Greensand, the name given to two divisions of the Cretaceous System (q.v.). They are so called from the occurrence in some of the strata of numerous small green specks of glauconite (a hydrous silicate of iron, alumina, and potash; see GREEN EARTH), sometimes so abundant as to give a green colour to them. The term is, however, far from being descriptive of the various included strata; it must be considered simply as a name. In some districts, especially on the Continent, the green particles are entirely absent from the strata. The petrographical character of the Upper Greensand is so like that of the Lower, that it is scarcely possible to separate them when the intermediate Gault is absent, except by their organic remains, which are very distinct; so much so, indeed, as to have caused the placing of the one series in the Lower Cretaceous group, and the other in the Upper.
The Upper Greensand consists of beds of sand and sandstone, generally of a green colour, with beds and concretionary masses of calcareous grit, called firestone, and chert. In the Wealden district the average thickness of the formation is about 60 feet. It is only doubtfully present north of Folkestone; in Sussex it reaches 20 feet, and in the Isle of Wight 100 feet in thickness. This formation is supposed to have been a littoral or shore deposit of the cretaceous sea. While the chalk was being deposited out at sea these sands were being laid down along the shore contemporaneously with the chalk, although they appear inferior to it. Their position would necessarily result from the cretaceous sea widening its area; as the shore was submerged the greensand became covered with the chalk, and thus appears as an older and underlying deposit. The beds of this series are rich in fossils, abounding especially in the remains of sponges, mollusca, and echinodermata.
The Lower Greensand consists chiefly of yellow, gray, white, and green sands, but includes also beds and bands of clay, limestone, and ironstone. It attains a thickness of 500 or so feet. The sands preponderate in the upper, and the clays in the lower portion of the formation. In Surrey, Kent, Sussex, &c. it is subdivided as follows:
- 4. Folkestone beds.
- 3. Sandgate beds.
- 2. Hythe beds.
- 1. Atherfield clay.
Some beds of clay of considerable thickness, occasionally as much as 60 feet, are used as fuller's earth. The calcareous stone is a highly fossiliferous band of limestone, locally called Kentish rag, much used for building in Kent and Sussex. The formation was formerly known as the iron-sand, because of the sands being cemented together by an abundance of oxide of iron; this gives them a reddish colour. The Lower Greensand contains numerous fossil mollusca and other remains. It is a marine-deposit, and rests on the fresh-water Wealden strata, showing that while it was being accumulated the sea made considerable encroachments on the land. In the Isle of Wight the strata are well developed, reaching a thickness of some 800 feet. In the Midland counties the same beds are recognised and have assumed various names, such as 'Faringdon beds,' 'Shotover sands,' 'Woburn sands and Wicken beds.' The Tealby series is the name given to the Lower Greensand beds of Lincolnshire. Near Flamborough Head the Lower Greensand and Wealden beds are represented by the Speeton clay.