Excursion to the Isle of Thanet. Easter Monday, March 29th, 1875, and following day - Geologists' Association excursion: Difference between revisions

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[[File:GA Image 0464.jpg|thumb|Figure 30 General vertical section of the Margate Chalk.]]
[[File:GA Image 0464.jpg|thumb|Figure 30 General vertical section of the Margate Chalk.]]


'''Director: George Dowker, F.G.S. (Report by W. H. Hudleston, F.R.S.) (Proc. Vol. iv. p. 254.)'''


'''Note:''' [Besides the excursion to Pegwell Bay and Richborough on Easter Tuesday, 1885, the Isle of Thanet was visited by the association on Easter Monday and Tuesday (March 29th and 30th), 1875. Below is the report of this excursion. No record exists of a visit to the Isle of Thanet which took place on August 4th, 1866.]
'''Note:''' [Besides the excursion to Pegwell Bay and Richborough on Easter Tuesday, 1885, the Isle of Thanet was visited by the association on Easter Monday and Tuesday (March 29th and 30th), 1875. Below is the report of this excursion. No record exists of a visit to the Isle of Thanet which took place on August 4th, 1866.]
==Excursion to the Isle of Thanet. Easter Monday, March 29th, 1875, and following day==
'''Director: George Dowker, F.G.S. (Report by W. H. Hudleston, F.R.S.) (Proc. Vol. iv. p. 254.)'''


The party quitted the railway station, at Ramsgate, shortly after one p.m. on Easter Monday, and proceeded at once towards Pegwell Bay. Arrived on the shore, Mr. Dowker began by pointing out, in the first place, an irregular mass of gravel and brick-earth, generally believed to be of fluviatile origin. It rested, in this place, directly upon the Chalk, and was considered to he of the same age as certain high level gravels resting on the Lower Tertiaries near Herne Bay, in which palœolithic implements had been found. Some geologists had referred its deposition to the Mid-Glacial period, but there seemed to be much doubt as to this. ''Buccinum'' and ''Ostrea'' had recently been found in the deposit, but whether these got in from above or were really a part of the deposit there was no evidence to show.
The party quitted the railway station, at Ramsgate, shortly after one p.m. on Easter Monday, and proceeded at once towards Pegwell Bay. Arrived on the shore, Mr. Dowker began by pointing out, in the first place, an irregular mass of gravel and brick-earth, generally believed to be of fluviatile origin. It rested, in this place, directly upon the Chalk, and was considered to he of the same age as certain high level gravels resting on the Lower Tertiaries near Herne Bay, in which palœolithic implements had been found. Some geologists had referred its deposition to the Mid-Glacial period, but there seemed to be much doubt as to this. ''Buccinum'' and ''Ostrea'' had recently been found in the deposit, but whether these got in from above or were really a part of the deposit there was no evidence to show.

Revision as of 19:53, 6 February 2022

From: A record of excursions made between 1860 and 1890. Edited by Thomas Vincent Holmes , F.G.S. and C. Davies Sherborn, F.G.S. London: Edward Stanford [For the Geologists’ Association], 1891. Source: Cornell University copy on the Internet Archive (Public domain work)
Figure 30 General vertical section of the Margate Chalk.


Note: [Besides the excursion to Pegwell Bay and Richborough on Easter Tuesday, 1885, the Isle of Thanet was visited by the association on Easter Monday and Tuesday (March 29th and 30th), 1875. Below is the report of this excursion. No record exists of a visit to the Isle of Thanet which took place on August 4th, 1866.]

Excursion to the Isle of Thanet. Easter Monday, March 29th, 1875, and following day

Director: George Dowker, F.G.S. (Report by W. H. Hudleston, F.R.S.) (Proc. Vol. iv. p. 254.)

The party quitted the railway station, at Ramsgate, shortly after one p.m. on Easter Monday, and proceeded at once towards Pegwell Bay. Arrived on the shore, Mr. Dowker began by pointing out, in the first place, an irregular mass of gravel and brick-earth, generally believed to be of fluviatile origin. It rested, in this place, directly upon the Chalk, and was considered to he of the same age as certain high level gravels resting on the Lower Tertiaries near Herne Bay, in which palœolithic implements had been found. Some geologists had referred its deposition to the Mid-Glacial period, but there seemed to be much doubt as to this. Buccinum and Ostrea had recently been found in the deposit, but whether these got in from above or were really a part of the deposit there was no evidence to show.

The Isle of Thanet is a somewhat oval mass, of moderately high ground, separated from the mainland of Kent by a belt of marsh, which, though now banked in and drained, has within the historical period been a shallow strait into which the estuary of the river Stour discharged, and which communicated freely with the sea, on the north, at Reculver (Regulbium), and on the south-east near Richborough (Rutupiœ) The chief rock of the isle is Chalk, which rises in a sort of gentle anticlinal swelling, having an axis with a mean direction, E.N.E.—W.S.W. It falls away also to the westward, whilst on the east its section by the sea prevents us from forming an idea as to how far the axis might be prolonged in this direction. This Chalk of Pegwell Bay is the same as that of Margate, and is the highest in the county, perhaps as high as any in England, the general Jades of its fauna having much resemblance to that of Norwich. It is remarkable for the small amount of flint contained, and this circumstance induced geologists formerly to suppose that this Thanet Chalk was really Lower Chalk. It also contains Ammonites; indeed there seems to be some connection between flintless Chalk and Ammonites which may be more than accidental. It is to be noted that the Ammonites occur in casts. From the appearance of large Ammonites, Prestwich was led to conclude that the Kentish Chalk represented the Medial Chalk of England, but their appearance at Dover and Margate in widely separate beds will not warrant this conclusion. The few flints there are appear, for the most part, to be fossil sponges.

There is a slight fault in the cliff at Pegwell Bay, which brings up the Chalk against the Thanet Beds; considerable irregularity in the dip is noticeable, with a balance towards the S.W. There is, generally, however, a very marked southerly dip of t1 e Chalk on this side of the Isle of Thanet, in the direction of the Stour Marshes, which lie in a well marked synclinal filled with Tertiary beds. Pegwell Bay itself may be viewed as a seaward continuation of this synclinal hollow. The junction of the Chalk and Thanet Sands may be seen in the low cliffs of Pegwell Bay. The well-known green-coated flints, so constant an attendant upon this junction, not only with the Thanet Sands, but with other permeable beds of the Lower Tertiaries, came in for a large share of attentipn. " It is a peculiar feature of these junction sections of Chalk and Thanet Sands, as seen in East Kent, that the green-coated flints nearly always rest on a semi-tabular mass of Chalk flint." (Dowker, Geol. Mag., 1866, p. 210.) This peculiarity was well observed on this occasion by the party in Pegwell Bay. The green coating is said to be due to a deposit of hydrous silicate of iron as a sort of glaze.

The lower beds of Thanet Sand, at Cliff End, are very fossiliferous. Dentalium nitens, Natica subdepressa, Cyprina Morrisi, Cytherea orbicularis, Nucula Bowerbanki, Ostrea bellovacina, Panopea granulala, Pholadomya cuneata, Pholadoneya Konincki, Thracia oblata, are the most abundant and characteristic species.

There is one bed seen in the low cliffs about four inches thick, which is a mass of shells, chiefly Cyprina Morrisi. This species, with many others, also occurs throughout, the shells being sometimes very fine, and in good preservation.

At the "Sportsman," (Red Cliff End Point) the whole party proceeded in conveyances by a road made along a remarkable pebble beach which extends from the Isle of Thanet almost to Sandwich. At a break in this pebble beach, called Ebsfleet, the Ordnance Map has the following legend, " Here the Saxons first landed."

The party dismounted at the Salterns, and crossed, the Stour, a sluggish river some thirty yards wide, in which a feeble neap tide was slowly rising. A walk of less than a mile across the marshes, but still close to the river, brought them to the foot of the singular island on which the Roman castrum of Richborough (Rutupim) was constructed. This is an island of Thanet Beds, capped by Woolwich Beds. About the junction is a persistent line of Corbula and other shells in a silicified condition. The railway to Deal shaves off a portion of this hill, and whilst the line was being made specimens were easily obtained; not so at present.

Mr. Dowker remarked on the singular fact that, although the Norwich Chalk contained fossils very similar to that of Thanet, we have here, immediately reposing on the Chalk, the lowest of the Tertiaries known in England; whereas, in Norfolk, we have for the most part the highest and newest Tertiaries in a similar position. He again also took the opportunity of showing that the existence of Ammonites, made known to the members by the Paper of Mr. Bedwell, was- no longer to be deemed a proof that the Thanet Chalk occupied a lower zone. Speaking of the Tertiaries he observed that nowhere else in England could such low beds of Thanet Sands be seen, and these contained very numerous marine remains. The Woolwich Beds, of which a cap existed here.(pointing to the platform forming the summit of the island), were also marine, and less characteristically developed than in the neighbourhood of London, where, as was well known, they were markedly estuarine in character. The President (Mr. W., Carruthers) had formerly determined for the speaker a very interesting specimen of Pinites, found about the junction between the Thanet and Woolwich Beds. Alluding now to the remarkable deposits of gravel which occur in East Kent at considerable elevations, both upon the Chalk and upon the Tertiaries, and which contain worked flints, Mr. Dowker stated his belief that these were of high antiquity. Dr. J. Geikie considers them to occupy an Inter-Glacial period, and one anterior to the date in which the Boulder Clay was deposited; they occur in the southeastern portions of England, and he considers their abscence further north and west is to be accounted for by their having there been obliterated by the Glacial sea which deposited the-Boulder Clay, which latter is not found in the east of England south of the Thames. The palœolithic gravels of East Kent must have been, according to Dr. Geikie, elevated above the reach of this Glacial sea. Kent would thus present to us a very old land surface. From many considerations it is evident that since the deposition of these gravels the Thames must have deepened its bed by too feet.

The party then proceeded to Richborough Castle. A low rectangular wall, about fifteen feet in thickness, composed chiefly of flints, encloses a flat quadrangle about five acres in extent, now wholly given up to the plough. A sort of pit towards the middle of the enclosure conducts to an underground passage. The view from this isolated table must, on a clear day, be very instructive physiographically, as it commands the synclinal hollow of the Stour marshes, with the great curve of Thanet on the north, and the still bolder swell of the long range of Chalk heights, forming the backbone of East Kent, on the south.

On the following morning the low cliffs of the Margate Chalk occupied the attention of the party.. Much interest in these beds had been excited by the publication of Mr. Bedwell's paper on "Ammonite Zones in the Isle of Thanet " (Proc. Geol. Assoc., vol. iii. p. 217). It may be as well to note a few peculiarities in this Margate Chalk. For the general vertical section, see J. W. Wetherell "On Some Fossils from the Margate Chalk" (Proc. Geol. Assoc., vol. iii. p. 192—reproduced here, p. 62); and, generally, G. Dowker "On the Chalk of Thanet, &c." (Geol. Mag., 1870, p. 466).

It would appear that the Margate Chalk has a thickness of about thirty feet; its lower limit may be taken at the line of the three-inch flint band which terminates the great thickness of flinty Chalk. It is soft, and makes good whitening. In purity it is perhaps not equal to the Chalk with flints, as the silica is most probably disseminated instead of being collected in masses. Of mechanical sediment there is not very much. The fossils are to a great extent indicated, rather than actually preserved, by ferruginous stains, and to judge from the frequency of these markings, these are plentiful. Ventriculite structure is not uncommon; some nice pieces were picked up here and at Pegwell Bay, also Brachiolites, Coscinopora, and fragments of Bryozoa. Beleninitella is not unfrequent. The Ammonite Zone was found as indicated by Mr. Bedwell, and as far as may be judged from a very limited walk, the Ammonites lie in one stratigraphical horizon, though, as there is a slight curvature in the beds, the height of this line varies. Thus, the first Ammonite discovered could only be assailed by missiles from below. The next one found was at a more convenient level. These Ammonites are probably compressed externally, and existing only as casts, are not easy to study with a view to the determination of the species. If there is any truth in Ammonite Zones, that is to say, in particular species being confined to particular zones, it is rather surprising that names applied to Lower Chalk forms, such as A. Lewisiensis and A. peramplus, should also be quoted for Ammonites in such high beds as the Margate Chalk. It is true that in extreme old age indifferently preserved specimens of large Ammonites, occurring as casts in calcareous beds, have a strong family likeness.

The Ammonites leptophyllus of Sharpe, quoted as occurring in the Upper Chalk of Brighton, Ramsgate, Greenhithe, &c., is probably the nearest to, and may actually be the same as the Margate species. "This species reaches a diameter of more than 2½ feet; in shape it is discoidal, flattened at the sides, and rounded at the back." (Pal. Soc., " Chalk Cephalopoda," p. 48). This description answers roughly to the specimen seen in the cliff. The diameter of the 'outer whorl of this specimen must have been upwards of three feet; many fragments of the middle whorl were obtained, and also a portion of the little inner whorl with some of the shell on, a rare circumstance in the Chalk. This inner whorl, which is only in a fragmentary condition, has the back very round, and the sides much compressed, with faint indications of numerous costar of little elevation barely reaching the centre of the back. The figure given by Sharpe (Pal. Soc., " Chalk Cephalopoda," plate xxii.) might very well represent the middle age of such a specimen. Much interest naturally attaches to these Ammonites, which occupy the highest position of any members of that great genus, so characteristic of the Secondary rocks. On or about this horizon, in England at least, they seem to die out.

The party then took the train to Herne Bay, and hastily inspected the sections between that place and Reculver before returning to London. (See Herne Bay and Reculver excursions, p. 57.)

References

Maps

Ordnance Survey. Geological. Sheet 3. 8s. 6d.

New Ordnance Survey. Sheets 274, 290. 1s. each.

Books

See p. 60.

G. Dowker, On the Chalk of the Isle of Thanet, and its connection with the Chalk of East Kent, 8vo, London (Geol. Assoc.), 1870 [out of print].

J. Prestwich, On the Equivalent in England of the "Sables de Bracheux," and on the Southern Limits of the Thanet Sand, Rep. Brit. Assoc., 1882 (1883), p. 538.