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III.—Railway Geology, No.I.—From Exeter to Newton-Bushell and Moretonhampstead

Published online by Cambridge University Press:  01 May 2009

Extract

The district selected for this article embraces a very unusual variety of geological phenomena, consisting of different kinds of recent gravels and Tertiary deposits, Greensand table-lands and patches, Triassic sandstone and conglomerate, rocks belonging to the interval between the Trias and uppermost Silurian strata, trap, and granite.

Type
Original Articles
Copyright
Copyright © Cambridge University Press 1867

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References

page 391 note 1 Sir H. de la Beche long ago noticed the absence of flints from large areas lying between flint-strewn surfaces as a difficulty in the way of the atmospheric theory.

page 392 note 1 Sir H. de la Beche and Mr. Godwin-Austen both believed that the excavation of the valleys commenced later than the distribution of the gravel on the ancient Black-down and Haldon area.

page 391 note 1 The most recent writers on this formation are Mr. Pengelly, Mr. Key, and Dr. Heer. The following is a very condensed statement of some of the principal facts contained in Mr. Pengelly's paper read before the Royal Society, November, 1861. The Bovey basin, exclusive of the part south of Newton, is four miles in greatest breadth, and six in length. In the “Coal-pit” (about half-a-mile from Bovey) the beds dip at 12½° towards S.W., the strike being N.W. and S.E. In Mr. Pengelly's principal section there were 72 distinct beds of lignite, clay, and sand, including the “head” or an unconformable covering, the thickness of which was 7½ft. The upper series of beds (order descending) consisted of clay, sand (one bed of sand 6ft. 3in. thick and thinning out eastwards or along strike); many beds of clay and lignite, one of the beds of clay containing lenticular patches of sand, most of them containing fragments of lignite, and one bed of lignite 6ft. 2in. thick. Between the upper and lower series there was a bed of sand about 11 ft. thick, coarse in upper part, finer towards base, containing patches of clay, and thinning out eastwards. Then came the lower series of beds containing no sand, but consisting of many alternating beds of clay and lignite, the former containing fragments of lignite. Towards the base the beds of lignite were very close to each other, and the lowest 4ft. thick. Some of the lignite beds consisted of, or contained, “ board coal.” Out of 27 lignite or coal beds only seven were more than lft. 8in. thick. In some placea rings of annual growth of trees were seen pressed into ellipses. The beds are known to be 300 feet deep. A short distance E. of the pit, there is a fault running N.E. and S.W., which proves a vertical displacement of the beds amounting to at least 100 feet. The beds N.W. of this fault must have extended 100 feet higher than at present, making the thickness of the deposit previously to denudation at least 400 feet. The covering called “head,” resting on the denuded edges of the beds beneath, consists of sand and clay, with large and small stones of granite, metamorphic rock, carbonaceous grit, trap, and flint and chert, the latter increasing in number eastwards. No stones have been found under the “head.”The Bovey beds must have been accumulated in a lake; the clay and sand must have come from Dartmoor; and the stones in the surface-covering must have been brought by a current from the north.—In a paper by Mr. Key, of Newton, read before the Geological Society in November, 1861, it is stated that the “Bovey deposit” rises from under high tide level near Newton to 151 feet above mean tide on Knighton Heath. He mentions three parallel beds of clay on the E. side of the basin, associated with muddy clay, silt, sand, and gravel, dipping west. The clay beds thin out S. of Newton station, and occur again at the Decoy, where they dip E., and where several associated seams of lignite, parted by dark clay and vegetable matter, stand nearly perpendicular. The pipe clay at the Decoy has been worked about 90ft. deep. Traces of clay may be found as far in tbe direction of Torquay as the Old Atmospheric Engine House. At the Bovey Pottery (coal-pit) the beds dip to S.E. about 1 lin. in a fathom (an old man who has worked among the beds nearly 40 years informed the author that about the Pottery they nowhere dip in any direction but S.W). Mr. Key gives a section taken by the late justly lamented Dr. Croker, of Bovey, in which some of the beds are represented as very unequal in thickness and of variable dip. Others have their abrupt ends apparently resting on beds less inclined, while an immense basin denuded out of the beds is filled with “head.” Mr. Key believes that the Bovey deposit, both body and head, originated in the river Bovey, discharging various kinds of sediment into a deep lake. The river once ran through the lake from near Bovey to near Newton, and thence, by way of Torr, to Torbay—the estuary of the Teign having been subsequently excavated.—Dr. Heer (paper read before Royal Society, November, 1861) states that the dwarf birch found in the “head” is an arctic plant, and, along with the associated willows, points to a cold climate. In the body of the deposit the following sub-tropical plants of Lower Miocene Tertiary age have been classifie:——Cinnamon, laurels, fig, palm, tree-ferns, etc. But the woods that mainly furnished the lignite consisted of a huge sub-tropical coniferous tree, Sequoia Couttsiæ, resembling the present Sequoia of California. The lower lignite beds consist almost entirely of stems of this and other trees, with brownish black clay. The trees never occur upright, and must have been drifted, the majority from a distance beyond the shores of the hike in which the beds were accumulated. The occurrence of seeds of Nymphtæa shows that it must have been a freshwater lake.

page 396 note 1 At a lower level, by the side of the railway, a deposit of much finer gravel may here and there be seen, part, if not all, of which may be river shingle.

page 396 note 2 This igneous rock I at first took for a fine-grained granite, and on the Geological Map it seems to come within the granite area. I saw afterwards a stone on the roadside to the east of the Blackenstone rock, one part of which consisted of this rock separated by a straight and very distinct line from the other part, which was very decided granite. In the present unsettled state of petrology, it is perhaps better to be cautions in applying names.

page 400 note 1 Since writing the above, I have found that the ingenious author of “Frost and Fire,” briefly mentions the Hey Tor Rocks as having been denuded by ice floating from the north-east, without referring to the direction of the tails and scattered wrecks. He likewise notices the ice-ground contour of the Blackenstone Rock, but leaves its tail out of consideration.

page 400 note 2 These blocks and fragments consist of, at least, two kinds of granite: a reddish fine-grained variety, and the common coarse-grained variety with large oblong crystals of felspar.