GREENSAND, in geology, the name that has been applied to no fewer than three distinct members of the Cretaceous System, viz. the Upper Greensand (see GAULT), the Lower Greensand and the so-called Cambridge Greensand, a local phase of the base of the Chalk (q.v.). The term was introduced by the early English geologists for certain sandy rocks which frequently exhibited a greenish colour on account of the presence of minute grains of the green mineral glauconite. Until the fossils of these rocks came to be carefully studied there was much confusion between what is now known as the Upper Greensand (Selbornian) and the Lower Greensand. Here we shall confine our attention to the latter. The Lower Greensand was first examined in detail by W. H. Fitton (_Q.J.G.S._ iii., 1847), who, in 1845, had proposed the name "Vectine" for the formation. The name was revived under the form "Vectian" in 1885 by A. J. Jukes-Browne, because, although sands and sandstones prevail, the green colour has often changed by oxidation of the iron to various shades of red and brown, and other lithological types, clays and limestones represent this horizon in certain areas. The Lower Greensand is typically developed in the Wealden district, in the Isle of Wight, in Dorsetshire about Swanage, and it appears again beneath the northern outcrop of the Chalk in Berkshire, Oxfordshire and Bedfordshire, and thence it is traceable through Norfolk and Lincolnshire into east Yorkshire. It rests conformably upon the Wealden formation in the south of England, but it is clearly separable from the beds beneath by the occurrence of marine fossils, and by the fact that there is a marked overlap of the Lower Greensand on the Weald in Wiltshire, and derived pebbles are found in the basal beds. The whole series is 800 ft. thick at Atherfield in the Isle of Wight, but it thins rapidly westward. It is usually clearly marked off from the overlying Gault. In the Wealden area the Lower Greensand has been subdivided as follows, although the several members are not everywhere recognizable:-- Isle of Wight. Folkestone Beds (70-100 ft.) Carstone and Sand rock series. Sandgate Beds (75-100 ft.) Ferruginous Sands (Shanklin sands). Hythe Beds (80-300 ft.) Ferruginous Sands (Walpen sands). Atherfield Clay (20-90 ft.) Atherfield Clay.