• Abstract and Figures
• Public Full-text

78 EXCURSION TO PLYMOUTH, EASTER, 1907. contrary to expectation, the chalk-mine has not been opened this winter. A large part of the chalk raised in 1906 still remains in stock and until it is used up no work will be done in the mine. The proprietor has, however, kindly promised to allow a party to go down as soon as work in the mine is resumed next winter. All those who had, in compliance with the request in the Circular, notified to the Excursion Secretary their intention of attending the Excursion, duly received notice that the Tertiary and Chalk sections in the East Wickham valley would be visited instead of the mine. A party of 13 met at Plumstead and the excursion of last April was practically repeated. The sections were in very good order and the weather was all that could be desired for our first field excursion of 1907. A hearty vote of thanks was accorded to the Director. EXCURSION TO PLYMOUTH, EASTER, 1907. Director: W. A. E. USSHER, F.G.S. Excursion Secretary: G. E. DIBLEY. (Report by THE DIRECTOR.) THE members assembled at Plymouth on Thursday, March 28th, and took up their quarters at Farley's Hotel. Friday, March 29th.-Favoured by splendid weather the party, numbering 49, proceeded to the starting place for the Turnchapel steamboats. Whilst waiting for the boat the Director gave a short address on the early readings of the Devonian section. The Devonian limestones were the first member of the series to receive special attention. The Plymouth limestone was described by Hennah in the early part of the nineteenth century and later on Lonsdale assigned to the limestones a position between the Carboniferous and Silurian, as a marine equivalent to the Old Red Sandstone. Sedgwick and Murchison extended this correlation to the pre­ carboniferous slates and grits in which the limestones occurred, thus establishing the Devonian System and including in it the Paleeozoic rocks which emerged in North and South Devon from beneath the central trough of Culm Measures which they had shown to be the main structure of Devon. Owing to the north and south post-carboniferous contraction, called the Armorican movement, proceeding from the south the axes of the folds were bent northward on either side of the Culm trough. This fact favoured the normal descending succession of the rocks of the 0\ .... r-, o 0\ ... r£ ~ f­ en -<: ... £ f­ ;:J 8,<; >< .... 0- o '"z o iii ~ L=:±:44 J. O W LIf CvlJi'l ;:J P~ u . UI' D erO/l(lAN ~ N"A"O~ "".., ... LJHI SS'."lr M,~ SM ro-s l ~£VOMJW M 1', f\ ~Do AI L o w~ 1f. (; Jll r s } BASI C It:NEou$ R4c"s~ L. I "'''AD''O<IT CiI'O UP J>S>'ONIA'" ___ F .. - rrL~/r'c HD(:ItS rH~ J)7JD7> D Ao4I"TJ'l10VTN .JLAT4S ApPIiO~ /It4A~ LIMIT cJI" G. C/fANIT£ 'f I II i , ~ "' ~I("" S' 'nJ'''''J FIG. 3.-GEOLOGICAL MAP OF PLY MOUTH DJ5TRICT.-W. A. E. Uss/zer, F.G.S. Scale, 4 miles = I inch. By Pelt" is,;on 0/ the Director of Ihe Geological Su",ey. 80 EXCURSION TO PLYMOUTH, EASTER, 1907. northern outcrop from the Culm Measures northward, which is so weIl described in De la Beche's Report. But notwithstanding the recognition by Phillips of the Upper beds on the southern margin of the Culm trough at South Petherwin, the oldest Devonian strata in the southern outcrop were placed between the Plymouth limestone and the southern lip of the Culm trough, whilst from the limestone southward Sedgwick and Murchison gave an ascending series up to the metamorphic rocks of Hope Cove. Although this error was no doubt largely due to the prevalent southerly dip of the cleavage planes and of the bedding through overfolding, it is very probable that the more detailed work of De la Beche in Cornwall and of Godwin Austen in South Devon, by introducing fresh sources of confusion, tended to perpetuate it. Pointing to the outcrop of the Lower Devonian rocks on Staddon heights the Director explained that this outcrop throughout South Devon maintains an east and west strike (see Map iii, Fig. 4),· but that on crossing to Cornwall it is shifted northward by a series of north-west and south-east faults, so that it reaches the north coast of Cornwall at Bedruthan steps much farther to the north. This structure inferred by him in 1890 (see Map ii, Fig. 5) t was subsequently proved by detailed mapping to be the Key to the Devonian stratigraphy of Cornwall, and furnished the explanation of the blending of Upper and Lower Devonian rocks, distinct in lithological types and fossil contents, in De la Beche's minor subdivisions, as he had traced the rocks now known to be Upper Devonian westward into the Lower Devonian districts without perceiving the intervening dislocations. Further­ more the variegated Lower Devonian slates of Watergate Bay (the Dartmouth slates), the lowest known division of the Devonian (which would be seen at Andurn Point), were correlated by De la Beche with the purple and green Upper Devonian slates, the series which corresponds to the Cypridinen Schiefer of the Continent and which would be seen at Wivelscombe quarry and in the Ince Castle coast on Saturday's excursion. Godwin Austen in the Torquay area further complicated the geology by regarding the repetition of the limestones through faults and folds as successive outcrops. Holl's maps in 1868 (see Map i, Fig. 6),t notwithstanding his recognition of inverted folding, shows the lower rocks on the north, "the Lower South Devon group," and the" Upper South Devon group" on the south, of the limestones, with an infold of the lower group. Clearly recognising the obstacle to this view presented by the age assigned to the Petherwin Beds, bolder than his predecessors he endeavoured to explain away the fossil evidence on which it was based. The preliminary notice for the Long Excursion in 1884 (Proc. • Somerset A rch, and Nat. Hist. Soc. 'go,. t Trans. Roy. Geol. Soc.• Cornwall. ,89" ~ Based On Holl's map In Quart. Jour". Geol, Soc., vol, xX!\', PI. XVI. EXCURSION TO PLYMOUTH, EASTER, 1907. 81 Sc~,lclInd",:O~ 1''$ Culm Aleasures ~. UpperDevonian ··········· MiddleDevonian··· . Lower {~~~;/ ~ 0 evo- onrnan ' Oartmouth: Granite. •• ~ Slates FIG. 4.-MAP III. USSHER (After Survey), 1900. CulmMeasures.......• UpperDevunian . MiddleDevonion I Stoddon . II" Meadfoot ...... , Lower DeWJnian Dortmuuth .... ~ Granite .....Ci:IIJ Looe.•...•...•.. Scale. / Iruik -/ O.'£les FIG. 5.-MAP II. USSHER (Before Survey), 1890. EXCURSION TO PLYMOUTH, EASTER, 1907. Ceol. Assoc., vol. viii, p. 442) proves no advance on earlier ideas. As regards the special study of the rocks and de­ posits of the area the papers written by the late R. N. Worth are invaluable.. The arrival of the steamer having put an abrupt termination to the discourse the party observed on passing the limestone cliffs of Cat down the comparatively horizontal bedding of the central parts of the limestone mass, which was best enabled to resist the folding. Landing at Turnchapel in order to traverse the inverted down­ ward succession from the limestone to the Dartmouth slates, a short halt was made to inspect a cutting in the lower (Eifelian) slates of the Middle Devonian near Fort Charles. The Staddon grits were next encountered on their salient outcrop feature. Closer inspection was debarred by the War Department, which had annexed the path, open to the public when the Director was stationed in Plymouth. Consequently a fine section of Staddon grits with several conglomeratic beds in the tabooed ground could not be seen, and a considerable detour involving some scrambling amid furze and brambles had to be made before reaching Bovisand Bay. At Bovisand Bay the Middle Division of the Lower Devonian, the Meadfoot group, succeeds the Staddon grit. In a short walk by the cliff path Andurn Point was reached. The Director here showed the purplish and green Dartmouth slates, with a mass of Diabase. These slates form the low coast of Andurn Point and with inter­ bedded grits extend thence to Westcombe beach, near Ringmore. He had found traces of Pteraspis in them near Langdon Court inland, and at Piskey's Cove, near Revelstoke, but not elsewhere in Devon. Faults, well marked by the beach reefs, separate the Dartmouth slates from the lower part of the Meadfoot group, which here resembles them in colour and corresponds to the Taunusien of the Continent. Between the faults a mass of breccia of local fragments under Head or old screes was pointed out, as the outlier of New Red mentioned by Sedgwick and Murchison in their description of this coast. From the fault proceeding northward to Crownhill Bay calcareous slates associated with hard grits and sandy, decomposed, friable beds full of casts of irrecognisably distorted fossils were seen to be repeated by numerous folds. The Director pointed out the occurence of numerous thin bands of limestone veined and mottled by irregular patches of calcspar, evidently of organic origin and perhaps Monticuliporoid corals. These limestones, he said, were met with in the fine section on the Tregantle coast and at East Looe j they are characteristic of the Looe beds on the coast, and had furnished him with a valuable horizon in correlating the different sections on either side of the Dartmouth slate anticlinals in South Devon and South-east Cornwall. These ~ -••-- --;I. EXCURSION TO PLY:lIQUTH. ". remarks were punctuated by the dis­ covery on the spot of a recognisable Monticuliporoid coral by Mr. E. T. Newton. On reaching Crownhill Bay the cal­ careous rocks were seen to terminate, plunging steeply under a series of grey slates, the junction being complicated by thrust faulting on .

Load more

  16

Copy Link