« PreviousContinue »
overlaid by the Chillesford Beds. It was these sections, first noticed by Mr. Prestwich, which led to the relations of the Norwich (Fluvio-marine) Crag and Red Crag being determined--the former being the equivalent of the higher portion of the latter. (See p. 289.)
From a careful study of the Crag district Mr. S.V.Wood, jun., concludes that the northern part of the Red Crag area continued to receive accumulations up to and during the time when the Fluvio-marine Crag was deposited; these beds were overlaid conformably by the Chillesford Beds, while in the southern part of the Red Crag area, as at Walton-on-theNaze, the Chillesford Beds overlapped and rested unconformably upon the lower portion of the Red Crag.
NORWICH CRAG SERIES.
LAMINATED SERIES. (Gunn.)
The seams of shelly sand or crag which occur in Norfolk are of a very impersistent character, but the researches of the Messrs. Wood, of Mr. F. W. Harmer, and of Mr. J. E. Taylor, have demonstrated that there are at least three horizons of crag which are termed the Fluvio-marine Crag or Norwich Crag proper, the Chillesford Crag, and the Bure Valley Crag; and that these are marked by some differences in the assemblage of mollusca.
The formation, to which the term Norwich Crag series' seems applicable, comprises a variable set of beds, of sands, laminated clays, and shingle, with in places seams of shells, which rest on the Chalk, attain a thickness of about 30 feet, and are overlaid by the Glacial deposits.
[In this section No. 5 is the representative of the Bure Valley Beds, No. 4 embraces the Chillesford Beds, and Nos. 2 and 3 include the Fluvio-marine Crag. The beds are very variable, and attain a total thickness of about 20 feet. The disturbance in the Chalk so conspicuous in 1868, is not very marked in the portions of Chalk now exposed (1876).]
5. Pebbly gravel and sand.
3. Sand and gravel, false-bedded. 4. Laminated clay and shelly seam. 2. Shell-bed and large flints.
i. Chalk (disturbed).
The Fluvio-marine Crag consists of buff-coloured shelly sands and shingle, often false-bedded, having a thickness of
| This is by no means a good name, as the Chillesford Beds are in places fluvio-marine, and the Bure Valley Beds contain some freshwater molluscs.
5 or 10 feet, and containing an admixture of marine and freshwater mollusca. It is probably of the same age as the upper part of the Red Crag, as both deposits in places are overlaid conformably by the Chillesford Beds.
Mr. J. E. Taylor first drew prominent attention to the fact that at Bramerton, where two layers of Crag are to be seen,
division contained shells of a more northern or arctic character than the lower one; and this Upper Crag (or “Taylor's Bed”) was identified by the Messrs. Wood with the shell-bed above the Red Crag at Chillesford.
At the base of the Fluvio-marine Crag is a bed termed by Mr. John Gunn the “Stone bed.' It rests on the Chalk the surface of which is occasionally bored by Pholas, and much furrowed and eroded. This bed yields numerous Mammalian remains, including Mastodon Arvernensis, Elephas meridionalis, Hippopotamus major, Rhinoceros leptorhinus, Cervus, Equus, Bos, Trogontherium Cuvieri, &c.
The Fluvio-marine Crag beds are well shown at Thorpe, Postwick, and Bramerton near Norwich, at Bulchamp, Wangford (10 feet), and Thorpe near Aldborough in Suffolk; they have also been exposed at Bungay, as Mr. C. Reid has determined.
Among the common fossils are Tellina obliqua, T. lata, T. prætenuis, Cardium edule, Mytilus edulis, Mactra (several species), Litorina litorea, Conovulus pyramidalis, Purpura lapillus, Trophon scalariforme, T. antiquum, Turritella communis, Cerithium punctatum, Paludina media, Hydrobia ventrosa, Balanus crenatus, Fish-remains, &c. Scrobicularia plana occurs at Bramerton.
Jet and Beekite are occasionally met with in the Fluviomarine Cray.
1 Mr. Charlesworth gave the name Mammaliferous Crag to the entire series of beds, but it is now kuown, through the researches of Mr. Guon, that the remains belong properly to the basement- or stone-bed, and that when found higher up in the series they show signs of being derived.
The Chillesford Beds (so named from Chillesford near Orford, in Suffolk) are subdivided by Mr. Wood, jun., into:
2. Chillesford Clay.
The Chillesford Sand consists of micaceous sand with a shelly bed (not constant); this passes upwards into a bed of laminated micaceous clay, containing a few shells—the Chillesford Clay. The entire thickness is from 20 to 25 feet.
The fossils include Astarte borealis, Tellina obliqua, T. lata, T. prætenuis, Cardium edule, Cyprina Islandica, Nucula Cobboldiæ, Mactra ovalis, Mya truncata (= Mya bed of the Rev. 0. Fisher), Purpura crispata, Litorina litorea.
Mr. Wood observes that the Chillesford Clay varies from a dark blue tenacious laminated clay, as at Aldeby and Easton Cliff, to a loamy micaceous sand, more or less interbedded with seams of laminated clay, as on the immediate west of Beccles, and on the south of Norwich, but it is easily recognizable everywhere.
In the pit at Bramerton, near Norwich, the following beds are seen resting upon the Chalk :
Between the Fluvio-marine Crag and Chillesford Beds at this locality is a layer of unfossiliferous sand about 12 feet in thickness, which Mr. Wood considers as equivalent to the Scrobicularia Crag of the Chillesford Section, which is as follows:
10 At Bramerton and Chillesford the Chillesford Beds, according to Mr. Wood, are marine; while at Easton Bavent, Burgh Kiln (near Aylsham), Coltishall, Horstead, Aldeby! (Toft Monks), the Chillesford Beds are fluvio-marine in character.
The Chillesford Beds are overlaid by the Bure Valley Beds, which form a connecting link between the Pliocene and Glacial series.
Bure Valley Beds.
These beds—the pebbly sands and pebble-beds—have been elucidated and described by Messrs. S. V. Wood, jun., and F. W. Harmer. The sands are mostly of a deep orange colour, and in them are seams and beds of rolled pebbles, which sometimes so predominate as to form masses of shingle: seams of laminated clay are not uncommon.
Near Halesworth and Henham they are described as forming true beaches, and at the latter place they attain a thickness of 25 feet.
The base of the Sands, when resting on the Chalk, is often occupied by an accumulation of shell-patches, known as Crag (Tellina-Balthica Crag): recognizable forms occur at Crostwick, Rackheath, Spixworth, Wroxham, Belaugh, Weybourn, Runton, and Trimmingham.
According to Messrs. Wood and Harmer they form the base of the whole Glacial series, and indicate the first setting
| The fossils of the Aldeby Beds have been most carefully worked out by Messrs. W. M. Crowfoot and E. T. Dowson.