1911 Encyclopædia Britannica/Wealden

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25718971911 Encyclopædia Britannica, Volume 28 — WealdenJohn Allen Howe

WEALDEN, in geology, a thick series of estuarine and freshwater deposits of Lower Cretaceous age, which derives its name from its development in the Weald of Kent and Sussex. In the type area it is exposed by the denudation of a broad anticlinal fold from which the higher Cretaceous beds have been removed. The Wealden rocks he in the central part of this anticline between the escarpments cf the North and South Downs; they extend eastwards from the neighbourhood of Haslemere and Elland Chapel to the west between Pevensey and Hythe. This formation is divisible into two portions, the Weald Clay above and the Hastings Sands below. The Weald Clay which occupies the central, upland part of the area from Horsham to the sea coast consists of dark brown and blue clays and shales, occasionally mottled in the neighbourhood of sandy lenticles, which together with calcareous sandstones, shelly limestones and nodular ironstones take a subordinate place in the series. About Horsham the Weald Clay is 1000 ft. thick, but it decreases in an eastward direction; at Tunbridge it is only 600 ft. Certain subordinate beds within the Weald Clay have received distinctive names. " Horsham stone " is a calcareous flaggy sandstone, often ripple marked, usually less than 5 ft. thick, which occurs at about 120 ft. above the base of the Clay. " Sussex marble " is the name given to more than one of the high limestone beds which are mainly composed of a large form of Paladin (P. fluviorum); some of the lower limestone layers contain a small species (P. sussexiensis). The Sussex marble (proper) occurs about 100 ft. below the top of the clays; it is the most important of the limestone bands, and its thickness varies from 6 ft. to 2 in.; it is known also as Bethersden marble, Petworth marble, Laughton stone, &c. It has been widely used in the Weald district in church architecture and for polished mantelpieces. The ironstones were formerly smelted in the western part of the area.

The Hastings Sands are divisible into three main subdivisions: the Tunbridge Wells Sand, the Wadhurst Clay and the Ashdown Sand. Like the overlying Weald Clay this series thickens as a whole towards the west. In the west, the Tunbridge Wells Sand is separated into an upper.and lower division by the thickening of abed of clay—the Grinstead Clay—which in the east, about Rye, &c., is quite thin; at Cuckfield a second clay bed 15 ft. thick divides the upper division. The upper beds of the lower Tunbridge Wells Sand cause good landscapes around West Hoathly and near East Grinstead. The Wadhurst Clay is very constant in character; near the base it frequently contains clay-ironstone, which in former times was the main source of supply for the Wealden iron industry. Much of the higher portion of the Hastings Sand country is made of the Ashdown Sands, consisting of sand, soft sandstones and subordinate clay bends; in the east, however, clay is strongly developed at the base of this group, and at Fairlight is more than 360 ft. thick, while the sandy portion is only 150 ft. These clays with sandy layers are known as the Fairlight Clays. Beds of lignite are found in these beds, and a calcareous sandstone, called Tilgate stone, occurs near the top of the Ashdown Sands and in the Wadhurst Clay. The old town of Hastings is built on Ashdown Sand, but St Leonards is mainly on Tunbridge Wells Sand.

Wealden beds occur on the southern side of the Isle of Wight and in the Isle of Purbeck in Dorsetshire. The Wealden anticline can be traced across the Channel into the Bas Boulonnais. A separate Wealden area exists in north Germany between Brunswick and Bentheim, in the Ostervald and Teutoberger Wald, where the Deister Sandstone (150 ft.) corresponds to the Hastings Sands and the Wälderthon (70–100 ft.) to the Weald Clay. The former contains valuable coal beds, worked in the neighbourhood of Obernkirchen, &c., and a good building stone.

The fossils of the Wealden beds comprise freshwater shellfish, Unio, Paladin, Melanopsis, Cyrena; and estuarine and marine forms such as Ostrea, Exogyra and Mytilus. An interesting series of dinosaurs and pterodactyies has been obtained from the Wealden of England and the continent of Europe, of which Iguanodon is the best known—a large number of almost entire skeletons of this genus were discovered in some buried Cretaceous valleys at Bernissart in Belgium; other forms are Heterosuchus, Ornithocheirus, Ornithopsis, Cimoliosaurus and Titanosaurus. Among the plant remains are Chara, Bennettites, Equisitites, Fittonia, Sagenopteris and Thujites. The fishes, plants and reptiles of these formations possess a decidedly Jurassic aspect, and for this reason several authorities are in favour of retaining the Wealden rocks in that system, and the close relationship between this formation and the underlying Purbeckian, both in England and in Germany, tends to support this view.

See Cretaceous, Neocomian, Purbeckian; also W. Topley, " Geology of the Weald," Mem. Geol. Survey (London, 1875).  (J. A. H.)