Silver Pit

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The Silver Pit is a long valley in the bed of the North Sea, 45 km (27 miles) east of Spurn Head in England. In origin, it is probably a tunnel valley (Benn & Evans fig.9.27) which was kept free of periglacial deposits by the Wash River when the sea level was lower, towards the end of the Devensian glaciation. However, it may date partially or largely from the Wolstonian glaciation.

Main article: Silverpit crater

The Outer Silver Pit or Silverpit crater, a separate formation, 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.

There are several theories as to how the Outer Silver Pit came to exist. It may have been formed by an asteroidal impact or more prosaically, by the dissolution of a thick bed of salt which permitted the upper strata to collapse. The Silverpit Formation is salt and other deposits laid down in a desert lake (like the Great Salt Lake or the Dead Sea) during the early Permian, before the influx of the Zechstein Sea. (Cameron et.al. p.42)

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[edit] Origins of the Silver Pit

The 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 is named by geologists the Urstrom, "ancient current". The older theory, that it flowed by way of the Outer Silver Pit is not a likely explanation as before it got that far west, the Urstrom would have found an outlet southwards over the Weald-Artois Anticline to the valley at the Strait of Dover, or northwards under the sea ice. There is no reason to think that it flowed along the top of this ridge, even with a retaining ice sheet to the north. That would have required 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.

[edit] Rivers

In the Cromerian interglacial, before more recent glaciation had influenced the area, a ridge of high ground, the Weald-Artois Anticline, 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, the Outer Silver Pit was the bed of the combined Maas, Rhine, Scheldt and Thames. Since at that stage, the ridge had probably not been breached by the glacial lake outburst floods 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. 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.

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.

[edit] References

  • Benn, D.I. & Evans, D.J.A. Glaciers and Glaciation (1998) ISBN 0-340-58431-9
  • Cameron, Crosby, Balson, Jeffery, Lott, Bulat & Harrison. The Geology of the Southern North Sea (1992) ISBN 0-11-884492-X
  • Glennie, K.W. Lower Permian - Rotliegend in ed. Glennie Introduction to the Petroleum Geology of the North Sea. (1990) ISBN 0-632-02711-8
  • Imray, Laurie, Norie & Wilson pub. East Coast of England: Orfordness to Blythe marine chart (1980)

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