure 5) and mudstones. This was a landscape with active volcanism and both lavas and the sites of small volcanoes are found around Exeter. These were probably formed at the end of the period of magma intrusion that gave rise to the granite masses found further west. The upper part of this sequence of rocks, known as the Sherwood Sandstone, is approximately 250 m thick. These rocks are present at the surface from east of Exmouth in the south to Minehead in the north. The pores or voids between the sand grains and pebbles allow water to flow easily through the rock as well as flowing through fractures within it, making this unit a locally important aquifer used for drinking water supply.