Bagshot Beds, in geology, a series of sands and clays of shallow-water origin, some being fresh-water, some marine. They belong to the upper Eocene formation of the London and Hampshire basins (England), and derive their name from Bagshot Heath in Surrey; but they are also well developed in Hampshire and the Isle of Wight. The following divisions are generally accepted: -
Barton sand, and Barton clay.
Bournemouth beds, Alum Bay beds,
The lower division consists of pale-yellow, current-bedded sand and loam, with layers of pipeclay and occasional beds of flint 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 at Brentwood, Billericay and Highbeech in Essex; outliers, capping hills of London clay, occur at Hampstead, Highgate and Harrow. In Surrey considerable tracts of London clay are covered by heath-bearing Lower Bagshot beds, as at Weybridge, Aldershot, Woking, etc. The "Ramsdell clay," N.W. of Basingstoke, belongs to this formation. In the Isle of Wight the lower division is well exposed at Alum Bay (660 ft.) and White Cliff Bay (140 ft.); here it consists of unfossiliferous sands (white, yellow, brown, crimson and every intermediate shade), and clays with layers of lignite and ferruginous sandstone. Similar beds are visible at Bournemouth, and in the neighbourhood of Poole, Wareham, Corfe and Studland.
The leaf-bearing clays of Alum Bay and Bournemouth are well known, and have yielded a large and interesting series of plant remains, including Eucalyptus, Caesalpinia, Populus, Platanus, Sequoia, Aralia, Polypodium, Osmunda, Nipadites and many others. The sands and clays of Bovey Tracey (see Bovey Beds) are probably of the same age. The clays of this formation are of great value for pottery manufacture; they are extensively mined in the vicinity of Wareham and Corfe, whence they are shipped from Poole and are consequently known as "Poole clays"; similarly, "Teignmouth clay" is obtained from the Bovey beds. Alum was formerly obtained from the clays of Alum Bay; and the lignites have been used as fuel near Corfe and at Bovey.
The Bracklesham beds (q.v.) 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. The Upper Bagshot beds, Barton sand and Barton clay, are from 140 to 200 ft. thick in the Isle of Wight.
The Agglestone (or Haggerstone) rock and Puckstone rock, near Studland in Dorsetshire, 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 the sarsen stones or greywethers of S.E. England have been derived from Bagshot strata.
See Memoirs of the Geological Survey (England): - "Geology of the Isle of Wight," new edition (1889); "The Geology of London and Part of the Thames Valley," vol. i. (1889); and "The Geology of the Country around Bournemouth" (1898).