Page:VCH Bedfordshire 1.djvu/45

 GEOLOGY sufficient depth to prove that it thus thinned out, his ' Geological Inquiry- respecting the Water-bearing Strata or the Country around London ' is a classic of geological literature replete with valuable information. Near Leighton the Lower Greensand consists for the most part of white and light-coloured sand known as ' silver sand,' which has an industrial value. 1 It is now chiefly obtained for filter-beds, but it is in places so free from iron and other colouring matter that it has been used for glass-making. The larger quartz grains show signs of attrition, being rounded and polished, like the sands of the sea-shore. At Leighton and near Silsoe a bed of dark-brown ferruginous sand- stone is exposed which is sufficiently indurated to be of service for building purposes, several churches in this part of the county being built of it, but it weathers rather rapidly and very unevenly. This ' carstone,' as it has been called, is a local feature which has been stated to be dependent on the presence beneath the sand of beds of clay which have arrested the percolation of ferruginous water. 3 Opposite the Castle Hill near Clophill there is a very instructive section showing 10 feet of dark-coloured clay, called by the workmen ' black clay ' to distinguish it from the ' blue clay ' of the Gault, occurring in three distinct beds of about equal thickness with thin layers of sand between them, dark-red carstone being above the clay and light-coloured sand beneath it ; but elsewhere the carstone is frequently seen resting on light-coloured sand with no trace of clay. The sand is, wherever exposed, seen to be partially or wholly false-bedded, showing that it was deposited in a shallow sea with shifting currents. The false-bedding is also evident in the carstone whether it occurs in a continuous layer or in isolated masses which are called ' doggers.' Concretions of brown iron-oxide are of frequent occurrence. At the base of the series near Woburn, and at a rather higher horizon near Potton, there is a peculiar bed of variable thickness (6 inches to 2 feet) consisting for the most part of pebbles with water-worn fossils derived from a distant source, and containing also numerous (so-called) ' coprolites.' The pebbles are of quartz, quartzite, limestone, ironstone, slate, etc., and they are sometimes cemented into a hard rock by car- bonate of lime. Sub-angular fragments of rock also occur. Most of the fossils are derived from older beds, the majority of Upper Jurassic and some of Lower Cretaceous age. With these may sometimes be found fossils proper to the formation, that is which lived in Vectian times. The indigenous fossils are chiefly brachiopods and lamellibranchs ; the derived fossils, teeth and bones of saurians and fishes. At Mill- brook have been found remains of the saurians Ichthyosaurus, P/esiosaurus, and Dakosaurus, and of the fishes Sphcerodus, Pycnodus, and Acrodus. Most of these also occur in the neighbourhood of Potton ; and in addition a previously undescribed brachiopod, Terebratula dallasii, and 1 Analyses of this sand from Heath near Leighton are given in E. W. Lewis's Lectures on the Geology of Leighton Buzzard, p. 6 1 (1872). a H. B. Woodward, Geology of England and Wales, ed. 2, p. 379 (1887). II