On the Structure of the Crag-beds of Norfolk and Suffolk, with some Observations on their Organic Remains.—Part II. The Red Crag of Suffolk
Authors: Prestwich Joseph 
Published: October 2009

Abstract

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superposition of the Red Crag to the Coralline having been clearly shown by previous writers, the author confines his paper to those questions on which differences of opinion still exist, namely, the structure of the Red Crag, its affinities with the Coralline, and its exact relation to the Mammaliferous Crag of Norfolk. The Red Crag of Suffolk is described as occupying an excavated area in the Coralline, wrapping round the isolated reefs of the latter, filling up the hollows between them, and occupying a similar, and sometimes a rather lower level than the summits of these older reefs. It forms such an extremely variable series of beds, that the author has been unable to observe any definite order of succession in the greater part of it; but he remarks that oblique lamination is most strongly developed in the lower and central portions, and that almost everywhere there occurs at the base a bed of phosphatic nodules, although deposits of that nature are by no means confined to one level. Old sea-cliffs of Coralline Crag, and remains of old sea-beaches at their base, are described by Mr. Prestwich as occurring at Sutton; and he also gives detailed descriptions of the numerous pits in the Red Crag of Suffolk where the phenomena which he describes may be observed. Dividing the Red Crag into an upper, frequently unfossiliferous member (the fossils of which, being most frequently in the position in which they lived, may be regarded as truly representing the fauna of the period), and a lower, fossiliferous portion (in which the shells are found mostly in a broken and comminuted state, and mixed largely with fossils derived from the older Coralline Crag), the author describes their distribution in Suffolk, and their mode of occurrence on the eroded Coralline Crag, referring more especially to the difficulty in drawing the line between them in many cases.

In treating of the organic remains of the Red Crag, Mr. Prestwich gives lists of the shells found at the different localities, which had been prepared with the aid of Mr. Gwyn Jeffreys. Taking the local conditions into consideration, eliminating the extraneous fossils of the Red Crag of Sutton, Butley, &c., and excluding the freshwater fossils of the more northern districts, the author regards the remaining fossils of the two divisions of the Red Crag as being so closely related that the whole group must palæontologically be treated as one. Mr. Searles Wood has given the total number of species of its Mollusca as 239; to these Mr. Gwyn Jeffreys has added six additional species; on the other hand, he regards ninety-nine of them as varieties and extraneous fossils, leaving 146 species belonging to the Red Crag. Of these Mr. Jeffreys has identified 133, or 92 per cent., with living species, 115 still being inhabitants of British seas, 15 being found in more northern seas, and 3 in more southern.
From the Mammaliferous Crag of Norfolk and the Red Crag of Suffolk never having been found in superposition, from the circumstance that just at the point where the latter ceases the former begins, as well as from the community of so many species of organic remains, the author regards the two deposits as equivalent; and he attributes their distinctive characters partly to the extraneous fossils in the Red Crag, and partly to the difference in the conditions which prevailed in the two areas at that time, and especially to the more littoral and brackish-water conditions which prevailed in the Norfolk area. In conclusion, Mr. Prestwich gives a sketch of the physical history of the Red-Crag period, describing the mode in which the various phenomena which he notices have been produced.