Chap11

[152]

CHAPTER XI.

NEW RED SANDSTONE AND MARL, AND RHÆTIC BEDS.

THE NEW RED SANDSTONE SERIES, or TRIAS, succeeds the Permian strata. It has received the name of Trias from the fact that when fully developed, as in Germany, it consists of the three great divisions of Keuper marls, Muschellkalk, and Bunter Sandstein. Comparatively few genera and no species of bivalve shells pass thus far upwards. The majority of the old genera of Brachiopoda disappear, and the whole grouping of the fossils now ceases to be Palæozoic, and assumes a character common to the Mesozoic or Secondary strata. The British section, with the exception of the Muschel kalk, is as follows :

These beds, with variations, occupy the undulating lands from Devonshire along the banks of the Severn, round the eastern borders of the Palaeozoic rocks of Herefordshire and North Wales. From thence they stretch eastward to the Permian and Carboniferous rocks of Lancashire, North Staffordshire, and Derbyshire.

[New Red Series. 153]

They surround all the midland coal-fields and Permian beds between Shrewsbury, Coventry, and Derby, and from thence, everywhere unconformably overlying the Permian rocks, they stretch north in a long band from Nottingham to the river Tees.¹ The general arrangement of these strata will be easily understood by help of the diagram, p. 154.

No fossils are known in the New Red or Bunter Sandstones of England, but a few marine shells are found in equivalent strata on the Continent.

In England, above the Upper soft red sandstone are beds of red, white, and brown (Keuper) sandstone, with interstratifications of red marl, often ripple-marked, and containing bones and footprints, chiefly of Labyrinthodont reptiles, together with a few plants and a peculiar fish, Dipteronotus cyphus, found near Bromsgrove, in Worcestershire. The larger impressions of footprints are 8 to 10 inches in length, and in front of each there often is a smaller one made by the forefoot, fig. 33.

In beds of Magnesian conglomerate at the base of, and associated with the New Red Marl at the edge of the Mendip Hills, Dr. Riley and Mr. Stutchbury discovered

¹ The Muschelkalk (absent in Britain) may be well seen, among other places, near Gotha, and at Eisenacli, in Thuringia. It is a grey shelly limestone, rich in Terebratulœ, Trigoniœ, Myœ, Plagiostomas, Aviculœ, Oysters, and Pectens. The genus Ceratites, closely allied to, if not a true Ammonite, occurs here. Lamellibranchiate molluscs, some of new genera, abound as individuals, while Brachiopoda (excepting Terebratulœ) sink in the scale.

At Guttenstein and Werfen, in the Austrian Alps, there are strata at the base of the New Red Sandstone which are not Permian, and which contain a rich and peculiar fauna—Ammontes, Belemltites, and other secondary forms, being mixed with Orthoceratites, Goniatites, and other genera usually considered characteristic of Palaozoic times.

[154 New Red Series.]

FIG. 32.

Typical section across the New Red series, Shropshire, showing their relation to the Permian and Coal-measure formations.

[Rock Salt. 155]

the bones of land lizards, Thecodontosaurus antiquus, Palœosaurus Cylindrodon, and P. Platyodon.

The rock salt of England lies above these beds in the great many plains of Lancashire, Cheshire, and Worcestershire. It is found at varying depths, in interrupted lenticular beds, ranging from a few feet to about 120 feet in thickness. No fossils occur in the salt. The mass is usually of a reddish colour, due to the presence of ferruginous impurities.

For long there was a total absence of any rational account of the manner of deposition of rock-salt, but I think few geologists now doubt that it was precipitated in supersaturated salt lakes during the Keuper period; and this could only have been done by evaporation, due to solar heat acting on the waters of salt lakes which had no outflow, like the Great Salt Lake of Utah, for example, or the salt lakes of Central Asia and of the Sahara.¹ The red marl varies from 500 to 2,000 feet in thickness, and contains a thin band of white sandstone, often with pseudomorphs of crystals of rock-salt, and also bearing a small bivalve crustacean, Estheria minuta, a lamellibranchiate small bivalve shell, Pullastra arenicola, a fish, Hybodus Keuperi, footprints of Labyrinthodon giganteus, and others, also bones of reptiles, and traces of land plants, fig. 33. Teeth also of a small Marsupial mammal, Microlestes antiquus, occur in the red marls near Watchett in Somersetshire. This is the oldest known mammalian relic. In Scotland, at Lossiemouth, Keuper sandstones contain scutes and bones of a crocodile, Stagenolepis Robertsoni, Hyperodapedon, and a land lizard, Telerpeton Elginense.²

¹ See memoir 'On the Physical Relations of the New Red Marl Rhætic Beds, and Lower Lias:' Geological Journal, 1871: Ramsay.

² On the Continent, near Strasburg, about thirty species of plants

[156 Physical Geography.]

On the whole, the same kind of arguments already applied to the Permian strata, may, with increased force, be used in relation to the New Red Sandstone and marl, especially the occurrence of rock-salt, gypsum, the red colour of the rocks, and the prevalence of the footprints and bones of Labyrinthodont Amphibia, and the remains of crocodiles, land lizards, Deinosauria, and plants. To me there remains no trace of a doubt that the New Red Sandstone was deposited in an inland lake, or lakes, possibly fresh, but probably brackish, and that

FIG. 33.

Triassic Fossils.

the overlying Keuper or New Red Marl beds were formed in a great salt lake, or lakes, if we take all Europe into account.

But inferences still more striking may be drawn respecting the Physical Geography of the time.

By referring to the descriptions of the Old Red

are known in the Bunter beds, chiefly Ferns, Calamites, (Jycads, and Conifer, and with them fish and Labyrinthodont arnphibia, and marine mollusca of the genera Tigonia, Mya, Mytilus, and Posidonia, so few in number, that in connection with the Labyrinthodonts, &c., they suggest the idea not of an open ocean, but of a salt lake. Teeth of a Marsupial mammal (Microlestes antiquus) occur in a bed between the Keuper and Liassic strata in Würtemburg.

[Physical Geography. 157]

Sandstone, Carboniferous, Permian, and New Red formations, it will be seen that, by the writer, they are all considered to afford evidence of continental as opposed to purely marine conditions; for the Old Red Sandstone was deposited in fresh water, the Coal-measures, whether below, interstratified with, or above the Carboniferous Limestone, on the edges of, and to a great extent on, a continent with large rivers, marshes, and beds of peat, and the Permian and New Red series both in salt lakes; in other words, a great continental epoch in Northern Europe (and in other regions), lasted from the close of the Upper Silurian epoch down to the end of the deposition of the New Red .Marl, one main feature of which was the abundance of reptilian life, partly Amphibian. Those parts of it in which the Permian and New Red strata were deposited can be best compared physically to the great area of inland drainage of Central Asia, so dry and arid where not artificially irrigated by rivers, and in which, from the Caspian Sea for 3,000 miles to the east, and far south towards the Himalayah, in a comparatively rainless district, all the lakes are salt, excepting those which have an outlet into some lower lake.

I specially draw attention to these remarkable inferences, for surely they give something like a broad view of an old phase of a long-enduring physical geography, so long, indeed, in my opinion, 'that the great continental era, which began with the Old Red Sandstone and closed with the New Red Marl, is comparable, in point of geological time, to that occupied in the deposition of the whole of the Mesozoic or Secondary series (later than the New Red Marl) and to the whole of the Cainozoic or Tertiary formations, and, indeed, to all the time that has elapsed since the begining

[158 Rhætic Beds.]

of the deposition of the Lias down to the present day.¹

When portions of geological history can be reduced to some such form as this, it seems to possess a kind of human interest in its resemblance more or less to the physical geography of today.²

THE RHÆTIC BEDS occupy only a small space in England, estimated by superficial area; for in general they run in a mere narrow strip between the New Red Marl and the Lower Lias, and in fact form true beds of passage from the Marl to the Liassic strata. To make this statement clear it is necessary to allude to a part of the geology of the Alps and of Italy.

Professor Stoppani has described a series of strata on the river Esino in Italy, which he considers to be equivalent in geological time to the Red Keuper marls north of the Alps. These strata, which he calls the Infra-Lias, contain about 200 species of fossils, chiefly mollusca, with a few Echinodermata and sponges, and at the top lie the well-known beds called the Avicula contorta zone, by Oppel, a name adopted in England for these strata by Dr. Wright, when he separated them from the ordinary beds of the Lias limestone and clay, and correlated them with their continental equivalents.

On the north side of the Tyrolese Alps, the Lower

¹ 'Proceedings of the Royal Society,' No. 152, 1874: Ramsay, 'On the Comparative Value of certain Geological Ages; or, Groups of Formation considered as Items of Geological Time.'

² Though I had often lectured on some of the questions respecting these old lakes and other points connected with the terrestrial conditions of the times, it was not till 1871 that I published anything on the subject in the papers alluded to in notes, and later, in 1874, in the 'Proceedings of the Royal Society.' Little or nothing is to be found in any Manual of Geology on the subject, except in the third edition of 'The Student's Manual of Geology,' by Professor Jukes, edited by Archibald Geikie, published in 1872.

[158 Rhætic Beds. 59]

St. Cassian and Hallstatt beds are believed by Hauer and Suess to represent the same strata; that is to say, they are the ocean representatives of the red marls of England and other parts of Europe, which I described as having been deposited in large inland salt lakes.

The Rhætic beds of England, which merely represent the very topmost part of the Italian series, seem to have been deposited in shallow seas and estuaries, or in lagoons or occasional salt lakes of small size, now and then separated from the sea by minor accidental changes in physical geography.

On the north shore of the estuary of the Severn, at Penarth, near Cardiff,¹ and elsewhere in England, there is a perfect physical gradation between the New Red Marl and the Rhætic Beds, shown by interstratifications of red, green, and grey marls, which, varying in different areas, pass upward by degrees into limestones, sandstones, and black shales. It is, therefore, impossible always to determine in this series precisely where the New Red Marl ends and the Rhætic Beds begin; and, indeed, all through the Red Marl, from bottom to top, there is a tendency to a recurrence of interstratified deposits that, lithologically, closely resemble the lower parts of the Rhetic beds, as, for example, at Penarth, near Cardiff. The 'White Lias' of Lyme Regis is now classed with this subformation.

All over England, wherever the base of the Lower Lias is well seen, the Rhætic beds, rarely more than 50 or 100 feet thick, are found to lie between the Lias and the New Red Marl. As a general rule they are seen to pass conformably and by easy gradations into each other, and they were, indeed, always classed with the Lias, till separated from these strata by Oppel.

¹ The Rhætic strata are sometimes called the Penarth Beds.

[160 Physical Geography.]

The succession of events that, through the Rhætic beds, marked the transition from the New Red Marl to the Lower Lias seems to have been as follows:

In the latter part of the Triassic epoch, as already stated, our Keuper, or New Red Marl, beds were deposited in an area that now forms part of England, and this area was in those days a great salt lake.

This lake gradually got partly filled with sediments, and by-and-by, through change in amount of rainfall, or through increase of heat, it ceased to have an outflow, evaporation being equal to, or greater than, the influx of water. Concentration and precipitation of salts ensued as already explained.

Subsequently, during deposition of the many sediments, by increase of rainfall, or climatic change of temperature, the water became somewhat less salt, but still sufficiently saline, by evaporation of the moisture on wet surfaces, to produce crystals of salt (now pseudomorphs) in sandy layers interstratifled with the marls, together with layers and nodular masses of gypsum, which state of affairs continued up to, and even during, the deposition of recognised Rhætic strata. That Rhætic areas got dried by temporary exposure is certain, for besides the pseudomorphs, sun-cracks are common in the strata.

In our area, sinking of the district took place at or about the time when the lake or lakes got nearly filled with sediment, and a partial influx of the sea over shallow bottoms was the result. The deposits that ensued, accompanied by a small migration of marine forms of life, constitute the Rhætic beds of England.

Many years ago, the late Professor Edward Forbes stated to me that the fauna of the White Limestone of Lyme Regis, then called White Lias, reminded him,

[Rhætic Fossils. 161]

in its assemblage of forms, of the molluscan fauna of the Caspian Sea, which is few in genera and species, and of an abnormal kind, in consequence of the brackish quality of the water. In the Black Sea also, there are misshapen forms, stated by Edward Forbes to be due to the gradual freshening of the water, because of the constant influx of rivers into it, and the current that runs through the Bosphorus into the Mediterranean Sea. Both of these cases relating to continental seas of a lake-like character, bear on the subject in question; especially seeing that these British beds of passage are also comparatively poor in genera and species, and that some of the species, to which special names have been given, are variable or even distorted in form. Others are hard to distinguish from shells common in the Lias, while some also occur in the great Marine Rhætic series of the Continent, and some pass upwards into the ordinary Lias. It is, indeed, difficult not to believe, that some of these forms are in reality abnormal and due to the locally unhealthy quality of the water in which they lived.

Though this volume has little to do with general palæntology, the following account of the fauna bears on these questions, and I therefore give it in some detail. It also helps to show that our Rhætic beds represent a set of local conditions that marked the passage of the Keuper marls into the undoubted Lower Lias, and, indeed, in places it is hard to separate them lithologically.

In these Rhætic beds there are now known two Crustacea, viz. Tropifer lœvis, from one of the Bone beds, and Estheria minuta, first known in the Keuper sandstones, and one Brachiopod, Discina Townshendi, the only one known in our Rhætic strata. Of the

[162 Rhætic Fossils.]

Lamellibranchiate molluscs, Lima prœcursor very much resembles Lima punctata of the Lias; Monotis decussata occurs at the top in thin limestone bands, which some have considered to form part of the Lower Lias. Ostrea fimbriata may possibly be O. irregularis of the Lias, but oysters are so variable in form that they are of small value in such an inquiry. Pecten Valoniensis, also a Rhætic shell, is a very variable form. Plicatula intusstriata passes into the Lower Lias. Anoplopliora musculoides, another Rhætic shell, occurs with Monotis decussata in the thin bands of limestone at the top, which some geologists call Lias. Modiola minima is found both in the Rhætic and Lower Lias strata. Figures of some other well known fossils are shown in fig. 34.

All the Gasteropoda of the Rhætic beds are said to be peculiar to that formation, and the same is the case with the fish; for, many years ago, Sir Philip Egerton declared 'that the beds in question, hitherto considered as belonging to the Lias, must be removed from that formation, inasmuch as they present a series of fishes not only specifically distinct from those of the Lias, but possess, in the Ganoid genera, the heterocerque tail, an organism confined to the fishes which existed anterior to the Lias.'¹ Of the Reptilia, Plesiosaurus costatus, P. Hawkinsii, P. trigonus, and, according to the late Mr. William Sanders, Ichthyosaurus platyodon, are common to these Rhætic beds, and to the basement beds of the Lower Lias. The discovery by Professor Boyd Dawkins of the small Marsupial mammal Microlestes antiquus, in the grey marls at Watchett, in Somersetshire, is not without significance, for it speaks

¹ A Notice on the Occurrence of Triassic Fishes in British Strata;' 'Proceedings of the Geological Society,' 1841, vol. iii., p. 409.

[Rhætic Fossils. 163]

of the neighbourhood of land that bordered the old Triassic lake, and the succeeding shallow Rhætic sea, part of which was the Mendip Hills, 'the home of the Microlestes.'¹

In the beds of passage, from 10 to 50 feet above

FIG. 34.

Group of Rhætic Fossils.

the Bone bed, there are certain thin bands in Gloucestershire, named by the Rev. P. B. Brodie, who

¹ For notices of this old land, see De la Beche 'On the Formation of the Rocks of South Wales and South-West of England'; and Ramsay, 'Denudation of South Wales and the Adjacent Counties of England,' Mem. Geol. Surv. vol. i. 1846; and 'Abnormal Conditions of Secondary Deposits,' &c., by Charles Moore, 'Quarterly Journal Geol. Soc.' vol. xxiii., 1867.

[164 Physical Geography.]

first described them, the 'insect limestones.' The fossilised contents of these bands throw some light on the physical geography of the lands that bordered the waters of the time, for in them have been found numerous elytra and other remains of Beetles, Grasshoppers, Cicadas, Dragonflies, and other neuropterous insects, associated with a fresh-water shell of the genus Cyclas, the shells of Cypris, and with ferns, Cycads, and leaves of Monocotyledonous plants. These beds, therefore, indicate either fresh-water strata, or else the immediate proximity of land, from whence streams washed into the sea insects, fresh-water Crustacea, shells, and land plants.

Sir Charles Lyell remarks that 'the size of the species (of insects) is usually small, and such as taken alone would imply a temperate climate; but many of the associated remains of other classes must lead to a different conclusion." This, however, seems to be explained by a remark long ago made to me by Edward Forbes, who, while working with Captain Graves, during the hydrographical survey of the Ægean Sea, observed that, during heavy rains, vast numbers of insects were washed into the sea, not such as inhabited the low hot shores of the Ægean, but those that lived in the high cool regions of the neighbouring mountains, which, caught in the floods of rain, were washed into rivers and borne onwards to yield food for fishes in the ocean.

In conclusion, if, as I believe, the New Red Marl was deposited in a salt lake, if it be the equivalent in time of the marine Infra-Lias beds of Stoppani in Italy, and of the Lower St. Cassian and Hallstatt beds of Hauer and Suess, then the Avicula contorta beds,

¹ 'Student's Elements of Geology,' p. 351, 1874.

[Physical Geography. 165]

being the natural marine successors of these strata on the Continent of Europe, are in like manner the natural marine successors of the lake-formed sediments of the red Keuper marls, and in reality are true passage beds from those red marls into the Lower Lias; and a candid consideration of the fossil fish, reptiles, shells, insects, and plants of the British Rhætic strata strengthens this view. When the waters of the old lakes were invaded by the sea, a migration of a few marine forms took possession of the old lacustrine area, and this depression gradually proceeding, culminated in the development of the great Liassic fauna, at a time when the old continent was submerged, and the mountain tracts were converted into groups of islands, the shores of which were washed by a broadening Liassic sea.