Midland Naturalist/Volume 01/Recent Discoveries in the Geology of Shropshire.—I

by Charles Callaway

Recent Discoveries in the Geology of Shropshire.—I.

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By Charles Callaway, M.A., B.SC. Lond., F.G.S.

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Introduction.

This paper, furnished at the desire of the Editors of this Journal, gives a brief outline of a paper read by me before the Geological Society, in March, 1877, and published in Vol. XXXIII. of the Society's Journal. Its object is to announce the discovery of a now area of Tremadoc and Pre-tremadoc rocks, near the Wrekin, with a fauna mainly composed of new species. Papers will probably be communicated on the quartzites of Shropshire, and on a recently discovered Pre-cambrian volcanic series of great interest and importance, when the rocks have been more completely worked out. Sir R. I. Murchison has described the area under consideration, from the Wrekin on the north-east to the May Hill sandstone at Kenley on the south-west, as composed oi strata of Caradoc age, the Wrekin itself being an igneous outburst altering the Caradoc sandstone an its flanks into quartzite.

The Geological Survey has followed Murchison, but has include, wader the name of "quartzite," certain sandstones in which I have detected fossils in abundance.

In the Journal of the Geological Society (Vol. X., p. 62,) Messrs. Aveline and Salter describe this area as Caradac, and Salter gives a list of fossils from {so-called} Lower Caradoc shales at Harnage and Shineton, mixing up Cambrian forms, such as Olenus, from Shineton, with Cambro-Silurian genera, such as Trinucleus, from Harnage, the shales at Shineton and at Harnage evidently being considered identical.

Salter, in the "Geological Magazine” for 1867, refers to the shales at Shineton, which he there records as "the top of the Llandeilo Flags proper." The same writer seems, in after years, to have been struck with the incongruous association of Cambrian and Cambro-Silurian forming; for, in "A Catalogue of the Collection of Cambrian and Silurian fossils contained in the Geological Museum of the University of Cambridge," published in 1873, while describing what he supposes to be a Triathrus from Shineton, he suggests, "it is possible that the locality may include some Tremadoc beds." With this exception, geologists have regarded the rocks of the area nuder consideration as of Caradoc age.

I shall endeavour to prove that the shales at Shineton are of Tremadoc age, and that a part of the so-called "quartzite” between the shales and the Wrekin represents the Hollybush Sandstone of Malvern. The true quartzites are probably Pre-cambrian; and the igneous chain of hills, from Lilleshall Hill through the Wrekin, the Lawley, Cacr Caradoc, and on to the south-west, are clearly stratified, and underlie unconformably the Cambrian racks,

Lower Caradoc Rocks.

Mr. Salter noticed at Harnage and on Cound Brook certain shales containing Trinucleus concentricus, Eaton, Beyrichia conmplicata, Salt, Diplograpsus pristis, His., Orthis testudinaria, Dalm., and other Cambro-Silurian fossils; and as these shales are very similar in lithological characters to the shales at Shineton, and have the same general strike, both shales were lumped together by him as Lower Caradoc. This lithological resemblance is evidently the chief cause of the errors of the surveyors. On closer inspection, however, it is seen that the shale at Harnage contains a distinct fauna from the Shineton shales. The most abundant fossils of the Harnage shales, collected near Boomcroft and in the Hacrnage and Cound-Brook area, are Trinucleus concentricus, Eaton, Beyrichia complicata, Salt., Primitia bicornis, R. Jones, Orthis testudinaria, Dalm., Theca, several species of Lamellibranchs, Diplograpsus pristis, His., and Favosites flbrsus, Goldf. These are common Caradoc forms, and it is perfectly clear that the shales containing them are at Caradoc age. In no case are these fossils found in the same beds as those which contain the older fauna presently to be described. It is necessary to call attention to this point, as the Rev. J. D. La Touche, president of the Caradoc Field Club, in his annual address in February last, has suggested some criticism on my conclusions, basing it on a supposed admixture of the older and younger faunas. There are no signs whatever of such admixture. The Tremadoc fauna ends abruptly upwards; the Caradoc fauna ends abruptly downwards; and not a single distinctively Arenig or Llandeilo species has been found in the district.

The Caradoc rocks of this area are much disturbed and faulted, and on Cound-Brook they are inverted, the older vesting to the younger at a considerable angle, and, in one or two spots, Shineton shales are strangely wedged in between Harnage shales. Farther details may he seen in my published paper (p. 656); but the Caradoc formation in South Shropshire deserves to be the subject of a separate memoir, recent observations haying considerably modified some of the conclusions of Aveling and Salter.

The Shineton Shales.

The locality where I first observed these shales is the spot near Shineton marked on the Geological Survey Map with an arrow dipping to the south-cast at 50’. The rocks are there exposed in two good sections on the left bank of the stream. It is from these sections that most of the characteristic fossils have been obtained; and I have, therefore, named the formation from its locality,

1.—Area.—These shales cover an area extending from near Evenwood, on the south-west, to within a mile of Wellington, on the north-east a distance of eight miles. Their greatest breadth, from Shineton to Dryton, is abort two miles; but where they range towards Wellington it is contracted almost to a point. The area is roughly triangular in shape, the apex of the triangle pointing to the north-cast. Its north-west side is bounded by a fault or faults for probably its entire length, various formations from the Hollybush Sandstone to the Trias shutting against the shales. On the south-east side the triangle is covered in by intrusive basaltic rocks for one-third of its distance from the apex, ant the remainder by the May Hill Sandstone. The base of the triangular area is limited by the Hoar Edge Grits, the lowest beds of the Caradoc. I have recently detected the shales in the hollow between the Lawley and Hoar Edge, an the south-eat side of Caer Caradoc, and west of the Longmynd, at the base of the Stiper Stones.

2.—Lithological Characters.— Shineton Shales are dark blue, Weathering to olive and yellow, the colouring iron-oxide sometimes separating as a stain or film. They are micaceous, thin bedded, soft, and rather fissile. I have rarely had any difficulty in distinguishing them from the Harnage Shales, either in situ or in hand specimens,

3.—Dip and Strike.—The general strike of the shales is about south-west, agreeing with the direction of the great fault and of the so-called igneous elevations of the district; but towards the south-west end of the area it bends round to the west, corresponding with the strike of the overlying Caradoc. The mean dip of the greater part of the shales is about 30° to the south-east; but in the lower part of the series, where they approach the fault, it becomes higher, then vertical, than dips steeply to the north-west, the evidence pointing towards the existence of an anticlinal. The thickness of the shales is probably not loss than 1,500 feet.

4.—Stratigraphical Position.—The Shineton Shales underlie the May Hill Sandstone unconformably; they are therefore older than that formation by an interval. They underlie the Caradoc, and are, of course, of greater antiquity. They overlie, probably unconformably, the Hollybush Sandstone. I shall endeavour to show that they are of Tremadoc age.

(a.)—Evidence from Fossils.—Most of the Shineton forms are new specifically, and some of the genera are also new. The species which are of geological value are the following:—Conocoryphe monile, Salter. Conocoryphe proper is distinctive of Lower Cambrian rocks, and this species is truly typical of the genus. Olenus Salteri, Callaway, and O. triarthrus, Call., new species of a genus which usually characterizes strata of the age of the Lingula Flags. Agnostus dux, Call, similar to certain St. David’s forms, (Menevian.) Lingulella Nicholsoni, Call, resembling L. lepis, a Tremadoc species. Asaphellus Honfrayi, Salt., common in the Upper Tremadoc at Portmadoc.

(b.)—Evidence from Correlation with Rocks in other Localities.—Dictyoneum beds at Pedwardine.—Shales identical lithologically with the Shineton Shaleos, and containing Lingulella Nicholsoni, are found at Pedwardine, twenty-five miles to the south-west of Shineton, on the same line of strike, They also contain Dictyoneum sociale, which has not yet been found at Shineton.

Dictyoneum Beds at Malvern.—Overlying the Olenus Shales near White-leaved Oak, are light-coloured shales, similar to the Shineton and Pedwardine beds, and containing two Shineton forms, Platypeltis Croftii, Call., and Conophrys salopiensis, Call., together with Dictyoneum sociale, Balt,

A comparison of the three formations at Shineton, Pedwardine, and Malvern is very interesting. The Shineton beds are connected with the Pedwardine shales by lithological resemblance, stratigraphical position, and the occurrence of Liagulella Nicholsoni. The Pedwardine rocks are correlated with the Malvern Dictyonema shales by lithological resemblance, stratigraphical position, and the link of Dietgouvem sociale. The Shineton Shales are directly connected with the Malvern beds by lithological resemblance, stratigraphical position, and the occurrence of two species of Trilobites in IommonI and indirectly througe their correlatioe with the Pedwardine Shales, I think I may fairly conclude that the Dictyonema beds at Pedwardine and Malvern are representatives of the Shineton Shales.

The occurrence of Dictyonema sociale in the Shineton Shales at Pedwardine and Malvern furnishes another link in the chain of palæontological evidence. This species is common at the base of the Lower Tremadoc of North Wales, and helps to connect that formation with the Shineton Shales. Taken by itself, the occurrence of a single species may not be decisive; but, when other lines of evidence converge to the same point, this fact is of value.

The Black Shales of Malvern are correlated by their fossils with the Dolgelly group, the uppermost zone of the Lingula Flags.

It may be concluded from a review of the evidence that the Shineton Shales are at least as old as the Lower Tremadoc.

The Hollybush Sandstone.

Forming a continuous band between the Shineton Shales and the quartzite which rests upon the Wrekin, is a series of thin-bedded, migaceous, green sandstones, holding the same geographical relation to the Shineton Shales as the Hollybush Sandstone of Malvern holds to the black Olenus Shales. The identification of this rock with the Hollybush is placed beyond doubt by the further evidence of Kufaryine eiegnlate and Serpulites jistula, which occur in good preservation at Nevos Castle, at the south-west end of the Wrekin. The sandstone is also found at Lilleshall, five miles to the north-oast of the Wrekin, where it forms an inlier a mile long by a quarter of a mile wide. Since the reading of my paper I have also discovered it on the south-east dank of Caer Caradoc, near Church Stretton. It is well exposed in a quarry at the north-cast end of the hill, and contains a thin band of limestone with Kutorgina cinguluta, Serpulites fistula, and other fossils, and holds its normal place between the quartzite and the Shineton Shales.

Details of the Hollybush Sandstone as well as of the Shales will be found in my published paper.

Fauna.

I append a list of the Upper Cambrian fossils found in South Shropshire, most of which it will be seen are new to science. They are described and figured in my paper:—

Crustacea.

Asaphus (Asaphellus) Homfrayi, Salt, Shineton Shales,

Asaphus (Platypeltis) Croftii, Call. gen. et sp. Shineton Shales,

Agnostus dux, Call. Shineton Shales.

Conocoryphe monile, Salt. Shineton Shales. Olenns Salteri, Call. Shineton Shales.

Olenns— — — triarthrus, Call. Shineton Shales.

Conophrys salopiensis, Call. gen. et sp. Shineton Shales.

Lichapyge enspidata, Call., gen. et sp. Shineton Shales.

Primitia, sp. (more than one.) Shineton Shales.

Annelida.

Serpuiites fistula, Holl. Hollybush Sandstone.

Pteropoda.

Theca lineata, Call. Shineton Shales.

Heteropoda.

Bellerophon shinetonensis, Call. Shineton Slates.

Brachiopoda.

Lingulella Nicholsoni, Call. Shineton Shales.

Obotella sabrinæ, Call. Shineton Shales.

Kutorgina cingulata, Bill, Hollybush Sandstone,

Echinodermata.

Macrocystella Mariæ, Call., gen. et sp. Shineton Shales,

Hydrozoa.

Dictyonema sociale, Salt, Shineton Shales.

Dendrograptus, Shineton Shales.

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