Page:VCH Bedfordshire 1.djvu/50

A HISTORY OF BEDFORDSHIRE sandy clay containing phosphatic nodules and numerous organic remains. It is regarded as the westerly extension of the Cambridge Greensand, the fossils which it yields being identical with those found in this bed in Cambridgeshire. The phosphatic nodules, with their associated fossils, have in all probability been derived from the denudation of the Upper Gault, which appears to have extended before this period con- siderably to the west of its present limit. The Chloritic Marl extends from Arlesey in the extreme east of the county, westerly by Shillington, Barton, and Sharpenhoe, and probably so far as Harlington, towards which place it is concealed by newer beds. During the period in which the coprolite-pits were being worked, numerous fossils were brought to light, especially in the neighbourhood of Barton and Sharpenhoe. That the assemblage of organisms was very remarkable may be seen from its inclusion of the reptilian genera Ich- thyosaurus, P/esiosaurus, and Pterodactylus, and the cephalopods Ammonites auritus, A. studeri, A. mante//i, and Belemnites minimus. Some of these were derived from the Gault during the extensive denudation to which it was subjected, and others are of the age of the bed in which they occur ; but when the attempt is made to distinguish between the two series, the difficulties which present themselves are well nigh insuperable. An attempt to do so has however been made by Mr. A. J. Jukes-Browne in a paper ' On the Relations of the Cambridge Gault and Greensand.' 1 The Chalk Marl is an impure limestone, dark in hue and some- what plastic in texture, which features are due to a small admixture of silt. Its mineral composition indicates that it was deposited in much deeper water than the Gault. It attains its maximum development over an area which extends due east from Berks through the counties of Herts and Beds, north and south of which it rapidly diminishes in thickness. This change is accompanied by differences in its mineral constituents, that to the north indicating an increase in the depth of the sea in which it was laid down, and that to the south-west a decrease in the depth. During the progress of the works connected with the extension of the Midland Railway, numerous fossils were found in this stratum, some of which are worthy of mention. Two specimens of a previously un- described crustacean were obtained (one from the dark grey portion of the bed and the other from the higher and lighter-coloured part of it) which were figured and described, with specimens from elsewhere, by Dr. Henry Woodward under the name of Palcega carteri. 2 The cephalopods include Ammonites varians in abundance, Actinocamax {Belemnites) lanceo- latus, a rare species, and Turrilites mantelli, previously recorded only from the south of England. Of reptilian remains a portion of a jaw of Ichthyosaurus campylodon with teeth in situ is the most noteworthy. The Chalk Marl, as well as some other divisions of the Chalk, contains numerous nodules of iron-pyrites, a mineral which is locally 1 Quart. Journ. Geo/. Soc. xxxi. 256-316. 3 Geo/. Mag. vii. 496, pi. xxii (1870). 16