Gault

Gault, in geology, one of the members of the Lower Cretaceous System. The name is still employed provincially in parts of England for a stiff blue clay of any kind; by the earlier writers it was sometimes spelt “Galt” or “Golt.”

The formation now known as Gault in England has been variously designated “Blue Marle,” “Brick Earth,” “Golt Brick Earth” and “Oak-tree-soil.” In certain parts of the south of England the Gault appears as a well-marked deposit of clay, lying between two sandy formations; the one above came to be known as the “Upper Greensand,” the one below being the “Lower Greensand” (see Greensand). Since the typical clayey Gault is continually taking on a sandy facies as it is traced both horizontally and vertically; and since the fossils of the Upper Greensand and Gault are inseparably related, it has been proposed by A.J. Jukes-Browne that these two series of beds should be regarded as the arenaceous and argillaceous phases of a single formation, to which he has given the name “Selbornian” (from the village of Selborne where the beds are well developed). Lithologically, then, the Selbornian includes the blue and grey clays and marls of the Gault proper; the glauconitic sands of the Upper Greensand, and their local equivalent, the “malm,” “malm rock” or “firestone,” which in places passes into the micaceous sandstone containing sponge spicules and globules of silica, the counterpart of the rock called “gaize” on the same horizon in northern France. In Yorkshire, Lincolnshire and parts of Norfolk the Selbornian is represented by the Red Chalk. The malm is a ferruginous siliceous rock, the silica being mainly in the colloidal condition in the form of globules and sponge spicules; some quartz grains, mica and glauconite are usually present along with from 2 to 25% of calcareous matter. Chert-bands and nodules are common in the Upper Greensand of certain districts; and calcareous concretions, locally recognized as cowstones (Lyme Regis), doggers or buhrstones, are not infrequent.

The principal divisions of the Selbornian stage with their characteristic zonal fossils are as follows:—

Warminster Beds | Pecten asper and Cardiaster fossarius.  
---|---  
Upper Gault | Devizes Beds or Merstham Beds with Schloenbachia rostralus.  
Lower Gault | Hoplites lautus.  
H. interruptus.  
Acanthoceras mammillatum.  
  
The Gault (with Upper Greensand) crops out all round the Wealden area; it extends beneath the London basin and reappears from beneath the northern scarp of the Chalk along the foot of the Chiltern Hills to near Tring. In the south of England the Gault clay is fairly constant in the lower part, with the Greensand above; the clay, however, passes into sand as it is followed westward and, as already pointed out, the clay and sand appear to pass into a red chalk towards the north-east. The Gault overlaps the Lower Greensand towards the east, where it rests upon the old Paleozoic axis; it also overlaps the same formation towards the west about Frome, and thence passes unconformably across the Portlandian beds, Kimeridge Clay, Corallian beds and Oxford Clay; in south Dorsetshire it rests upon the Wealden Series. The Gault (with Upper Greensand) passes on to the Jurassic and Rhaetic rocks near Axmouth, and oversteps farther westward, in the Haldon Hills, on to the Permian. A large outlier occurs on the Blackdown Hills of Devonshire. Good localities for fossils are Folkestone—where many of the shells are preserved with their original pearly nacre,—Burnham, Merstham, Isle of Wight, the Blackdown and Haldon Hills, Warminster, Hunstanton and Speeton, Black Venn near Lyme Regis, and Devizes (malmstone and gaize). The beds are well developed in the vale of Wardour, and in the Isle of Wight; the Gault forms the so-called “blue slipper” at Ventnor which has been the cause of the landslip or undercliff.

The Gault of north France is very similar to that in the south of England, but the French term Albien includes only a portion of the Selbornian formation. The Gault of north-west Germany embraces beds that would be classed as Albien and Aptien by French authors; it comprises the “Flammenmergel”—a pale siliceous marl shot with flame-shaped darker patches—a clay with Belemnites minimus, and the “Gargasmergel” (Aptian). In the Diester and Teutoberger Wald, and in the region of Halberstadt, the clays and marls are replaced by sandstones, the so-called Gault-Quader. Continental writers usually place the Gault or Albian at the summit of the Lower Cretaceous; while with English geologists the practice is to commence the Upper Cretaceous with this formation. In addition to the fossils already noticed, the following may be mentioned: Acanthoceras Desmoceras Beaudanti, Hoplites splendens, Hamites, Scaphites, Turrilites, Aporrhais retusa, Trigonia aliforme, also Ichthyosaurus and Ornithocheirus (Pterodactyl). From the clays, bricks and tiles are made at Burham, Barnwell, Dunton Green, Arlesey, Hitchin, &c. The cherts in the Greensand portion are used for road metal, and in the Blackdown Hills, for scythe stones; hearthstone is obtained about Merstham; phosphatic nodules occur at several horizons.

See Cretaceous System; Albian; Aptian; also A.J. Jukes-Browne, “The Gault and Upper Greensand of England.” vol. i., Cretaceous Rocks of Britain; Mem. Geol. Survey, 1900.

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