Page:VCH Norfolk 1.djvu/38

 A HISTORY OF NORFOLK acterized by Belemnitella {Actinocamax) plena. This division has been quarried at Stoke Ferry and Marham, at Hillington, Dersingham, Heacham and Hunstanton. At Marham the hard chalk has been used for building-purposes, and elsewhere for lime and whiting. Among other fossils in the Lower Chalk are remains of fishes also Ammonites mantelli, A. rotomagensis, Inoceramus latus, Ostrea vesicularis, Rhynchonella mantelliana, 'Terebratula biplicata, T. globosa and Discoidea cylindrka. In the beds at Gayton Mr. Whitaker noticed small boulders of granitic and other igneous rocks, a fact of considerable interest as such erratic materials in the Chalk are by no means of frequent occurrence, and they suggest transport by icebergs. The boundary between the Lower and Middle Chalk extends from Hockwold-cum- Wilton to Narford and Gaytonthorpe. The Middle Chalk comprises hard bedded white chalk with a few seams of marl and many layers of flint, including tabular flints and occasional Paramoudras (see p. 9). Traces of a hard band representing the Melbourn Rock of Cambridgeshire have been noticed at its base at Shernborne and to the south-east of Heacham. The Middle Chalk has been well exposed at Methwold, and again at Sedgeford, Hillington, Great Massingham, Harpley and Docking. Among the fossils of this division are Ammonites peramplus (which occasionally measures 2 feet in diameter), Inoceramus cuvieri, I. mytiloides, Rhynchonella cuvieri, R. plicatilis and Echinoconus subrotundus. It has been found difficult in Norfolk to draw a definite line of boundary between the outcrop of Middle and Upper Chalk owing to the want of evidence of the Chalk Rock, which in many localities further south and south-west forms a marked horizon. This rock has been observed in Norfolk only in one locality west of SwafFham, where it is but a foot thick. The Upper Chalk is for the most part a soft chalk, characterized by numerous nodular flints and by huge forms known as Paramoudras. These flints were in all cases formed subsequently to the deposition of the chalky mud ; some originated during its consolidation, others like the occasional tabular flints fill horizontal, oblique and vertical planes of shrinkage and faulting. The siliceous matter of the nodules is of organic origin due to the presence and decay of siliceous organisms, and especially sponges. Not only sponges, but also mollusca, echinoderms, and occasionally reptilian teeth, acted as nuclei around which the flinty matter was concentrated. The nodules, like septaria in clay formations, usually coincide with the planes of stratification, because the superficial layers of chalky mud became saturated with silica due to the subsidence and decay of siliceous organisms. Hence the nodular layers were probably formed stage by stage during the gradual accumulation and consolidation of the Chalk. Dr. G. C. Wallich suggested that the Paramoudras, which sometimes occur in vertical rows, were due to sluggish flows of highly siliceous layers into hollows of the sea-bed. Other observers have suggested that 8