Geological Prehistory
Parallel Valleys
Parallel Valleys are organised systems of impact valleys. They occur both on land ,and in locations that are now submarine.
They reflect the retreat of a melting edge of the ice sheet during a period of extreme heat input into the environment.
Each valley marks the retreat of the icesheet over a period of one year.
Impact valleys cover most of Britain and the North Sea.
Owing to the position of the British Isles in relation to the Atlantic Ocean and the European Continent, and to its relatively small land mass, many of the impact valleys are not parallel to each other, but are rather chaotic in their orientation.
Also parallel valleys are formed in specific locations, and in specific conditions. They are commonly orientated along contours on rising land in the foothills of mountains.
They also orient in relation to warm air sources. These would include proximity to warm water, direct from the Atlantic Ocean and retreating from warm air crossing the area from the south-west to the north-east.
Additionally, warm air and river water from mainland Europe would also have affected the locations and orientation of some impact valleys.
The following survey of parallel valleys begins on the south coast of England and follows the English and Scottish coasts to northern Scotland, returning overland from the Highlands through England to southern Wales.
The English Channel
The English Channel is possessed of many ridges and troughs. Some of these are believed to have been sculpted by water flowing as huge quantities of water flowing from the west to east and back again every day.
A section across a palaeochannel rhat is in an equivalent position the Lobourg Channel, has been sampled and defined. A section across it has been developed ,and dates for the materials in its makeup have been acquired. [Denudation of the continental shelf between Britain and France at the glacial–interglacial timescale by Claire L. Mellett et al]
The palaeovalley has been cut through material deposited in 83,000BP, and the date of the first deposit in the valley is 21,200BP which is roughly when the last ice age peaked, and when the ice sheets began to retreat.
The basal lag indicated in the drawing is a thin bedload that is deposited in the floor of the valley, coarsely grained and poorly sorted, and consisting of gravels and pebbles
It is deposited under uniform energy conditions in highly sediment charged environments, and generally indicates transport capacity-limited or competence-limited conditions due to oversupply of sediment.
The basal lag may represent residual deposits reflecting rapid deposition and/or prolonged winnowing.
This suggests, but cannot fully confirm, that the deepest troughs in the English Channel were cut at the height of the last ice age, and at the beginning of the deglaciation phase.
The Norfolk Banks
The Southern North Sea contains a group of shallow undersea features, of which the most famous is Dogger Bank.
In addition to this is the deep trough the Outer Silver Pit which separates it from the shallows that occupy the seabed between England Holland and France. The shallows host multiple valleys and ridges, many of which would until relatively recently have been islets with their surfaces raised above local sea levels.
The major valley through the Dover Strait, and into the North Sea is the Lobourg Channel, and this connects directly with a series of valleys and ridges off the coast of Norfolk, the Norfolk Banks.
Figure above shows a strip of shallow sea between Holland and Eastern England. The Norfolk Banks rest on this seabed surface.
The Norfolk Banks rest on a relatively flat surface at 20 to 30 m depth (see above). They consist of accumulations of sand on an erosional surface of Pleistocene deposits that in the interbank areas is either exposed or covered by a thin lag gravel. The largest of the Norfolk Banks, Well Bank, is over 50 km long, 1.7 km wide and rises to 38 m above the adjacent sea floor, although some banks are even higher at over 42 m (Gaston, 1972).
Owing to their position in the English Channel region of the North Sea where the huge volumes of tidal waters swing back and forth East to west and west to east every day the Norfolk Banks are believed to have been built up as undersea dunes.
Although this is a plausible explanation for their existence it is not likely to be accurate.
The Norfolk Banks are the latgest sea floor features of this type in the world, and they are much too well organised to be formed by the movement of water longitudinally along them.
This is probably a unique situation, and the Norfolk Banks are a unique set of features, and their orientation, parallel to the Norfolk coast suggests that they are parallel valleys and ridges in the floor of the southern North Sea.
As parallel valleys they represent one of the leading edges of the retreat of ice sheet on Britain.
The causation of these edge collapses in the English Channel may have been the emergence of rivers flowing through unfrozen France, and coming up against the edge of the ice sheet on Britain.
As I say, the Norfolk Banks lay parallel to the Norfolk coast and, as raised landforms, they are likely to be the last vestiges of a land surface that extended between Holland and the east coast of England until early neolithic people started to migrate into Britain.
They are described as having bioturbation in their upper layers which is usually suggested to be from burrowing crustaceans, but they might equally have hosted rooted trees or shrubs.
Parallel Valleys on the Kent coast suggest the possibility that the south downs in Kent also carried a thickness of ice sheet.
The map above is from "Geology of the country around Ramsgate and Dover. Memoir for 1:50 000 geological sheets 274 and 290", where the presence of these dry valleys is mentioned, with their association to local "bournes", but their creation is not otherwise explained.
The survival of the Norfolk Banks as raised sandbanks is probably because a local ice-sheet on southern and eastern England extended out from the present coastline.
This ice-sheet may not have been very thick, 50 to 100 metres, perhaps. Where it lay over the land east of Norfolk it consolidated the underlying sediments sufficiently that while softer sediments, not consolidated, were quickly eroded by encroaching raised sea-level the Norfolk Banks remained as a seabed feature to present times.
While snow cover was present over most of the rest of England it was absent at the south coast and of variable thickness elsewhere. Thick ice sheets were restricted to northerly, easterly, and mountainous locations.
Dogger Bank and the North Sea
Moving north from Kent, a group of parallel valleys are offshore from northern England, that have been mapped by Dave Roberts et al in: "The mixed‐bed glacial landform imprint of the North Sea Lobe in the western North Sea"
The valleys are offshore from Berwick in the north, and the Farn Deeps are the deep feature in the south. This group may also be an impact valleys
Again, [3] a significant series of undersea parallel valleys are adjacent and parallel to the Aberdeenshire coast....
.....and further parallel valleys [4] extend around the North Coast of Aberdeenshire.
Returning to the southern North Sea, the Outer Silver Pit and Dogger Bank are cut from the south-west and from the northwest by a series of parallel valleys [Large-scale glacitectonic deformation in response to active ice sheet retreat across Dogger Bank (southern central North Sea) during the Last Glacial Maximum by Emrys Phillips et al]
Moraines are also present on the surface of the southern tip of Dogger Bank.
The Devil's Hole at a distance off the Scottish coast, is a group of parallel valleys which, like those northwest of Dogger Bank retreated to the east towards the Norwegian Coast.
Further north are the Witch Ground, and the Fladen Deeps which contain parallel valleys that also retreat in an easterly direction towards Norway.
On land the presence of impact valleys is a feature of the landscape here in Scotland, but they are chaotic; parallel, but also chaotic.
This may be because they were developed on high plateau geology rather than on rising slopes, which often define parallel valleys.
In south Wales a more organised array of parallel valleys has developed, facing off against the Gulf Stream weather from the mid Atlantic Ocean and the equatorial Americas.
In England one of the most clear demonstrations of the parallel valley, and the impact valley, is the Trent Valley that curves round the east and south sides of the Pennine Hills. It is over a million years old and is more deeply cut at its midpoint than it is at either end. For this reason it contained a lake at various times in geological prehistory.
All opinions my own, unless referenced otherwise.
Jeffery Nicholls
South Ronaldsay
Orkney
#BIIS #LGM #Quaternary #Geology #glaciation #IceAge #ice-sheet
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