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Published online by Cambridge University Press: 01 May 2009
During the past few years Mr. Charles Moore and Mr. J. McMurtrie have called attention to a remarkable inversion of strata, which was indicated by the working of coal under the Mountain Limestone near Vobster, on the north of the Mendip Hills. And this fact renders necessary an important modification in the geological sections of the district.
page 149 note 1 Moore, C., Geol. Mag. for 06, 1866, p. 331Google Scholar, and Quart. Journ. Geol. Soc., vol. xxiii., 1867, p. 453.Google Scholar In the latter publication, referring to his interesting discovery of a trap dyke, near Stoke Lane, on the Mendip Hills, Mr. Moore remarks that, by its protrusion, the Old Red Sandstone and Carboniferous rocks have been carried forward in a northerly direction, and are not only left standing vertically, but are in some instances folded over upon themselves. In illustration of this he mentions the working of coal beneath the Mountain Limestone of Luckington.
J. McMurtrie, “Faults and Contortions of the Somersetshire Coal-field,” a paper read before the Bath Nat. Hist, and Antiq. Field Club, 24th Feb., 1869.
Mr. S. W. Brice, in his “Essay on the Coal-field of North Somersetshire,” 1867, mentions that at Vobster branches have been driven right under the Mountain Limestone in search of coal. And he suggests that when the Mendip range was considerably higher than it is now, masses of the Limestone might easily have been dislocated from their parent rock, and have rolled down a species of inclined plane into the positions they now occupy.
page 150 note 1 Two of these, the one at Luckington and the other at Upper Vobster, were laid down on the Geological Survey Map, sheet 19. During a re-survey of the ground, Mr. W. A. E. Ussher has carefully traced out the boundaries of the three. They were very accurately laid down by Mr. W. Sanders, F.R.S., in his “Map of the Bristol Coal Fields, and country adjacent,” 1864.
page 150 note 2 A similar section was given by Buckland and Conybeare. Trans. Geol. Soc., series 2, vol. i.Google Scholar
page 150 note 3 Trans. Geol. Soc., series 2, vol. i., p. 255Google Scholar. See also Conybeare, and Phillips, , “Geology of England and Wales,” 1822, p. 428.Google Scholar
page 151 note 1 It has been remarked that the disturbances in the Somersetshire Coal-field furnish “our nearest approach in Great Britain to the abrupt foldings which are so remarkable in the Coal-field of Belgium.” (W. W. Smyth, etc.)
page 151 note 2 And Mr. McMurtrie kindly informed me in a letter dated March 8th, 1870, that so far as he was aware the Millstone Grit has never been detected between the Limestone and the Coal-measures at Luckington and Vobster.
page 151 note 3 This gradual passage is clearly shown in Vertical Sections (sheets 11 and 12) published by the Geological Survey of England. Indeed, it is well known that from the Old Red Sandstone to the Coal-measures in this area, there is no physical break whatever.
page 151 note 4 On this subject, see also the remarks of ProfessorRamsay, , Mem. Geol. Survey, vol. i., p. 312.Google Scholar
page 152 note 1 McMurtrie, —Lecture on the Carboniferous Strata of Somersetshire, 1868, p. 14.Google Scholar
page 152 note 2 Map of the Bristol Coal Fields, etc.
page 152 note 3 De la Beehe, , Mem. Geol. Survey, Vol. i., 1846, p. 413.Google Scholar
page 152 note 4 Vide Sanders' Map. The only localitywhere anything like an inversion has been observed, is by the road-side at Churchill Batch, at the western edge of the Mendip range. Proceeding along the high road in a northerly direction, the Mountain Limestone is seen dipping at a high angleto the north, then it becomes vertical, and a little further on the beds dip to the south. The whole is seen clearly in one section, and it forms an excellent example of “fan-shaped” structure. This was originally noticed by Buckland and Conybeare, who regarded it as a sharp synclinal. Mr. McMurtrie admits that the Mountain Limestone has not as a rule been folded back, regarding the Vobster patches as exceptions.
page 152 note 5 I find that the same idea is expressed by Conybeare, and Phillips, , op. cit. pp. 347 and 360.Google Scholar
page 154 note 1 This is the kind of structure so characteristic of the Alps—as noticed by Studer, Desor, Lory, and others.
page 154 note 2 Greenwell, G. C., “Notes on the Coal-field of East Somerset.” Trans. North of England Inst. of Mining Engineers, vol. ii., 1854, p. 255.Google Scholar
Greenwell, and McMurtrie, , “On the Radstock portion of the Somersetshire Coal-field.” 1864. p. 17.Google Scholar
McMurtrie, “Faults and Contortions,” etc.
The faults in the Somersetshire Coal-field are worthy of much study from their variety and remarkable characters. Mr. McMurtrie mentions an instance of beds thinning out towards a fault, a fact that may sometimes be noticed in the Lias where it is similarly affected.
