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
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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![with flints, and 284 feet of Grey Chalk. Add to this, the 64 feet extra of Lower Chalk existing at Calais, and it will there give to the base of the Chalk a depth of about 615 feet beneath the sea- level, or 410 feet higher than at Calais; on the English coast, another boring Sir John Hawltshaw had made at St. Margaret’s Bay—about 4 miles eastward of Dover—after traversing 240 feet of White Chalk and 296 feet of Grey Chalk, reached the Gault at a depth of 554 feet (Appendix, p. 32). IVith a persistent trend of so wide a range, and with a strike subject to so little variation, there is reason to believe that the main axis of the Palaeozoic rocks, after their exhibition in the Boulonnais, passes under tlie Channel, and that the crest of the same ridge would probably be found underground somewhere near Folkestone. The elements of greater uncertainty are—1st. The depth at which the older rocks lie; 2nd. The line along which theWealden and Lower Cretaceous strata thin out against the older rocks. The Lower Greensand, and the Wealden strata have at Folke- stone and Hythe dimensions far in excess of the same beds at Wissant. Still there is some evidence of their thinning out; for, on the western side of the Weald, the Lower Greensand has a thickness of as much as 500 feet, which is diminished to 260 feet at Folkestone. Of the thickness of the W’^ealden series in the Hythe district there is no positive information. We have assigned to it a thickness of 600 to 800 feet; but there are no borings or wells to give exact measure, and the only guide in this case has been the dip and range of the beds between that town and Appledore—a fallacious guide in a case like this, where the beds are very obscure and where the lower divisions may be affected without the upper or surface beds showing it. Any estimate, therefore, of the dimensions of the Wealden and Jurassic series under Folkestone must be conjectural. Looking at the tolerably certain range of the Palaeozoic rocks westward from the Boiilonnais, and the partial thinning out of the Lower Green- sand at Folkestone, the Author would be disposed to expect a some- what like thinning out of the underlying Wealden and Jurassic formations in the English as in the French area ; and that further northward, at Dover or St. Margaret’s, the Gault may overlie almost directly the Palaeozoic rocks. ^ If such should be the case, ^ Fragments of mica slate have been found in the Lower Chalk near Folkestone. “ Mem. Geol. Survey,” vol. iv., p. 33. These all tend to show the proximity of an old land surface of Palaeozoic rocks. In the Lower Greensand at Maidstone, pebbles (some of large size) of Palaeozoic rocks are occasionally found, and in one ])art of the Wealden series they are not uncommon.](https://iiif.wellcomecollection.org/image/b2244614x_0019.jp2/full/800%2C/0/default.jpg)
