Page:VCH Bedfordshire 1.djvu/42

 A HISTORY OF BEDFORDSHIRE abundant. Right across Bedfordshire, from Leighton to Potton, the Lower Greensand covers much of the upper portion of the Oxford Clay as well as the succeeding Ampthill and Kimeridge Clays, overlapping these formations, and more to the north the lower portion is overlaid by boulder clay. This makes its precise limits difficult to ascertain in some districts, but it is known to extend over the greater part of North Beds. There are many brickfields in it, and where boulder clay overlies it an admixture of the two clays is found to be advantageous in brick-making. Boulder clay over Oxford Clay makes a very tenacious soil, retentive of moisture and well suited to the growth of corn. The Ampthill Clay was formed under much the same conditions as the Oxford Clay below it, and the Kimeridge Clay above, and consti- tutes a passage-bed between them, having an admixture of their fossils. Being a comparatively deep-sea representative of the Corallian beds, which owe their origin to the destructive action of the sea on coral-reefs, it is frequently called Corallian, but the term is scarcely applicable. The two life-zones of the Corallian stone-beds are very distinct, but although both are probably present in the Ampthill Clay, they cannot be dis- tinguished in either Beds or Cambs owing to the Oxfordian and Kime- rigian fossils being mixed. The Ampthill resembles the Oxford Clay in not being a bed of clay only. In the cutting north of Ampthill railway station it consists of a thin septarian band with Ostrea discoidea in and beneath it ; 50 feet of marly shale and stiff clay, with selenite, a layer of calcareous nodules with Iscbyodus, and a rusty band with Ammonites cordatus and other fossils ; a thin band of pale earthy limestone ; 6 feet of grey and yellow marly shale ; and a rubbly rock-bed at the base, 4 ft. 6 in. thick, and containing numerous fossils ; the whole (given in de- scending order) resting upon Oxford Clay and being surmounted by Kimeridge Clay. 1 The outcrop of the Ampthill Clay is concealed for the most part by the overlapping of the Lower Greensand. In this section at Ampthill there appears above the Ampthill Clay a bed of clay and dark-blue shale, 1 o feet thick, with Ammonites biplex, Ostrea deltoidea, and other fossils, which is of Kimeridge Clay age, and has boulder clay resting unconformably upon it. The Kimeridge Clay must here have suffered very considerable denudation. Elsewhere it usually varies from 400 to 1,000 feet in thickness, but near Aylesbury it is not more than 100 feet thick. Here only its base is seen, the higher beds having been washed away, together probably with the succeeding Portland Beds, of which the only trace in the county consists of fossils in the Lower Greensand which have been derived from Portlandian strata. It was probably the presence of these fossils which led Professor A. C. Ramsay to say that outliers of the Portland Stone occur in Bedfordshire, rightly adding that ' the whole has evidently been exposed to denudation before the deposition of the Cretaceous rocks.' 3 1 Jurassic Rocks pf Britain, v. 13;. 2 Physical Geology and Geography of Great Britain, ed. 5, p. 191.