Outer Silver Pit
Encyclopedia
The Outer Silver Pit is a west-to-east valley in the bed of the North Sea
. Its widest part is 125 to 175 km (75 to 105 miles) east of Flamborough Head
in England
. It lies between the Dogger Bank
and the ridge dividing the northern from the southern North Sea basins, which runs between Norfolk
and Friesland
.
When the sea level was lower (such as in the Ice Age
) it was a lake in Doggerland
, and sea-bed-penetrating sonar bathymetry
has found its shorelines and courses and estuaries of rivers that ran into it from the high land of the Dogger Bank
; it overflowed into a river at its west end.
Whatever the facts concerning the course of the southern North Sea rivers, the Outer Silver Pit will not have been initiated by the rivers. One of the other theories must be looked to for that.
, joined the Upper Cretaceous chalk
in Kent
, England
to that of the Boulonnais
at Cap Blanc Nez, in the Pas de Calais, France
. It is possible that in the Cromerian Stage, the Outer Silver Pit was the bed of the combined Maas, Rhine, Scheldt
and Thames. Since at that stage, the Weald-Artois Anticline had probably not been breached by the glacial lake outburst flood
s that scoured the Strait of Dover, the southern North Sea basin held a freshwater lake impounded by the ridge of the Weald-Artois Anticline and by the Norfolk-Friesland ridge. Whether the lake spilled north through the Outer Silver Pit would depend on the extent to which the modern sea bed to the north is formed by later marine and glacial sediment, and how much the various parts of the area were raised by forebulge
effect caused by the weight of the Scandinavian ice cap. The eastern end of the Outer Silver Pit is aligned with the lowest part of the Norfolk-Friesland ridge. However, the outlet at the western end of the pit appears to be blocked by somewhat higher ground.
Bed of the Urstrom
There is an older theory that the Outer Silver Pit was part of the valley of the great ice-age river Urstrom
, during some of the Ice Ages when the Scandinavian ice did not meet the British ice, leaving the North Sea bed with open drainage northwards. Maps by some geologists show rivers from the German part of the Urstrom's catchment area flowing northwards east of the Dogger Bank
highland area; but Scandinavian ice reaching the Dogger Bank would have forced the Urstrom into another route.
When the Scandinavian and Scottish ice caps first met, the Urstrom (unless it found a way northwards under the ice) would have backed up into a huge lake in the southern North Sea, which finally found an outlet southwards over the Weald-Artois Anticline
and helped to cut the valley of the Strait of Dover: see Strait of Dover#Geological formation.
There is no reason to think that the Urstrom flowed along the top of this ridge, even with a retaining ice sheet to the north. That would have needed the ice to be grounded in the middle of the North Sea basin but not at the western edge, near the source of its British component.
North Sea
In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...
. Its widest part is 125 to 175 km (75 to 105 miles) east of Flamborough Head
Flamborough Head
Flamborough Head is a promontory of on the Yorkshire coast of England, between the Filey and Bridlington bays of the North Sea. It is a chalk headland, and the resistance it offers to coastal erosion may be contrasted with the low coast of Holderness to the south...
in England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
. It lies between the Dogger Bank
Dogger Bank
Dogger Bank is a large sandbank in a shallow area of the North Sea about off the east coast of England. It extends over approximately , with its dimensions being about long and up to broad. The water depth ranges from 15 to 36 metres , about shallower than the surrounding sea. It is a...
and the ridge dividing the northern from the southern North Sea basins, which runs between Norfolk
Norfolk
Norfolk is a low-lying county in the East of England. It has borders with Lincolnshire to the west, Cambridgeshire to the west and southwest and Suffolk to the south. Its northern and eastern boundaries are the North Sea coast and to the north-west the county is bordered by The Wash. The county...
and Friesland
Friesland
Friesland is a province in the north of the Netherlands and part of the ancient region of Frisia.Until the end of 1996, the province bore Friesland as its official name. In 1997 this Dutch name lost its official status to the Frisian Fryslân...
.
When the sea level was lower (such as in the Ice Age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
) it was a lake in Doggerland
Doggerland
Doggerland is a name given by archaeologists and geologists to a former landmass in the southern North Sea that connected the island of Great Britain to mainland Europe during and after the last Ice Age, surviving until about 6,500 or 6,200 BCE, though gradually being swallowed by rising sea levels...
, and sea-bed-penetrating sonar bathymetry
Bathymetry
Bathymetry is the study of underwater depth of lake or ocean floors. In other words, bathymetry is the underwater equivalent to hypsometry. The name comes from Greek βαθύς , "deep", and μέτρον , "measure"...
has found its shorelines and courses and estuaries of rivers that ran into it from the high land of the Dogger Bank
Dogger Bank
Dogger Bank is a large sandbank in a shallow area of the North Sea about off the east coast of England. It extends over approximately , with its dimensions being about long and up to broad. The water depth ranges from 15 to 36 metres , about shallower than the surrounding sea. It is a...
; it overflowed into a river at its west end.
Whatever the facts concerning the course of the southern North Sea rivers, the Outer Silver Pit will not have been initiated by the rivers. One of the other theories must be looked to for that.
Bed of the Rhine?
In the Cromerian Stage, before more recent glaciation had influenced the area, a ridge of high ground, the Weald-Artois AnticlineWeald-Artois Anticline
The Weald–Artois anticline is a large anticline, a geological structure running between the regions of the Weald in southern England and the Artois in northeastern France. The fold formed during the Alpine orogeny, from the late Oligocene to middle Miocene as an uplifted form of the Weald basin...
, joined the Upper Cretaceous chalk
Chalk
Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....
in Kent
Kent
Kent is a county in southeast England, and is one of the home counties. It borders East Sussex, Surrey and Greater London and has a defined boundary with Essex in the middle of the Thames Estuary. The ceremonial county boundaries of Kent include the shire county of Kent and the unitary borough of...
, England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
to that of the Boulonnais
Boulonnais
The Boulonnais, also known as the "White Marble Horse", is a heavy draft horse breed. It is known for its elegant, though large, appearance and is found in many colors. The breed's origins trace to a period before the Crusades and, during the 17th century, Spanish Barb, Arabian and Andalusian blood...
at Cap Blanc Nez, in the Pas de Calais, France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
. It is possible that in the Cromerian Stage, the Outer Silver Pit was the bed of the combined Maas, Rhine, Scheldt
Scheldt
The Scheldt is a 350 km long river in northern France, western Belgium and the southwestern part of the Netherlands...
and Thames. Since at that stage, the Weald-Artois Anticline had probably not been breached by the glacial lake outburst flood
Glacial lake outburst flood
A glacial lake outburst flood is a type of outburst flood that occurs when the dam containing a glacial lake fails. The dam can consist of glacier ice or a terminal moraine...
s that scoured the Strait of Dover, the southern North Sea basin held a freshwater lake impounded by the ridge of the Weald-Artois Anticline and by the Norfolk-Friesland ridge. Whether the lake spilled north through the Outer Silver Pit would depend on the extent to which the modern sea bed to the north is formed by later marine and glacial sediment, and how much the various parts of the area were raised by forebulge
Forebulge
In geology, a forebulge is a flexural bulge in front of a load on the lithosphere. This load causes the lithosphere to flex by depressing the plate beneath it. Because of the flexural rigidity of the lithosphere, the area around the load is uplifted by a height that is 4% of that of the depression...
effect caused by the weight of the Scandinavian ice cap. The eastern end of the Outer Silver Pit is aligned with the lowest part of the Norfolk-Friesland ridge. However, the outlet at the western end of the pit appears to be blocked by somewhat higher ground.
Bed of the UrstromUrstromThe Urstrom is a geologists' name for a great glacial age river of the Polish and north German plain, which drained the combined melt-waters from the northern headwaters of the Alps and the southern part of the Scandinavian ice during the Devensian ice age....
?
There is an older theory that the Outer Silver Pit was part of the valley of the great ice-age river UrstromUrstrom
The Urstrom is a geologists' name for a great glacial age river of the Polish and north German plain, which drained the combined melt-waters from the northern headwaters of the Alps and the southern part of the Scandinavian ice during the Devensian ice age....
, during some of the Ice Ages when the Scandinavian ice did not meet the British ice, leaving the North Sea bed with open drainage northwards. Maps by some geologists show rivers from the German part of the Urstrom's catchment area flowing northwards east of the Dogger Bank
Dogger Bank
Dogger Bank is a large sandbank in a shallow area of the North Sea about off the east coast of England. It extends over approximately , with its dimensions being about long and up to broad. The water depth ranges from 15 to 36 metres , about shallower than the surrounding sea. It is a...
highland area; but Scandinavian ice reaching the Dogger Bank would have forced the Urstrom into another route.
When the Scandinavian and Scottish ice caps first met, the Urstrom (unless it found a way northwards under the ice) would have backed up into a huge lake in the southern North Sea, which finally found an outlet southwards over the Weald-Artois Anticline
Weald-Artois Anticline
The Weald–Artois anticline is a large anticline, a geological structure running between the regions of the Weald in southern England and the Artois in northeastern France. The fold formed during the Alpine orogeny, from the late Oligocene to middle Miocene as an uplifted form of the Weald basin...
and helped to cut the valley of the Strait of Dover: see Strait of Dover#Geological formation.
There is no reason to think that the Urstrom flowed along the top of this ridge, even with a retaining ice sheet to the north. That would have needed the ice to be grounded in the middle of the North Sea basin but not at the western edge, near the source of its British component.
External links
- Silver Pit chart The Outer Silver Pit is further to the north-east, at the eastern end of the Skate Hole.
- Outer Silver Pit chart It lies between the Dogger BankDogger BankDogger Bank is a large sandbank in a shallow area of the North Sea about off the east coast of England. It extends over approximately , with its dimensions being about long and up to broad. The water depth ranges from 15 to 36 metres , about shallower than the surrounding sea. It is a...
and the NorfolkNorfolkNorfolk is a low-lying county in the East of England. It has borders with Lincolnshire to the west, Cambridgeshire to the west and southwest and Suffolk to the south. Its northern and eastern boundaries are the North Sea coast and to the north-west the county is bordered by The Wash. The county...
to FrieslandFrieslandFriesland is a province in the north of the Netherlands and part of the ancient region of Frisia.Until the end of 1996, the province bore Friesland as its official name. In 1997 this Dutch name lost its official status to the Frisian Fryslân...
ridge.