245 THE LIASSIC ROCKS OF GLAMORGAN. By· A. E. TRUEMAN, D.Se., F.G.S., University College of Swansea. Read June 9th, 19zz. PLATES 8-10. PAGE I. INTRODUCTION 245 II. HISTORY OF RESEARCH •• 247 III. GENERAL SEQUENCE IN THE LIAS OF GLAMORGAN •• 248 THE AMMONITE SEQUENCE 249 IV. STRATIGRAPHICAL DETAILS 254 A. COAST SECTIONS NEAR DUNRAYEN 254­ B. CoAST SECTIONS EAST OF DUNRAYEN 264 C. INLAND EXPOSURES OF NORMAL LIASSIC ROCKS 268 D. INLAND EXPOSURES OF LITTORAL LIAS "170 V. THE ORIGIN OF THE NODULAR LIMESTONES 274­ VI. CONCLUSIONS AND GENERAL CONSIDE R ATIO N S 276 VII. NOTES ON THE AMMONITES 280 VIII. BIBLIOGRAPHY 283 1. INTRODUCTION. THE Liassic rocks of Glamorgan occur in several small out- liers west of Cardiff, and in a broad tract stretching between Bridgend and Cowbridge and southwards to the sea (fig. 55). This area is known as the Vale of Glamorgan, and is a beautiful agricultural district, which has not shared theindustrial develop­ ment of the adjacent coalfield.* The "Vale" is a low plateau, generally a little over one hundred feet above sea-level, but ranging to three hundred feet above sea-level in several tracts of Carboniferous Limestone. The northern margin is crossed near Bridgend by the main South Wales line of the Great Western Railway, while the only other railway of importance, the Vale of Glamorgan Railway, which runs near the coast and connects Barry and Bridgend, affords the easiest means of reaching many of the exposures to be described. Liassic rocks, with very low dips, crop out over the greater portion of the Vale. In the east of the area near Cardiff, and elsewhere, they are underlain by the Rhretic and Keuper Marls, but at many places further west these formations are overlapped by the Lias, which then rests unconformably on the underlying Palseozoio rocks, chiefly Carboniferous Limestone. The Triassic and Liassic rocks of the Vale of Glamorgan were laid down near a' shore lirie in a region of steady and fairly rapid subsidence. As the sea encroached on the land, which lay to • A. E. Trueman... Population Changes in the Eastern Part of the South Wales Coalfield," Geog,,,phicalJOl4mal, 1919, p. 410. PROC. GEOL. Assoc., VOL. XXXIII., PART 4. 1922. 16 i .&Y:~~};i;*?:~: :·:··:·; · Ohahns ?­ Soutne rn do,", l't1 un r cv an Trwyn y Wife" >-l ic" ~ c: tIl CWMNcuh. :;"" ..Llol\t"'lt f"'\(\,.r Z .. ~ ,Dona.t,5; - 1'1"8" Po ..." S",ITUT\tr,"u"l,),C 'to\1\t 8"...., \.lon4 Sco.l. o~ Mol•• o 1 ~ 3 4 ~ 8 ,J t J 1 I R Eo l- S T o L c 1-\ K " '"' ~. ~ ' :' ~. .. g ~ EZJ'.:... ~ EJ 0... S,luro"n Old RedSandstont Carben i ferous M illston. Gr it & Iie upu& low.rlio~ Alluv ium 810'"'" Sand ~s li mestone COAl•M~(l ~L ~ r Rhett ic F IG. 5 5 .-GEOLOGICAL S KE TCH l\!AP OF T ilE V ALE OF GL\ MORGA:<I. (Adap ted from th e one-inch maps of the Geological Survey, Nos. 2 6 1- 2 , 263, New Series.) THE LIASSIC ROCKS OF GLAMORGAN. the north and north-west, there were at various stages groups of islands, generally composed of Carboniferous Limestone, situated near the mainland. Around such islands, and along the shore of the mainland, littoral deposits of various kinds accumulated. Two chief types of littoral deposits have been described among the Liassic rocks of the area, the Sutton Stone, a whitish con­ glomeratic limestone, and the Southerndown Beds, which are usually darker in colour and often consist of limestone and cherty breccias. Oolites and chertylimestones also occur in some parts of the area. The Liassic rocks of Glamorgan represent only the lowest few zones of the Lower Lias, but these are of more than usual thickness, and owing to the gentle dips which prevail over the greater part of the area, the rocks have an extensive outcrop. These rocks form the cliffs which stretch almost continuously from Sutton to near Barry, a distance of about twenty miles. Such a great extent of almost horizontal rocks is somewhat monotonous, but undoubtedly these cliffs offer. an unrivalled opportunity to study the sequence offaunasinthelower portions of the Lower Lias and also to investigate the relations of the littoral deposits. Indeed, it is doubtful whether anywhere in Britain there are similar sections in any formation which show such rapid lateral changes in lithology, or in which the various stages of a marine transgression can be so completely studied. II. HISTORY OF RESEARCH, The Liassic rocks of littoral character in Glamorgan attracted the notice of geologists at a very early date, and more attention has been paid to them than to the larger areas of normal Lias. De la Beche pointed out that various limestones become con­ glomeratic in the west and that on the coast near Sutton there is at the base of the series a white limestone, known as the Sutton Stone.* In 1863 and for several years later the Sutton Stone and the associated beds were studied by numerous workers. A summary of their views has been given by the late H. B. Woodward] in "The Lias of England and Wales," and it does not appear to be necessary to discuss them in detail here. R. F. Tomes: and E. B. Tawney§ maintained that the conglomerate series of the Sutton and Southerndown district was of Rhsetie age, while C. Moore.] W. H. Bristow.j] and R. Tate** held that the fossils were characteristic of the lowest zones of the Lias. Tate, in particular, showed that the fossils considered by Tawney as • 8 (see Bibliography, p. 283). t 81 p. 99 (reprinted in 1'7 p. 61.) t 10 § 20 1112. '2. ··18. A. E. TRUEMAN. new species, are relatively common in Hettangian rocks in this country and on the Continent, and proved the Liassic age of the deposits. P. M. Duncan studied the corals from the Sutton Stone and his account of the age and relations of these littoral deposits is more accurate than subsequent writers have realised.* Since that time the Liassic rocks have been examined by H. B. Woodward] and by other officers of the Geological Sur­ vey. t In the Cardiff and Bridgend Memoirs a summary of their observations is given; but as no detailed examination of the fossils was made at that time, and as very few, if any, accur­ ate records of ammonites within the area are available, further investigations appeared desirable. Before the exact age and relations of the littoral deposits could be satisfactorily deter­ mined, it has been necessary to obtain more detailed and exact information of the faunal succession in those parts of Glamorgan where the Liassic rocks are of normal character. The writer has already published an account of the succession in the lowest zones, as seen in the cliffs of Lavernock and Penarth.§ In the present paper some additional facts concerning the faunal sequence are recorded, and the relations of the littoral deposits are examined. III. GENERAL SEQUENCE IN THE LIAS OF GLAMORGAN. The Liassic rocks of Glamorgan are probably not less than 350 feet in thickness, and consist, where normally developed, of nodular limestones and shales. The following are the only divisions that have hitherto been recognized:- Nodular limestones with some shales about 200 feet. Shales with some nodules. (The lower part are called Lavernock Shales) "roo feet. Alternating limestones and shales ,,45 feet. The lower divisions were described and the faunal sequence in them was discussed in the Cardiff paper. The upper series is not seen in the outliers near Cardiff, but in the cliffs westwards from Barry they are clearly exposed. Frequently they form the upper portion of the cliffs, and only at a few looalities are even the lowest beds of these limestones accessible. It has proved to be almost impossible to subdivide this limestone series by any lithological characters. Throughout most of the two hundred feet the limestones are nodular and thickly-bedded, and the interbedded shales are usually thin. At some localities. a few limestone bands of more massive type form a horizon that • 4, 15, (See especially the Appendix to II). t 29.81. t Ie,17. These will be referred to in this paper as the CardiU and Bridgend Memoirs respec­ tive1y~ • 2lI, l!lI. The fir&t paper willbe referred to as the CarditJ paper. THE UASSIC ROCKS OF GLAMORGAN. 249 can be traced along the coast, sometimes for a few miles, but it is rarely possible to trace such a band unless there are con tinuous exposures unbroken by faults. On the whole, the series is more calcareous in the west, and nodularlimestones with inter­ bedded shales become more massive in appearance as they are traced westwards. Further, what appears to be a massive limestone band when exposed in a freshly-broken cliff face is usually seen to be made up of nodular limestones when it is weathered in the cliff or on the beach (see Plate SA). About 80 feet above the base of the upper limestone series the limestones are less massive and the series becomes more argillaceous, and it is usually possible to identify this horizon in the cliffs. Similar changes at other levels, however, are much less constant when traced along the cliffs for a few miles. The changes are indicated in the vertical sections in figure 56. Even approximate correlation by lithological characters is thus rarely possible in Glamorgan. In these circumstances it is, therefore, unfortunate that ammonites and other fossils are by no means common in these beds.