Cement Stone at Nacton and Septaria Generally

CEMENT STONE AT NACTON AND SEPTARIA GENERALLY. It may be worth while to record the existence in the London Clay of the Orwell N.E. bank of what amounts to a regulär bed of cement stone, a kind of septarian mass. Guided by Mr. E. C. Green I recently explored the shore on the south of Orwell Park, Nacton. There we found this rock in more or less tabular form, not perhaps quite in situ, for it had slumped down somewhat towards the river, but not greatly disturbed.

Septarian nodules proper are concretions formed, long after deposition, in a clay or marl by the percolation of water carrying calcium carbonate in Solution and sometimes iron. By a process not yet thoroughly understood this C a C0 3 , having been picked up by the water during part of its infiltration through the rock or some overlying bed, is redeposited between the particles of clay, generally at certain definite levels, the concretion starting at various nodes or foci, often at a fossil, and gradually extending outwards, tili it forms a rounded mass of clay-limestone (cement stone) perhaps the size of a football or larger, or clay-ironstone. They generally have either a radiating or concentric structure, or both.

Any lines of bedding in the rock are commonly preserved, running through the concretion, a proof that the latter did not exist as an indurated mass prior to deposition like the stones in a conglomerate or fossil beach. A further frequent development is that the septarium, on shrinking, cracks in a peculiar way from the centre outwards, producing radiating fissures diminishing outwards which may or may not reach the surface and which become filled with calcite or other mineral so that the whole may be divided into more or less regulär sections by the mineral veins, often assuming an appearance not unlike a tortoise shell.

Septaria occur quite often in the London Clay and were much used by the Romans in building and sometimes by the medieval masons who probably filched them from Roman ruins : one may see them in churches.

The special interest of this Nacton variety lies in the fact that it does not consist of a mere line of nodules but of a more or less continuous tabular bed, a dark grey and iron-stained rock some two feet thick. It was doubtless formed in a somewhat similar way. Its fissures, though not regulär, may be two or three inches across, filled with the fibrous calcite known commonly as " beef " from its resemblance to a rather stringy steak.

Another such tabular concretionary layer occurs at Stoke-byNayland where recently a contractor, sinking a pit for a sewerage plant, had to resort to blasting. But that was a sarsen sandstone,

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considerably harder than any calcareous cement stone yet formed in a similar way in Reading Beds. There cannot be many such examples of sheets of rock in the populär sense of the word at or near surface in Suffolk—excluding erratics.

Miss Willis has kindly pointed out to me a short reference to septaria in our Proceedings for 1938, Vol. I, Pt. 1, xxx.

F.

H. A.

ENGLEHEART.

NOTES ON CRAG PALAEONTOLOGY—1

By P.

G.

CAMBRIDGE

SYNOPSIS : The genera Ostrea, Cyprina and Teilina as understood by Searles V. Wood are reviewed and a species new to the Crag, Glibertia prosperi van Meulen, is recorded.

It is now over one hundred years since Searles V. Wood commenced his famous work on the Crag Mollusca and although considerable work has been done on the recent mollusca, involving many alterations to the older and better known names, no comprehensive work on the Plio-Pleistocene shells has been published in this country for many years. The author hopes to be able to review the Crag fauna in a series of articles of which this forms the first.

The term Crag is used loosely in these notes for convenience, in place of the more accurate but clumsy term Plio-Pleistocene, to indicate the related Coralline (Gedgravian) and Red (Ipsvician) Crag. The Norwich Crag (Icenian) is recorded separately.

As a matter of convenience the boundary between the Pliocene and the Pleistocene is placed by most modern writers at the base of the Waltonian. How long a period of time elapsed between the deposition of the Coralline Crag and the Waltonian Red Crag is not known, but certain Continental horizons are missing from the succession in this country. The occasional appearance of worn examples of Angulus benedeni (Nyst), Surculites intorta (Brocchi) etc., in the area of disturbed Crag between Felixstowe and Ipswich would seem to point to the destruction of some Crag beds and the incorporation of the remnants in the Newbournian Crag.

T h e writer has been greatly assisted by Dr. L. R. Cox and Mr. Castell of the British Museum, L. v. d. Slik of Rotterdam, Dr. M. Glibert of Brüssels, and others who have helped with