On the geological conditions affecting the construction of a tunnel between England and France / by Joseph Prestwich ; with an abstract of the discussion upon the paper ; edited by James Forrest.

  • Prestwich, Joseph, 1812-1896.
Date:
1874
Catalogue details

Licence: Public Domain Mark

Credit: On the geological conditions affecting the construction of a tunnel between England and France / by Joseph Prestwich ; with an abstract of the discussion upon the paper ; edited by James Forrest. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    It would be instructive to determine the levels of these various springs. This very commonly impermeable character of the Chalk Marl has given rise to the hope that it might prove compact enough for a submarine tunnel under the Channel between Cape Blanc- Nez and the South Foreland; but when it is considered that such a work would have to face the risks arising from the lateral passage of the inland springs,^ and from the chance fissures, so common to calcareous rocks communicating with the sea, it is feared that the difficulties would prove to be of a very formidable nature. It is to be observed that in the Channel the chalk is frequently bare, besides being unprotected by any overlying strata. The Upper Greensand is here too thin (1 to 3 feet) to require notice. It is a chalk marl, mixed with green grains. The Gault.—This formation consists of a bluish-grey clay, very tenacious and impermeable. As it is, however, only 130 feet thick on the English coast, and as this is reduced to about 40 feet on the French coast, the attempt to carry a tunnel through it under the Channel would hardly be feasible. The Lower Greensands are composed, 1st, of the Folkestone beds, which consist of extremely permeable sands; 2nd and 3rd, of the Sandgate and Hythe beds of mixed grey clays, sands, and ragstone; 4th, of the Atherfield clay. The latter alone is imper- meable ; but although these beds form on the English coast a mass of strata together, 260 feet in thickness,^ they thin off so much to the eastward, that on the French coast they are reduced to 30 or 40 feet, exclusive of the Atherfield clay,^ which is, however, only 30 feet thick at Hythe. The Wealden Strata, largely developed in England, where they attain a thickness of not less than 1,200 feet, are reduced to a thick- ness not exceeding 40 feet or 50 feet in the Boulogne district,^ and. ^ Although it might be possible to overcome many of the inland springs, there would be the danger that, owing to the ramifications of the fissures through which they pass, some may communicate with the sea-bed, so that draining of these lesser waters would only leave channels for the more dangerous influx of the sea waters through the very fissures by which the inland waters now find vent as submarine springs. - Dr. Fitton, “ Trans. Geol. See.,” 2nd Series, vol. iv., p. 319, and Drew, “ Mem. Geol. Survey,” Sheet 4. * There are specimens in the Boulogne Museum of the Ostrea Leymerii, from Wissaut, indicating probably the presence there of the Atherfield clay, which is, however, only occasionally exposed on the shore. '* For a description of these and the underlying rocks of the Boulonnais, see M. Rozet’s“ Description Gcoguostique duBassiu du Bas Boulounais,” Paris, 1828 ;