London and the Thames Valley - Geology - Revision history

Dbk: /* Geology at depth */

2021-11-03T12:31:08Z

Geology at depth

← Older revision		Revision as of 13:31, 3 November 2021
Line 15:		Line 15:
	[[Image:P902271.jpg\|thumb\|300px\|left\|Schematic cross-section of the geology underlying London and the Thames Valley. The alignment of this section and key are shown in '''Figure P902272'''. P902271]][[Image:P902250.jpg\|thumb\|300px\|right\|Schematic cross-sections of the geology underlying London and the Thames Valley. The alignment of this section and key are shown in '''Figure P902272'''.]]		[[Image:P902271.jpg\|thumb\|300px\|left\|Schematic cross-section of the geology underlying London and the Thames Valley. The alignment of this section and key are shown in '''Figure P902272'''. P902271]][[Image:P902250.jpg\|thumb\|300px\|right\|Schematic cross-sections of the geology underlying London and the Thames Valley. The alignment of this section and key are shown in '''Figure P902272'''.]]
	In some places in the south and west of the region, the younger sedimentary bedrock extends to more than 1 km depth. However, in most of the region the older sedimentary bedrock and basement rocks occur within 500 m of the surface, and lie as shallow as 150 m in some north-eastern parts. '''Figures P902271''' and '''P902250''' are vertical sections through the geology, referred to as geological cross-sections, and illustrate these variations.		In some places in the south and west of the region, the younger sedimentary bedrock extends to more than 1 km depth. However, in most of the region the older sedimentary bedrock and basement rocks occur within 500 m of the surface, and lie as shallow as 150 m in some north-eastern parts. '''Figures P902271''' and '''P902250''' are vertical sections through the geology, referred to as geological cross-sections, and illustrate these variations.

			[[Image:P902272.jpg\|thumb\|300px\|right\|Geological sketch map showing the range and distribution of different rock types in the London and Thames Valley region, in relation to major towns and cities. The extent of London and Thames Valley region is identified on the inset map of the United Kingdom. P902272]]

	The younger sedimentary bedrock occurs at the surface and dictates the broad variations in geography and land use across the region. It is composed of varied rocks formed from about 50 to 250 million years ago, mainly as sediment layers in shallow seas or on extensive coastal plains. While harder and more consolidated than the superficial deposits, many of the younger sedimentary bedrock are quite soft and easily eroded. The youngest, uppermost layers occur in the centre of the London Basin and are dominated by clays. The Chalk, which forms the surrounding Chiltern Hills, the North Downs and the downlands of Berkshire, north Hampshire and Wiltshire, also extends at depth beneath the London Basin forming a down-folded, basin-like form that geologists call a syncline (see '''Figure P902250'''). In the Upper Thames Valley, the younger sedimentary layers are tilted gently towards the south-east, descending beneath the Chalk. Progressively older rocks occur at the surface towards the north-west.		The younger sedimentary bedrock occurs at the surface and dictates the broad variations in geography and land use across the region. It is composed of varied rocks formed from about 50 to 250 million years ago, mainly as sediment layers in shallow seas or on extensive coastal plains. While harder and more consolidated than the superficial deposits, many of the younger sedimentary bedrock are quite soft and easily eroded. The youngest, uppermost layers occur in the centre of the London Basin and are dominated by clays. The Chalk, which forms the surrounding Chiltern Hills, the North Downs and the downlands of Berkshire, north Hampshire and Wiltshire, also extends at depth beneath the London Basin forming a down-folded, basin-like form that geologists call a syncline (see '''Figure P902250'''). In the Upper Thames Valley, the younger sedimentary layers are tilted gently towards the south-east, descending beneath the Chalk. Progressively older rocks occur at the surface towards the north-west.

	[[Image:P902272.jpg\|thumb\|300px\|right\|Geological sketch map showing the range and distribution of different rock types in the London and Thames Valley region, in relation to major towns and cities. The extent of London and Thames Valley region is identified on the inset map of the United Kingdom. P902272]]

	The older sedimentary bedrock is preserved in troughs or basins resting on the even older basement rocks and concealed beneath the younger sedimentary bedrock. Elsewhere, basement rocks lie directly below the younger sedimentary bedrock because the older sedimentary rocks had been eroded away before the younger sedimentary rocks were laid down.		The older sedimentary bedrock is preserved in troughs or basins resting on the even older basement rocks and concealed beneath the younger sedimentary bedrock. Elsewhere, basement rocks lie directly below the younger sedimentary bedrock because the older sedimentary rocks had been eroded away before the younger sedimentary rocks were laid down.

Dbk: /* Geology at depth */

2021-11-03T12:30:28Z

Geology at depth

← Older revision		Revision as of 13:30, 3 November 2021
Line 17:		Line 17:

	The younger sedimentary bedrock occurs at the surface and dictates the broad variations in geography and land use across the region. It is composed of varied rocks formed from about 50 to 250 million years ago, mainly as sediment layers in shallow seas or on extensive coastal plains. While harder and more consolidated than the superficial deposits, many of the younger sedimentary bedrock are quite soft and easily eroded. The youngest, uppermost layers occur in the centre of the London Basin and are dominated by clays. The Chalk, which forms the surrounding Chiltern Hills, the North Downs and the downlands of Berkshire, north Hampshire and Wiltshire, also extends at depth beneath the London Basin forming a down-folded, basin-like form that geologists call a syncline (see '''Figure P902250'''). In the Upper Thames Valley, the younger sedimentary layers are tilted gently towards the south-east, descending beneath the Chalk. Progressively older rocks occur at the surface towards the north-west.		The younger sedimentary bedrock occurs at the surface and dictates the broad variations in geography and land use across the region. It is composed of varied rocks formed from about 50 to 250 million years ago, mainly as sediment layers in shallow seas or on extensive coastal plains. While harder and more consolidated than the superficial deposits, many of the younger sedimentary bedrock are quite soft and easily eroded. The youngest, uppermost layers occur in the centre of the London Basin and are dominated by clays. The Chalk, which forms the surrounding Chiltern Hills, the North Downs and the downlands of Berkshire, north Hampshire and Wiltshire, also extends at depth beneath the London Basin forming a down-folded, basin-like form that geologists call a syncline (see '''Figure P902250'''). In the Upper Thames Valley, the younger sedimentary layers are tilted gently towards the south-east, descending beneath the Chalk. Progressively older rocks occur at the surface towards the north-west.

			[[Image:P902272.jpg\|thumb\|300px\|right\|Geological sketch map showing the range and distribution of different rock types in the London and Thames Valley region, in relation to major towns and cities. The extent of London and Thames Valley region is identified on the inset map of the United Kingdom. P902272]]

	The older sedimentary bedrock is preserved in troughs or basins resting on the even older basement rocks and concealed beneath the younger sedimentary bedrock. Elsewhere, basement rocks lie directly below the younger sedimentary bedrock because the older sedimentary rocks had been eroded away before the younger sedimentary rocks were laid down.		The older sedimentary bedrock is preserved in troughs or basins resting on the even older basement rocks and concealed beneath the younger sedimentary bedrock. Elsewhere, basement rocks lie directly below the younger sedimentary bedrock because the older sedimentary rocks had been eroded away before the younger sedimentary rocks were laid down.

Dbk at 16:24, 18 April 2016

2016-04-18T16:24:35Z

← Older revision		Revision as of 17:24, 18 April 2016
Line 58:		Line 58:
	In marked contrast to the London Platform, the younger sedimentary bedrock immediately to the south is founded on a distinctive basement rocks more comparable to the rocks occurring at the surface in Devon. Folded and faulted basement rocks of this type extend only a short distance into the southern fringe of the Thames Valley region, where they are found at depths of more than 500 m. This fringe is marked at the surface by features such as the Hog’s Back ridge of western Surrey.		In marked contrast to the London Platform, the younger sedimentary bedrock immediately to the south is founded on a distinctive basement rocks more comparable to the rocks occurring at the surface in Devon. Folded and faulted basement rocks of this type extend only a short distance into the southern fringe of the Thames Valley region, where they are found at depths of more than 500 m. This fringe is marked at the surface by features such as the Hog’s Back ridge of western Surrey.

	[[Category:~~13._London_and_the_Thames_Valley~~ \| 01]]		[[Category: London_and_the_Thames_Valley - summary \| 01]]

Dbk at 16:23, 18 April 2016

2016-04-18T16:23:20Z

← Older revision		Revision as of 17:23, 18 April 2016
Line 58:		Line 58:
	In marked contrast to the London Platform, the younger sedimentary bedrock immediately to the south is founded on a distinctive basement rocks more comparable to the rocks occurring at the surface in Devon. Folded and faulted basement rocks of this type extend only a short distance into the southern fringe of the Thames Valley region, where they are found at depths of more than 500 m. This fringe is marked at the surface by features such as the Hog’s Back ridge of western Surrey.		In marked contrast to the London Platform, the younger sedimentary bedrock immediately to the south is founded on a distinctive basement rocks more comparable to the rocks occurring at the surface in Devon. Folded and faulted basement rocks of this type extend only a short distance into the southern fringe of the Thames Valley region, where they are found at depths of more than 500 m. This fringe is marked at the surface by features such as the Hog’s Back ridge of western Surrey.

	Category:13._London_and_the_Thames_Valley \| 01]]		[[Category:13._London_and_the_Thames_Valley \| 01]]

Dbk at 16:23, 18 April 2016

2016-04-18T16:23:04Z

← Older revision		Revision as of 17:23, 18 April 2016
Line 58:		Line 58:
	In marked contrast to the London Platform, the younger sedimentary bedrock immediately to the south is founded on a distinctive basement rocks more comparable to the rocks occurring at the surface in Devon. Folded and faulted basement rocks of this type extend only a short distance into the southern fringe of the Thames Valley region, where they are found at depths of more than 500 m. This fringe is marked at the surface by features such as the Hog’s Back ridge of western Surrey.		In marked contrast to the London Platform, the younger sedimentary bedrock immediately to the south is founded on a distinctive basement rocks more comparable to the rocks occurring at the surface in Devon. Folded and faulted basement rocks of this type extend only a short distance into the southern fringe of the Thames Valley region, where they are found at depths of more than 500 m. This fringe is marked at the surface by features such as the Hog’s Back ridge of western Surrey.

	[[Category:13. ~~London and the Thames Valley~~ \| 01]]		Category:13._London_and_the_Thames_Valley \| 01]]

Jeth1 at 14:42, 20 October 2014

2014-10-20T14:42:44Z

← Older revision		Revision as of 15:42, 20 October 2014
Line 49:		Line 49:

	==Older sedimentary bedrock and basement rocks==		==Older sedimentary bedrock and basement rocks==
	[[Image:P902530.jpg\|thumb\|300px\|~~Bedded limestones in~~ the ~~Great Oolite, Oxfordshire~~. P902530.]]		[[Image:P902530.jpg\|thumb\|300px\|The extent of the London Platform. P902530.]]
	The older sedimentary bedrock is preserved at depth in concealed troughs or basins found in the Bedford—Hitchin area where the Carboniferous Limestone dominates and between Banbury in the north and Newbury and Reading in the south. Here the Carboniferous Limestone is overlain by thick Coal Measures composed mainly of sandstones, siltstones, mudstones and thin coals. These rocks also include igneous rocks formed by the cooling of molten rock from deep in the Earth either at surface as volcanic lava or under the ground as igneous intrusions. They are similar to those found at the surface in the South Wales Coalfield and are probably more than 1000 m in thickness.		The older sedimentary bedrock is preserved at depth in concealed troughs or basins found in the Bedford—Hitchin area where the Carboniferous Limestone dominates and between Banbury in the north and Newbury and Reading in the south. Here the Carboniferous Limestone is overlain by thick Coal Measures composed mainly of sandstones, siltstones, mudstones and thin coals. These rocks also include igneous rocks formed by the cooling of molten rock from deep in the Earth either at surface as volcanic lava or under the ground as igneous intrusions. They are similar to those found at the surface in the South Wales Coalfield and are probably more than 1000 m in thickness.

Jeth1 at 14:41, 20 October 2014

2014-10-20T14:41:40Z

Show changes

Dbk: /* Older Sedimentary Bedrock and Basement rocks */

2014-10-10T08:57:06Z

Older Sedimentary Bedrock and Basement rocks

← Older revision		Revision as of 09:57, 10 October 2014
Line 49:		Line 49:

	==Older Sedimentary Bedrock and Basement rocks==		==Older Sedimentary Bedrock and Basement rocks==
			[[Image:P902530.jpg\|thumb\|300px\|Bedded limestones in the Great Oolite, Oxfordshire. P902530.]]
	The older sedimentary bedrock is preserved at depth in concealed troughs or basins found in the Bedford-Hitchin area where the Carboniferous Limestone dominates and between Banbury in the north and Newbury and Reading in the south. Here the Carboniferous Limestone is overlain by thick Coal Measures composed mainly of sandstones, siltstones, mudstones and thin coals. These rocks also include igneous rocks formed by the cooling of molten rock from deep in the Earth either at surface as volcanic lava or under the ground as igneous intrusions. They are similar to those found at the surface in the South Wales Coalfield and are probably more than 1000 m in thickness.		The older sedimentary bedrock is preserved at depth in concealed troughs or basins found in the Bedford-Hitchin area where the Carboniferous Limestone dominates and between Banbury in the north and Newbury and Reading in the south. Here the Carboniferous Limestone is overlain by thick Coal Measures composed mainly of sandstones, siltstones, mudstones and thin coals. These rocks also include igneous rocks formed by the cooling of molten rock from deep in the Earth either at surface as volcanic lava or under the ground as igneous intrusions. They are similar to those found at the surface in the South Wales Coalfield and are probably more than 1000 m in thickness.

	Basement rocks underlie these basins, and elsewhere in the region, for example around and north of London, they rest directly below the younger sedimentary bedrock. The basement rocks are generally harder and less porous than the overlying rocks and mainly comprise grey and red slates, mudstones and sandstones (Devonian rocks and older rocks). The layers have been distorted by earth movements, in places tilting at steep angles. While the rocks themselves are less porous than the younger sediments they are cut through by fractures that do contain groundwater and include ancient geological faults where the rocks on each side of the fracture have moved relative to one another.		Basement rocks underlie these basins, and elsewhere in the region, for example around and north of London, they rest directly below the younger sedimentary bedrock. The basement rocks are generally harder and less porous than the overlying rocks and mainly comprise grey and red slates, mudstones and sandstones (Devonian rocks and older rocks). The layers have been distorted by earth movements, in places tilting at steep angles. While the rocks themselves are less porous than the younger sediments they are cut through by fractures that do contain groundwater and include ancient geological faults where the rocks on each side of the fracture have moved relative to one another.

	Most of the Thames Valley overlies an uplifted block of these old rocks called the London Platform (~~'''~~Figure ~~8'''~~). Here basement rocks occur at less than 500 m depth; indeed, in the northeast of the region, they are within about 150 m of the surface. The London Platform has been an area of geological stability for at least 250 million years. One consequence of this is that the younger bedrock formations found at surface in the Upper Thames Valley, and layers of a similar age at depth in the Wealden region (to the south), thin quickly towards the London Basin where they are mainly absent. Only the Chalk and the Gault Clay immediately below it form continuous layers across this London Platform.		Most of the Thames Valley overlies an uplifted block of these old rocks called the London Platform (Figure P902530). Here basement rocks occur at less than 500 m depth; indeed, in the northeast of the region, they are within about 150 m of the surface. The London Platform has been an area of geological stability for at least 250 million years. One consequence of this is that the younger bedrock formations found at surface in the Upper Thames Valley, and layers of a similar age at depth in the Wealden region (to the south), thin quickly towards the London Basin where they are mainly absent. Only the Chalk and the Gault Clay immediately below it form continuous layers across this London Platform.

	In marked contrast to the London Platform, the younger sedimentary bedrock immediately to the south is founded on a distinctive basement rocks more comparable to the rocks occurring at the surface in Devon. Folded and faulted basement rocks of this type extend only a short distance into the southern fringe of the Thames Valley region, where they are found at depths of more than 500 m. This fringe is marked at the surface by features such as the Hog’s Back ridge of western Surrey.		In marked contrast to the London Platform, the younger sedimentary bedrock immediately to the south is founded on a distinctive basement rocks more comparable to the rocks occurring at the surface in Devon. Folded and faulted basement rocks of this type extend only a short distance into the southern fringe of the Thames Valley region, where they are found at depths of more than 500 m. This fringe is marked at the surface by features such as the Hog’s Back ridge of western Surrey.

	[[Category:13. London and the Thames Valley \| 01]]		[[Category:13. London and the Thames Valley \| 01]]

Dbk: /* The Upper Thames Valley */

2014-10-09T09:31:46Z

The Upper Thames Valley

← Older revision		Revision as of 10:31, 9 October 2014
Line 40:		Line 40:

	[[Image:P211894.jpg\|thumb\|300px\|left\|Bedded limestones in the Great Oolite, Oxfordshire. P211894]]		[[Image:P211894.jpg\|thumb\|300px\|left\|Bedded limestones in the Great Oolite, Oxfordshire. P211894]]
	The Upper Thames Valley area is underlain by varied deposits that originated as sediments on the ancient sea-floor, and which now form successive layers beneath the Chalk. This sedimentary sequence includes four major layers composed mainly of mudstone (Gault Clay, Kimmeridge and Ampthill clays, Oxford Clay, and the Lias and Mercia Mudstone groups), separated by layers dominated by sandstone, siltstone or limestone (Upper and Lower Greensand, the Portland, Purbeck, Corallian , Great and Inferior Oolite groups (~~Figure~~ P211894), the Sherwood Sandstone Group and some un-named Permian sediments). Some of these intervening layers form aquifers of regional or local importance, and are characterised by flow between the rock grains or along fractures, or both. In some limestones (Corallian Group), the groundwater can be salty and some salt springs support rare and protected habitats. The mudstones are clays which have been consolidated into harder rocks over millions of years: they form very effective barriers to the movement of water between the various aquifer units in this area.		The Upper Thames Valley area is underlain by varied deposits that originated as sediments on the ancient sea-floor, and which now form successive layers beneath the Chalk. This sedimentary sequence includes four major layers composed mainly of mudstone (Gault Clay, Kimmeridge and Ampthill clays, Oxford Clay, and the Lias and Mercia Mudstone groups), separated by layers dominated by sandstone, siltstone or limestone (Upper and Lower Greensand, the Portland, Purbeck, Corallian , Great and Inferior Oolite groups (Plate P211894), the Sherwood Sandstone Group and some un-named Permian sediments). Some of these intervening layers form aquifers of regional or local importance, and are characterised by flow between the rock grains or along fractures, or both. In some limestones (Corallian Group), the groundwater can be salty and some salt springs support rare and protected habitats. The mudstones are clays which have been consolidated into harder rocks over millions of years: they form very effective barriers to the movement of water between the various aquifer units in this area.

	The youngest of the mudstone layers is the Gault Clay and is up to 100 m thick where it comes to the surface in the Upper Thames Valley, generally diminishing eastwards to about 50 m in Essex and north Kent. It occurs beneath the Chalk down to as low as 440 m below sea level in the deepest parts of the London Basin. The other mudstone intervals are also thickest in the west of the region, where they are between 100 and 230 m thick near the surface, but together with the intervening sandstone, siltstone or limestone layers, they become thinner to the east.		The youngest of the mudstone layers is the Gault Clay and is up to 100 m thick where it comes to the surface in the Upper Thames Valley, generally diminishing eastwards to about 50 m in Essex and north Kent. It occurs beneath the Chalk down to as low as 440 m below sea level in the deepest parts of the London Basin. The other mudstone intervals are also thickest in the west of the region, where they are between 100 and 230 m thick near the surface, but together with the intervening sandstone, siltstone or limestone layers, they become thinner to the east.

Dbk: /* The Chalk Downlands */

2014-10-09T09:31:29Z

The Chalk Downlands

← Older revision		Revision as of 10:31, 9 October 2014
Line 28:		Line 28:
	===The Chalk Downlands===		===The Chalk Downlands===
	[[Image:P209845.jpg\|thumb\|300px\|Chalk quarries in the face of the North Downs, near Dorking, Surrey. P209845.]]		[[Image:P209845.jpg\|thumb\|300px\|Chalk quarries in the face of the North Downs, near Dorking, Surrey. P209845.]]
	The London Basin is surrounded to the south by the North Downs of Surrey and west Kent (~~Figure~~ P209845), extending westwards into the downlands of northern Hampshire, and to the north by the Chiltern Hills and the Berkshire Downs, extending westwards into the Marlborough Downs of Wiltshire. All these areas of downland are formed by the Chalk, which is a very fine-grained white or pale grey limestone. The Chalk often contains nodules of flint, a very hard form of silica.		The London Basin is surrounded to the south by the North Downs of Surrey and west Kent (Plate P209845), extending westwards into the downlands of northern Hampshire, and to the north by the Chiltern Hills and the Berkshire Downs, extending westwards into the Marlborough Downs of Wiltshire. All these areas of downland are formed by the Chalk, which is a very fine-grained white or pale grey limestone. The Chalk often contains nodules of flint, a very hard form of silica.

	[[Image:P535262.jpg\|thumb\|300px\| left\|Chalk with flints beneath Palaeogene sediments, in a road cutting near Newbury. P535262.]]		[[Image:P535262.jpg\|thumb\|300px\| left\|Chalk with flints beneath Palaeogene sediments, in a road cutting near Newbury. P535262.]]
	[[Image:P902287.jpg\|thumb\|300px\|Water-filled fissures and pores dissolved in the Chalk, seen in a borehole, Height of image about 50 cm. P902287.]]		[[Image:P902287.jpg\|thumb\|300px\|Water-filled fissures and pores dissolved in the Chalk, seen in a borehole, Height of image about 50 cm. P902287.]]
	As mentioned above the Chalk (~~Figure~~ P535262) forms a continuous layer beneath the London Basin. It extends to depths of as much as 500 m in west Surrey and is typically about 200 m thick, although it reaches 275 m in east Essex.		As mentioned above the Chalk (Plate P535262) forms a continuous layer beneath the London Basin. It extends to depths of as much as 500 m in west Surrey and is typically about 200 m thick, although it reaches 275 m in east Essex.

	The Chalk is the most important aquifer, or source of underground water, in southern England. Most of the water flow in the Chalk is not through the pore spaces between the grains of the rock but along fine fractures within it (~~Figure~~ P902287). These fractures are both horizontal and vertical and connect together to make pathways for water to flow through. Because the Chalk is composed of calcium carbonate which can be slowly dissolved by groundwater, the fractures become wider over long periods of time leading to the quite rapid flow of water through some parts of the Chalk. Where the Chalk is covered by the younger sedimentary layers of the London Basin the thick clays form a barrier to the movement of groundwater, preventing both the upwards movement of water out of the Chalk and downwards percolation into the Chalk.		The Chalk is the most important aquifer, or source of underground water, in southern England. Most of the water flow in the Chalk is not through the pore spaces between the grains of the rock but along fine fractures within it (Plate P902287). These fractures are both horizontal and vertical and connect together to make pathways for water to flow through. Because the Chalk is composed of calcium carbonate which can be slowly dissolved by groundwater, the fractures become wider over long periods of time leading to the quite rapid flow of water through some parts of the Chalk. Where the Chalk is covered by the younger sedimentary layers of the London Basin the thick clays form a barrier to the movement of groundwater, preventing both the upwards movement of water out of the Chalk and downwards percolation into the Chalk.

	===The Upper Thames Valley===		===The Upper Thames Valley===