The Geology of South Shropshire, with Special Reference to the District to Be Visited During the Long Excur Sion

297 THE GEOLOGY OF SOUTH SHROPSHIRE, WITH SPECIAL REFERENCE TO THE DISTRICT TO BE VISITED DURING THE LONG EXCUR SION. By Prof. C. LAPWORTH, LL.D., F.R.S., and W. W. WAT TS, M.A., F.G.S. [Re ad 6th J uly, /89....] CO N TEN TS. PAGE I. Introduction 297 II. Physical Structure 298 III. Fundamental or Pre-Cambrian Rocks 3° 1 IV. The Cambrian System 308 V. The Ordovician System 3II VI. The Silurian System ... 32 1 VII. The Old Red Sandstone and Newer Rocks 330 VIII. The Igneous Rocks of Shropshire ..• 334 IX. H istorical Summary 343 X. Conclusion 354 I.-INTRODUCTION. R OM whatever point of view we regard the district to be F visited by the Geologists' Association in 1894, whether from its association with so many of the Fathers of British Geology, from the variety and nature of its rocks, the frequency and perfect preservation of its fossils, the complexity and systems of its tectonic structure, or from the beauty of its scenery, it is one of surpassing interest. In classic times it formed the eastern portion of the land of the Ordovices, while it was bounded immediately to the south by the realm of the Silures. W ithin the last half century its hills, its towns, and its very hamlets have become familiar to the geological student through out the world. The Longmynd, Rushton, Ur.iconium, Comley, Shineton, the Stiper Stones, Caradoc, Wenlock, Ludlow, Aymestry, and Downton all give names to important rock groups which have passed into common geological knowledge, while there yet remain many minor divisions which the progress of the science may prove to be type stages and zones! a system of units to which deposits of other areas will eventually be referred. There are comprised within the county representatives of almost every known horizon from the Longrnyndian to the Lias, brought into close approximation by a cluster of unconformities which Mr. D. C. Davies .first drew attention to in a paper to this Association" and upon which Professor Blake has justly remarked, • Pro c, Geol, A ssoc. vol. ii , , 873, p, 299. AUGUST, 1894. 2 I C. LAPWORTH AND W. W. WATTS ON There are folds, faults, and igneous rocks of all ages; scenery of all types, from the rolling mountain range of the Longmynd to the noble Wenlock Edge escarpments and the broad Midland Plain. The county is now fairly accessible to the geologist, and it only needs to be added that its structure has been studied and described by such men as Lewis and Murchison, Aikin, Prest wich and Lightbody, La Touche and Symonds, Allport and Bonney, Maw, Morton, Callaway, Blake, and Cobbold. It will be best, after giving a general notice of the principal physical features of the county, first to describe the sequence of rocks in Shropshire so far as it is at present understood, and sub sequently to give a brief historical summary in order bring out more clearly the successive stages by which the structure has been worked out. In this way attention can be drawn to the different series in order, in such a manner as to bring out those salient charac teristics which have rendered the individualization of each system a possibility. We do not propose, however, to enter into every detail of the history of geological discovery and controversy; nor have we space to do justice to all the individual workers who have completed the fabric of our present knowledge. II. PHYSICAL STRUCTURE. One of the greatest fracture-lines known in British Geology is that which sweeps, from N.E. to S.W. in direction, from Lilleshall Hill through the Wrekin and Stretton Hills down to Kington in Radnorshire. A fault of great throw, often splintering up into a. host of minor fractures, runs along this line and brings up ancient rocks into unwonted contact with suchhewer divisions as the New Red Sandstone, Carboniferous, and Silurian rocks. The physical features of Southern Shropshire may be regarded as built up on and around this line. Centrally we have the Longmynd, a great moorland relic of a plain of marine denudation, carved out by streams into deep valleys or "gutters" which have isolated the lumpy, apparently structureless, hills that diversify its surface. Here however, as elsewhere in the county, traces of a later marine denudation are to be seen in the old sea cliffs, low lines of hills sometimes two or three miles long, against which the Silurian floor FIG. I.-SECTIONS :-II, Fig. 6; III, Fig. 13; IV, Fig. g; V, Fig. 10; VI, Fig. 7 ; VII, Fig. 8. LOCALITIES :-(1) Wellington; (2) The Wrekin ; (3) Rushton; (4) Charl ton Hill; (5) Lea Rock; (6) Walcott; (7) Leebotwood; (8) The Lawley; (g) Caer Caradoc; (10) Church Stretton ; (II) Pontesbury ; (12) Nill's Hill; (13) Minsterley ; (14) Mytton Dingle; (IS) Roman Gravels; (16) Hope; (17) Chirbury; (18) Priest Weston; (Ig) Llan fawr; (20) Comdon ; (2 I) Whetstone; (22) Fishpool; (23) Craven Arms; (24) Horderley; (28) Cressage; (2g) Belswardine Brook; (30) Buildwas Abbey; (31) Lincoln Hill ; (32) Coalbrookdale. SHROPSHIRE 4 FIG. I.-The Map is based on the Index Map of Wales, Sheet IS, of the Geological Survey, with modifications taken from the papers of Dr. C. Callaway and Professor J. F. Blake, and from unpublished observations by Professor C. Lapworth and Mr. W, W. Watts. The scale does not admit of the introduction of more minute subdivisions, although these have been mapped, nor does it allow of more than a diagrammatic representation in many parts. 30 0 C. LAPWORTH AND W. W. WATTS ON lies as the sea under Beachy Head to-day. Round the Longmynd comes a ring of volcanic hills extending from the Wrekin to Church Stretton, and back from Chittol to Pontesbury, each hill an isolated hog-back with abrupt sides and more gently sloping N.E. and S.W. ends, giving rise to features which are always striking from the changing and unexpected nature of their outline. Beyond these, to the West, we have wave after wave of escarpments, all rangingN.E.and S.W.or N.N.E. and S.S.W., rising ridge beyond ridge right away into Wales, wherever a resisting plate of harder rock comes to the surface. The Stiper Stones, Cefn Gwynnlle, the Stapeley Hills, the ridges of Priest Weston, Hagley, and Whittery, the Long Mountain, and the Breiddens are some of these chains which will come under our direct observation. To the east of the Longmynd the structure is the same, and beginning with the escarpment of Hoar Edge we cross those of Chatwall, Church Preen, Wenlock Edge, and Weo Edge, until we reach the Old Red Sandstone county and the swelling heights of the Clees. The escarpments are broken by cross valleys, of which the most important are the Severn Gorge under Benthall Edge, at Iron bridge, and another hardly less interesting, that of the Onny to the south, and these are divided from each other by a typical series of longitudinal valleys, carved out into Apedale, Hopedale, Corvedale, and other dales by the denuding forces to which the relief of the scarps is due. To the north-east the ridges die down under the Shropshire coalfield; to the north-west they are covered up by the great Triassic plain, bearing on its surface the Liassic outlier of North Shropshire. The regularity of these features is to some extent broken by the intrusion of igneous rocks in a way which at first seems irregular, and also by faults, but it is not difficult to ascertain that even these irregularities conform to the general structure of the district, and are but another expression of the same earth move ments to which the main features of the county are due. The geographical order of description above follows to some extent the geological grouping and date of the rocks, the Longmyndian and U riconian volcanic hills being beyond doubt the oldest rocks, while newer and newer groups are met with as we pass westward in succession over Tremadoc, Arenig, Llandeilo, Bala, Llandovery, Wenlock, and Ludlow strata up to the Old Red Sandstone of the Long Mountain, where a syncline intervenes, and we begin to descend the series again to the Bala rocks of the Breiddens. On the east we cross the Olene/lus and Olenus zones of the Cambrian, the Bala or Caradoc rocks of the Ordovician, and then the divisions of the Silurian and Old Red Sandstone up to the Carboniferous of the Clee Hills and the coalfield of the Forest of Wyre. These relations will be easily understood by reference to the map (fig. r) and the long section (figs. <1 and 10). THE GEOLOGY OF SOUTH SHROPSHIRE. 3°1 III. FUNDAMENTAL OR PRECAMBRIAN ROCKS. The base of the Cambrian system of Shropshire cannot yet be said to he mapped out satisfactorily. There occur beneath the fossil bearing Cambrian Strata three, four, or possibly five groups of rocks whose relative age has not been determined to the satisfaction of all parties, namely the Rushton schists, the gneiss of Primrose Hill, the Uriconian Volcanic series, the Longmyndian Grey Shale series, and the Longmyndian Red Sandstone series. Opinion is much divided on this point, so that we will merely give an account of the character of the rocks, the grouping of the fundamental rocks advocated by Dr. Callaway " and by Professor Blake.j and the descriptions and arguments which they have adduced in support of their conclusions. AB Longmyndian Red Sandstone Series . 5 4 Longmyndian Flagstone and Shale Series 4 2 Uriconian Lavas and Ashes.

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