 | This articleis largely based on an article in the out-of-copyrightEncyclopædia Britannica Eleventh Edition, which was produced in 1911. It should be brought up to date to reflect subsequent history or scholarship (including the references, if any). When you have completed the review, replace this notice with a simple note on this article's talk page.(October 2015) |
| Bagshot Formation | |
|---|---|
| Stratigraphic range:Ypresian | |
| Type | Geological formation |
| Unit of | Bracklesham Group |
| Sub-units | Swinley Clay Member |
| Underlies | Windlesham Formation |
| Overlies | London Clay Formation |
| Thickness | up to 45 metres (150 ft) |
| Lithology | |
| Primary | Sand |
| Other | Clay,Silt,Gravel |
| Location | |
| Region | Europe |
| Country |  UK |
| Extent | London Basin |
| Type section | |
| Named for | Bagshot |
| Location | Heathland, near Bagshot |
Ingeology, theBagshot Beds are a series ofsands andclays of shallow-water origin, some being fresh-water, some marine. They belong to the upperEocene formation of theLondon andHampshire basins,[1] inEngland and derive their name fromBagshot Heath in Surrey. They are also well developed inHampshire,Berkshire and theIsle of Wight. The following divisions are generally accepted:
The lower division consists of pale-yellow, current-bedded sand andloam, with layers ofpipeclay and occasional beds offlint pebbles. In the London basin, wherever the junction of the Bagshot beds with the London clay is exposed, it is clear that no sharp line can be drawn between these formations. The Lower Bagshot Beds may be observed atBrentwood,Billericay andHigh Beach inEssex; outliers, capping hills ofLondon clay, occur atHampstead,Highgate andHarrow. In Surrey, considerable tracts of London clay are covered by heath-bearing Lower Bagshot Beds, as atWeybridge,Aldershot,Woking etc. TheRamsdell clay, N.W. ofBasingstoke, belongs to this formation. In the Isle of Wight, the lower division is well exposed at Alum Bay (200 m.) and White Cliff Bay (140 ft.). Here it consists of unfossiliferous sands (white, yellow, brown, crimson and every intermediate shade) and clays with layers oflignite and ferruginoussandstone. Similar beds are visible atBournemouth and in the neighborhood ofPoole,Wareham,Corfe Castle andStudland.[2]
The leaf-bearing clays of Alum Bay and Bournemouth are well known and have yielded a large and interesting series of plant remains, includingEucalyptus,Caesalpinia,Populus,Platanus,Sequoia,Aralia,Polypodium,Osmunda,Nipadites and many others. The clays of this formation are of great value for pottery manufacture; they are extensively mined near Wareham and Corfe, whence they are shipped from Poole and are consequently known as 'Poole clays'. Alum was formerly obtained from the clays of Alum Bay; and the lignites have been used as fuel near Corfe and at Bovey. TheBracklesham Beds are sometimes classed with the overlying Barton clay as Middle Bagshot. In the London basin the Barton Beds are unknown. In Surrey and Berkshire, the Bracklesham Beds are from 20 to 50 ft. thick; in Alum Bay they are 100 ft., with beds of lignite in the lower portion; and about here they are sharply marked off from the Barton clay by a bed of conglomerate formed of flint pebbles. In Berkshire, the Bagshot Beds are variegated quartzose sands with grey to umber, or occasionally cabonaceous clay bands, with pebble beds sometimes present near the base.[1] They are 30-36m (100-120 ft.) thick around Reading and near to Kintbury.[1] The Bracklesham Beds in Berkshire are generally variegated glauconitic clays, loams, and subordinate sands.[1] They are generally 12m (40 ft.) thick near Reading, with thin pebble beds sometimes present.[1] This geology generally creates undulating, gently sloped land.[1] The presence of glauconite in some of the soil's materials can provide adequate levels of potassium (Berkshire soils are generally deficient in potassium), but they are generally still lacking.[1] The Upper Bagshot Beds, Barton sand and Barton clay, are from 140 to 200 ft. thick in the Isle of Wight. TheAgglestone (or Haggerstone) rock and Puckstone rock, near Studland inDorset are formed of large indurated masses of the Lower Bagshot beds that have resisted the weather;Creechbarrow near Corfe is another striking feature due to the same beds. Many of thesarsen stones orgreywethers of S.E. England have been derived from Bagshot strata.[2]
One or more of the preceding sentences incorporates text from a publication now in thepublic domain: Chisholm, Hugh, ed. (1911). "Bagshot Beds".Encyclopædia Britannica. Vol. 3 (11th ed.). Cambridge University Press. p. 207.