Page:VCH Bedfordshire 1.djvu/38

 A HISTORY OF BEDFORDSHIRE The geological formations represented in Bedfordshire, with their chief lithological characters and approximate thickness, are given in the table on the preceding page, in descending order, the names of the for- mations which do not come to the surface in the county being printed in italics. In the following account of the Jurassic and Cretaceous rocks, at the head of each division are given the geographical names adopted for the formations or their subdivisions by continental and by some English geologists, the lithological names in general use in this country, and the life-zones represented in the county. LOWER JURASSIC Toarcian. . Upper Lias. . Zone of Ammonites communis The oldest formation of the occurrence of which within the county we have actual evidence is the highest division of the Lower Jurassic series, the Upper Lias, and that does not come to the surface, having only been met with in well-borings at Sharnbrook and Felmersham, where it appears as a blue clay. The greatest thickness passed into being 66 feet, it is not likely that the lower zone of the Upper Lias, that of Ammonites serpentinus, has been reached. The strata rise towards the north-west, and both zones crop out in Northamptonshire. The mud of which the Liassic clays are composed was probably derived from the erosion of Carboniferous shales, the sediment having been deposited in the sea whilst its bed was sinking, a process which had been going on throughout Permian and Triassic times. MIDDLE JURASSIC {Cornbrash .... Zone of Ammonites discus Great Oolite ... „ A. gracilis Upper Estuarine Series „ Modiola imbricata Bajocian Inferior Oolite ... „ Ammonites opalinus The Inferior Oolite is represented only by the Northampton Sand, a passage-bed between the Lias and the Oolites, and by some geologists considered to belong to the older formation. It has been sunk into at Wymington, yielding a supply of water. It comes to the surface in the adjoining county of Northampton, where it consists of sandy ironstones with a thin bed of coarse oolitic limestone on the top. It represents the Midford Sand of Dorset and Somerset and the lower part of the Inferior Oolite of Cheltenham, and indicates a change of conditions to a more shallow sea than that of the Liassic period. The Upper Estuarine Series is the oldest formation which comes to the surface in the county. At Bedford it is 70 feet beneath the surface, and its outcrop, which is not a continuous one, is some miles to the north-west. It consists of sand and sandy clay of various colours, with irregular layers of limestone here and there, and is of fluvio-marine origin, showing alternations of marine and freshwater conditions, and 4