Greensand

Greensand, a name applied to certain sandy beds belonging to the Cretaceous system (q.v.), as developed in the British Isles from the presence in them of grains of glauconite (q.v.), though the beds are often clays or limestones, and even more often not noticeably green. The marine beds that immediately overlie the Weald Clay, and are equivalent to the Upper Neocomian of the Continent, are known as the "Lower Greensand." Among their most characteristic fossils are Perna Mulletii and Exogyra sinuata. They have been subdivided in the south-eastern counties into four series or pheises: - Folkestone beds, silver and iron-shot, sharp, current-bedded sands, 70 to 100 feet thick.

Sandgate beds, less pure sands, with fuller's-earth and chert bands 75 to 100 feet.

The beds, dull and earthy buff and yellow sands with layers of sandstone in Surrey, passing into the Kentish Rag limestone and "hassock" or earthy sandstone in Kent, 80 to 300 feet.

Atherfield Clay, 20 to 60 feet thick. The Folkestone beds are overlaid by the Gault with some apparent unconformability, and only about 20 per cent, of the Lower Greensand fossils pass upward into the Upper Cretaceous.

The Lower Greensand has its most conspicuous development in a range of hills running through Kent and Surrey, S. of and parallel to the North Downs, and reaching in Leith Hill a height of nearly 1,000 feet.

The name "Upper Greensand" is applied to the often compact sandstone above the Gault, the base of the Cenomanian of the Continent. In Surrey it forms a narrow baud, usually of a marly character, but occasionally a deep orange sand. It is well developed in East Devon, Somerset, Wilts, and N. Berks, forming two zones: the lower, or Blackdown beds, the zone of Ammonitesinfatus; and the upper, or Warminster beds, that of Pecten aspcr, Holaster nodulosus, etc. To the latter belongs the Firestone of Merstham and Godstone, Surrey.

To a still higher level belongs the so-called "Cambridge Greensand," a layer of glauconitic marl containing -erratics and phosphatic fossils derived from the Gault. It is probably on the same horizon as the "Chloritic Marl" and the Red Chalk of Hunstanton.