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Sunday, 19 December 2010

Geology of the Wiltshire Downs

Geology of the Wiltshire Downs (from Natural England site)

Dominating almost two thirds of Wiltshire ...... is the Upper Cretaceous Chalk which forms the vast rolling landscape of Salisbury Plain with its seemingly endless arable fields and unimproved grassland punctuated only by small hilltop woodlands. During the Tertiary and Quaternary, erosion and weathering of the chalk produced the clay-with-flints deposits that occur across parts of the plateau surface and which today often support woodland vegetation, in contrast to the thin, dry soils of the Chalk. Scattered across the Plain and the downs are Sarsen stones, the weather-worn blocks of grey sandstone derived from the former cover of Tertiary deposits.


Though freshwater and terrestrial conditions continued through into the early Cretaceous once more sea-level rose and there was a return to shallow marine conditions. This is marked in Wiltshire by the sands and ironstones of the Lower Greensand, which outcrop in the Vale of Wardour and in the Vale of Pewsey to the west of Devizes. Ironstone within a small outcrop of the Lower Greensand at Seend was formerly quarried and the ore smelted on the spot.
The Lower Greensand is succeeded by the Gault Clay and the Upper Greensand, their outcrop zigzagging in a narrow strip around the edge of the Chalk escarpment and flooring the Vale of Pewsey east of Devizes. Exposures of the bluish-grey Gault Clay are rare, but quarries in the Upper Greensand near Warminster show that the warm, sub-tropical sea in which it was deposited teemed with life as large numbers of fossil bivalves, sea-urchins and ammonites can be found. In the south-west of the County, the Upper Greensand outcrop gives rise to an undulating, hilly landscape. The rounded hills and relatively free-draining and slightly acidic soils provide the attractive setting for the historic parks of Stourhead and Longleat.
The transition to the overlying Chalk can be observed in several old quarries located at the base of the scarp slope that dominates the skyline in much of west and north Wiltshire.

This leads up onto the large chalk plateau of Salisbury Plain which dominates the centre of the County, Cranborne Chase in the south and the Marlborough Downs in the north. The very pure limestone (up to 98% calcium carbonate) of the Chalk was deposited in a warm shallow tropical sea around 70-100 million years ago. Despite its extensive outcrop, natural exposures are not common, but it can be seen in road cuttings and old quarries such as those at Harnham just to the south of Salisbury. Throughout its area of outcrop, wherever agricultural practice is suitable, the chalk supports calcareous grassland vegetation that is rich in many plant species including orchids and herbs.

Tertiary (comprising Neogene and Palaeogene)

The main outcrop of Tertiary (65-2 million years ago) rocks occurs in the south-eastern corner of the County. These outcrop in a narrow belt to the north of Dean Hill in a shallow basin that forms a small extension of the much larger Tertiary Hampshire Basin to the south. There is also a small outcrop of Tertiary sediments at Great Bedwyn and Savernake Forest south-east of Marlborough at the far western end of the London Basin. These various clays and sands were laid down in shallow marine, coastal and fluvial (river) environments and include the Upnor Formation, Reading Formation, London Clay and Bracklesham Group.
Sarsen stones, isolated remnant blocks of weathered Tertiary sandstone with a hard silica cement, can be found over the surface of the chalk. These probably represent outlying deposits of sands within the Reading Formation and indicate that these Tertiary sediments formerly extended well beyond their present more limited outcrop. Cementation of the sands probably occurred just below the ground surface under an arid or semi-arid climate, perhaps 5-10 million years ago, and the surrounding uncemented sediments have long since been washed away. In some areas the surface of the chalk is covered by clay-with-flints. This represents the insoluble residue left after chalk has been eroded and weathered away, and has probably been formed over many millions of years. The clay-with-flints often gives rise to more acidic soil conditions which contrasts with the calcareous soils derived directly from the chalk bedrock.


Over the last two million years the climate of Britain has varied tremendously with periods of temperate climate interrupted by repeated advances and retreats of glaciers and ice sheets. Collectively these periods have become known as the Ice Age (we are still in one of the temperate phases) and the actions of the ice sheets and the climatic changes have been instrumental in forming the landscape we see today.
Ice did not reach Wiltshire during the Quaternary, although tundra-like conditions would have prevailed. Under these arctic conditions the dry valleys of the chalk probably formed. These features occur on what is a very permeable rock which does not generally support surface drainage systems. However, during the various glacial periods, deep permafrost would have made the ground impermeable and allowed gradual erosion of the frozen soil surface to occur, particularly during summer thaws. Much of the shaping of the present form of the Chalk landscape with its scarps, dip-slopes and valleys would have been produced through water erosion.


Some detail about the Chalk layers (acknowledgement to Wilts Geology Group):
The beds of the Chalk formation build some of the most spectacular scarp and downland scenery in Wiltshire. Mineralogically the transition from the Upper Greensand to Grey (Lower) Chalk is gradual, the basal beds being very sandy and glauconitic. These levels include the famous Warminster Greensand and the Cenomanian Basement Bed or Glauconitic Marl, (both these beds are now incorporated as the Melbury Sandstone).
"Proper" Chalk begins with the Zig Zag Chalk Member, which forms the basal third of the escarpment around Salisbury Plain. The lower beds have a grey appearance due to a silt content, but the chalk gradually whitens and becomes more massive upwards in the section known as the Plenus Marls, after a characteristic belemnite.
The base of the White (Middle and Upper) Chalk occurs at a hard nodular band known as the Melbourn Rock, whilst the remainder of the formation, 20 to 40 metres thick, is divided into the Holywell Nodular Chalk Member and the New Pit Chalk Member.
In the highest levels of the outcrop flints become common. The Lewes Nodular Chalk Formation caps the highest escarpments. At its base, the Chalk Rock is rich in glauconitised/phosphatised erosion surfaces (hardgrounds), four in the outcrop on White Sheet Hill. The topmost members of the White Chalk, some 60m thick, are typical firm, white chalk with regular courses of flint nodules.

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