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.

  • Joseph Prestwich
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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    thick, while in the neighbourhood of Marquise it is reduced to about 230 feet or 240 feet4 On the English coast it forms the eliffs at Kimmeridge in the Isle of Purbeek, where it is not less than 550 feet thick. It then passes eastward, and is lost under the Cretaeeous roeks. There is reason to believe, however, that, as it exists in the Boulonnais and reappears from under the Cretaceous series in Bedfordshire and Buckinghamshire, it passes underneath the Wealden area.^ In the Boulonnais, the general dip of the Kimmeridge clay is southward under the Cretaceous series, and westward under the Channel, across half of which on the French side it probably ex- tends ; for midway between the two coasts is the Varne Shoal, which, from trials made by M. Thome de Gamond, is formed, there is little doubt, of the hard concretionary blocks of sandstone, cha- racteristic of the Portland series at Gris-nez and Mont Lambert. This shoal probably marks the boundary of the Kimmeridge clay, which there passes with these overl}dng Portland beds under the Weald clay, specimens of which latter are stated by M. Thome de Gamond to have been brought up by him between the Yarne and the English coast, in depths of 16 fathoms. The Kimmeridge clay in the Boulonnais consists essentially of massive clays, indurated marls, and thin-bedded argillaceous lime- stones, with one subordinate central bed of sands and sandstone 12 to 15 feet thick. As a whole this series is impermeable, but still the sandstone and limestone bands give rise to small springs. In the Pays de Bray, three unimportant water levels were found in it. At Sotteville, nearEouen, a strong brine spring was met with at a depth of 670 feet. It is possible, therefore, that if a tunnel were driven into the upper clays at Andrecelles, between Boulogne and Gris-nez, or else in the lower beds near Boulogne, it might, by following the dip seaward of the Kimmeridge clay, be carried in that formation half-way across the Channel; but the presence of the Portland beds in mid-channel, and of the Weald clay further on, must there give ' M. Thome de Gamond makes it 183 feet thick at Cape Gris-nez.—Op. cit p. 23, and Table, pi. III. 2 Tlie Kimmeridge clay (and some of the underlying secondary strata) may be more or less redirced under the Wealden area. For, as mentioned above, the superposition of the Wealden on those strata, shows, in the Boulonnais, a con- siderable amount of preliminary denudation ; so that, looking at the much greater volume and depth of the Wealden in the English area, it is possible that it may liave been deposited in a channel or basin denuded partly out of the Portland, Kimmeridge au<l Oolitic strata, and that a portion of these formations may he removed from under the Wealden area.
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