Contributions to the Physical History of the British Isles. With a dissertation on the origin of Western Europe, and of the Atlantic Ocean. Illustrated by ... coloured maps

Front matter

CONTRIBUTIONS TO TUB PHYSICAL HISTOEY OF THE BEITISH ISLES. WITH A DISSERTATION ON THE ORIGIN OF AVE STERN EUROPE, AND OF THE ATLANTIC OCEAN. illustrated by twenty-seven coloured MAPS. BV EDWARD HULL, M.A., LL.D., F.R.S., DIRECTOR OF THE GEOLOGICAL SURVEY OF IRELAND, AND PROFESSOR OF GEOLOGY IN THE ROYAL COLLEGE OF SCIENCE, DUBLIN. LONDON: EDWARD STANFORD, 55, CHARING CROSS, S.W. 1882. INSCRIPTION. TO MY VALUED CONTINENTAL FRIENDS Db. LAURENT-GUILLAUME DE KONINCK, Professeur (Smerite a l'Universite de Liege ; Dr. FERDINAND ROMER, Geb. Bergratb und Professor an der Universitat, Breslau ; AND Dr. ARNOLD VON LASAULX, Professor der Mineralogie an der Universitat, Bonn ; THESE PAGES ARE RESPECTFULLY AND LOVINGLY INSCRIBED. E. H. Dublin, 29th September, 1882. a

Preface

. I had intended giving the title of " Palæo-Physio graphy of the British Islands " to this volume ; but certain friends in whose judgment I have the most implicit confidence assured me that no one would have the slightest conception from such a title of the contents of my book, and consequently that it would be useful only to adorn the shelves of my publishers, or of such of my private friends as I might feel disposed to present with a copy. I was aware that the term " Physiography " had been invented by an eminent naturalist, and I supposed that, having been popularised through the instrumentality of his pen, the reading public would have had no difficulty in discovering that " Palæo-Physiography " meant the physiography of past geological times. But I was assured that none but a few of the most profound Greek scholars would be able to elucidate the meaning of such a terribly long word ; and as I was aware that, in the first place, one-half of the reading public never learn Greek at all, and that of the other half only about one per cent, retain anything of their knowledge after leaving the halls of alma mater, I waived my predi lections in favour of the name at first proposed, and adopted the more commonplace designation which will be found on the title-page.

PREFACE.

. VI The volume itself, indeed, would never have seen the light had it not been for a fortunate concourse of circumstances which may be, in confidence, revealed to the inquiring public. For some years past I had contemplated the pre paration of a series of maps of the British Isles for the purpose of illustrating, as far as our knowledge and data would allow, the physical geography of successive geological periods ; thus showing the changes of form through which our little isles have passed from their first genesis down to the period in which we have the happiness to live. I was deterred, how ever, by the view of the cost of publication. To carry out my intentions as I desired would require a series of chromo-lithographic plates, which would prove so heavy an expense at the outset that I feared no pub lisher would-be sufiiciently courageous to undertake the cost of publication.

But what I despaired of having done through the ordinary channels I was enabled to accomplish with the aid of the Royal Dublin Society, in whose Transactions these little maps have already made their appearance. A fresh edition has been struck off for these pages, and it will give the reader some idea of the labour and cost of engraving when I state that no fewer than sixty two lithographic stones have been used in their preparation. To myself the labour of preparation of the plates has been one of love ; — a gratification arising from the prospect of carrying out a project I had long enter tained ; and as I have received from various quarters

PREFACE.

. VII some unexpected expressions of approval of this under taking, I have reason to hope that those interested in problems of physical geography will find the book useful. That the idea has occurred to others I am aware from the following interesting fact, which I may here be allowed to register. About a quarter of a century ago, the late Mr. William Longman, the then head of the eminent publishing firm of that name, proposed to my friend, the Rev. Dr. Haughton, Fellow and Professor of Trinity College, Dublin, that he should undertake the preparation of a set of maps of the British Isles, somewhat of the character of those now produced, with a view to publication. Circumstances prevented the execution of this work by one so well qualified to carry it out, but it may be affirmed with some degree of confidence that the time had then scarcely arrived in which the project could have been executed with that degree of exactness now attain able.

Our knowledge of the general geological struc ture of the British Isles has not been very materially extended since that period as far as regards the sur face distribution of the strata ; but another branch of inquiry, that regarding the interior structure, has received very important additions. This is due to the discoveries made during the last quarter of a cen tury by deep borings in search of coal or other minerals, and of water, over extensive districts, but chiefly in the central and south-eastern counties. These borings have been of the greatest interest to geologists, because they have revealed the internal

PREFACE.

. VIII structure of a large extent of country which would otherwise have been the subject only of conjecture, or of geological inference. For the present purpose the details of these borings have been of material use. Certainly without their aid it would have been impos sible for me to have shown with the exactness now attainable the underground limits of the Triassic, Liassic, and Oolitic formations in the direction of the Thames Valley and of the eastern coast, after they have been successively lost sight of beneath the more recent deposits. Amongst those to whom I am indebted for light thrown on the internal structure of the British Isles, there is one name which takes unquestioned pre cedence — that of Mr. R. Godwin-Austen — in whose elaborate essay, published in 1856 by the Geological Society of London, many of the problems since elucidated by experiment were investigated and solved.

In reperusing that essay by the light of sub sequent discoveries, I have been profoundly impressed by the knowledge of details which its author evinces regarding the geological structure of the region of which he treats, and the masterly manner in which, by the handling of these details, he has enabled us to understand the physical features of Western Europe in past times. I feel persuaded that that essay will ever be considered a masterpiece of geological induction. PAGE 1 CONTENTS.

Part I. Chapter

I. INTROD UCTION. HOW WE ENDEAVOUR TO REPRODUCE TO OUR MINDS THE PHY SICAL GEOGRAPHY OF SUCCESSIVE GEOLOGICAL EPOCHS ..

Chapter II. The General Principles To Be Observed In Attempting Restorations Of

OLD SEA-MARGINS. (a) Formation of sedimentary strata— (b) Changes in the characters of strata — (c) Interchange of deposits of organic and mechanical origin — (d) Shelving sea-bottoms and coasts — (e) Shore beds — (/) Discordances of stratification 6

Chapter III. Continuity

: ORGANIC AND TERRESTRIAL. («) Of the grouping of strata and their organisms — (b) Periodic disturbances of strata — (c) Volcanic rocks — (d) Metamorphic rocks 17

Chapter IV

Of THE "PERMANENCY OF CONTINENTS AND OCEANS " .. .. CONTENTS. X

Chapter V. Condition Of The Atlantic And European Areas In Geological Times

PAGE (a) The Laurentian epoch — (b) The Cambrian epoch — (c) The Lower Silurian epoch — (d) The Upper Silurian and Devono-Silurian epochs — (e) The Carboniferous epoch in Europe and the British Isles— (/) The Carboniferous epoch in North America 27

Chapter VI. The Genesis Of The North Atlantic Ocean

(a) Formation of ridges and depressions at the close of the Carboniferous period in Britain and Europe — (b) Ridges and depressions in the western hemisphere — (c) The Alleghanies, their structure — (d) The geological date of formation as proved by the relations of the Richmond coal-basin — Recapitulation of evidence CONTENTS. XI

Chapter XIII. The Glacial Or Post-pliocene Period

1. Epoch, of the Lower Boulder Clay .* («) Three main divisions — (6) The British area during the epoch of the Lower Boulder Clay — (c) Elevation of land — (d) Directions of ice flow 117 2. («) Inter-glacial Epoch : (6) Representative Post-Pliocene series — (c) Distribution of Inter-glacial beds — (d) Amount of submergence CONTENTS. VAGI. 3. Epoch of the Upper Boulder Clay : (a) Changes of physical conditions— (6) Character of the Upper Boulder Clay — (c) Localities — (d) North and east of England — (e) Ireland — (/) Scotland — (a) Local moraines of this epoch — (h) Gradual disappearance of Arctic conditions and advent of man 128 General Conclusions ERRATA IN PLATES. Plate III., Fig. 1. — The tinted portion along the north shore of Galway bay ought to be white. Plate X., Fig. 1. — The portions tinted yellow in the N.E. of Ireland ought to be white.

ILLUSTRATIONS. WOODCUTS. PAGE Fig. 1.— Sketch Map of the British Isles 40 „ 2. — Diagram explanatory of the Geological Structure of the United States between the Atlantic and Mississippi 48 PLATES. (Maps of the British Isles.) Plate I. Fig. 1. — Geological Map of the Laurentian Period. 2. — Distribution of land and sea. II. „ 1. — Geological Map of the Cambrian Period. „ 2. — Distribution of land and sea. III. „ 1. — Geological Map of the Lower Silurian Period. 2. — Distribution ofland and sen. „ IV. „ 1. — Geological Map of the Upper Silurian and Devono-Silurian Periods. „ 2. — Distribution of land and sea. „ V. ,, 1. — Geological Map of the Lower and Middle Devonian Periods. „ 2. — Distribution of land and sea. VI. „ 1. — Geological Map of the Old Red Sandstone and Lower Carboniferous Periods. ,, 2. — Distribution of land and sea.

VII. „ 1. — Geological Map of the Upper Carboniferous Period. „ 2. — Distribution of land and water. n VIII. „ 1.— Geological Map ofthe Permian Period. „ 2.— Distribution of land and sea. ILLUSTRATIONS. XVI Plate IX. Fig, 1. — Geological Map of the Triassic Period. 2.— Distribution of land and water. 1. — Geological Map of the Jurassic Period. 2. — Distribution of land and sea. „ X. 1. — Geological Map of the Cretaceous Period. 2. — Distribution of land and sea. XI. „ XIL 1. — Geological Map of the Lower and Middle Tertiary Period. 2. — Distribution of land and sea. „ XIII. ,, XIV. 1. — Physical Map of the Lower Glacial Stage. 1. — Physical Map of the Inter-glacial Stage. 2. — Physical Map of the Upper Glacial Stage. CONTRIBUTIONS to the PHYSICAL HISTOEY OF THE BRITISH ISLES.

PART I. CHAPTER

I. INTRODUCTORY. HOW WE ENDEAVOUR TO REPRODUCE TO OUR MINDS THE PHYSICAL GEOGRAPHY OF SUCCESSIVE GEO- LOGICAL EPOCHS. Geology is the geography of the past. As the geographer of the present day seeks both by the pen and pencil to produce a vivid picture of the physical features of some country, and to portray the characters and habits of the animals and plants which inhabit it, so is it the aim of the geologist to repro duce by similar agencies the past physical features of the earth's surface, to restore the outlines of land and sea, to recall the position of the mountains, valleys, and submerged depressions, and to re-people the several parts with the by-gone forms of the fauna and flora of successive periods. In this way, were i; our means of information sufficient, we might present for any special country, such as that of the British Isles, a series of pictures, representing from very early times the various phases of surface-sculpturing which it has undergone, and the changes which have succeeded each other in the animal and plant life, whether aqueous or terrestrial, of the corresponding periods.

It will be evident from the above statement that the subject of Palæo-physiography (or the geography of past geological times), divides itself naturally into two branches, one dealing with the physical, the other with the vital characters of special regions. For a considerable portion of the earth's surface, palæontologists have succeeded in reproducing the groups of animals and plants which have peopled the waters and decorated the lands of these regions at successive stages of by-gone time. Perhaps of all the attempts in this direction none have been so successful as that of Dr. Oswald Heer, who has graphically recalled for us the extinct fauna and flora of Switzerland, tracing them down from forms long passed away to those immediately preceding, and closely allied to, those of our own times.

With no less skill has he also endeavoured to bring to our minds the successive changes which the features of that wonderfully diversified country have undergone, corresponding to the changes observed in the animal and plant life. American geologists have also done much in the same direction, and with a firm hand and appreciative eye, have portrayed the examples OP THE BRITISH ISLES. of rock-sculpturing and illustrations of surface change which nature has so profusely disclosed amongst the mountains, plains, and canons of the Far West. Still, even in the restoration of the past life-history of any special region much remains to be done, from the fact that our knowledge of the animals and plants of geological periods is largely restricted to those forms which inhabited the lakes and seas, and whose remains are entombed in the strata that were deposited beneath their waters.

Of the land plants and animals our knowledge is com paratively limited, and depends on such fragmentary remains as may happen to have been drifted out to sea by streams or currents, subsequently pre served, and ultimately brought to the notice of the observer* The land forms of animal life during past geological periods, are probably destined ever to remain to a great extent unknown to us, and we must be fain to content ourselves with the rich, and often profuse, assemblages of the shells and skeletons of aqueous forms which are preserved for our study in the strata of the earth's crust. Being thus driven back by the force of circum stances from attempts to restore in imagination the fauna of the land of geological times, may we not essay to accomplish something in the way of physical restoration ?

Here surely we can find some little solace in our disappointment ; and, foregoing a line of * The small proportion ofland forms dredged up in the seas around the coasts of the British Isles, shows how entirely unrepresentative of such forms may be the fossil contents of the strata. B study from which nature has remorselessly debarred us, take up another for which she has furnished more ample materials.* If we are unable to people the lands ofthe Carboniferous, Triassic, Jurassic, and more recent periods with their successive forms of animal life, may we not, at least, attempt to ascertain where the lands of those periods lay over and around such a region (for instance) as that ofthe British Isles; where were the respective margins of land and sea, and where the areas deeply submerged, or, on the other hand, greatly elevated ?

Or, going into a still wider field, may we not seek to ascertain from the method of such an inquiry, whether any light is thrown on some of those interesting physical problems which are so intimately connected with biological study, such as the " permanency of oceans and continents," and the position of the lands from which were derived our sedimentary rocks ? Nay, may we not even attempt to represent these details on maps, showing as in those of the present day of the same region, but with * In reference to the question of the imperfection of the palæontological record, especially as regards land animals, Professor Haeckel has pointed to a source of such imperfection not usually recognised. He shows that permanent strata which enclose organic bodies can only be formed during the slow sinking of the ground under water ; but when the submerged floor is being elevated, the strata which may be deposited are liable to be constantly swept away in shallow water.

Hence we have no traces of the organic forms living during periods of elevation, although it can be theoretically proved that the variety of animal and plant life must have increased during these very periods of elevation. For, as new tracts of land are raised above water, new islands are formed. Every new island, however, becomes a centre from which fresh varieties have their origin ; but owing to the causes assigned, their records are lost to us. — 'The History of Creation ' (Schopfungsgeschichte), E. Ray Lankester's Trans., vol. ii. p. 23. OP THE BRITISH ISLES. 5 less detail and often doubtfully, the position of land and water, the heights of the one and the depths of the other, and thus trace through the long past of geological time the various phases of transmutation through which our little section of the globe has passed ?

Such is the task I have set before myself in the following chapters ; but before entering on a de scription of the maps themselves, and the details represented on them, it is necessary, in the first place, that I should state the general principles upon which these physical restorations have been at tempted ; and, in the second, the general conclusions regarding some of the problems above alluded to, which seem to be fairly deducible from these restorations.

CHAPTER II. GENERAL PRINCIPLES TO BE OBSERVED IN ATTEMPTING RESTORATIONS OF OLD

SEA-MARGINS. (a.) Formation of sedimentary strata. — From the re motest period which can be strictly comprehended within the range of geological time, the crust of our earth has been undergoing change. Ever since the consolidation of the crust, and the condensation of aqueous vapour on the surface, a period of whieh we have no record amongst the existing rock-masses of the globe, land and sea have been changing places somewhere or other ; and while this has been going on, the land surfaces in all districts, where rain has fallen and rivers have flowed, have been subjected to a process of waste, known amongst geologists by the term " denudation." The continuous action of rain, rivers, waves acting on coasts, snow and ice, and even winds, has been to denude (or lay bare) more and more of the land surface, and to transport the derivative material, in the form of gravel, sand, or mud, into the adjoining seas.

The distance to which these materials are carried seawards varies greatly. As a general rule, the coarser materials are deposited near the shore, but the finer may be carried by the agency of ocean currents for hundreds or thousands of miles out to sea, before finally settling over the bottom. Nevertheless sediment thus derived is not usually carried into mid-ocean, a fact determined by deep-sea soundings,* but is deposited within a zone of a few hundred miles from the shores We may suppose, however, that in all cases the sediment becomes finer as it increases in distance from its source, and that it tends to thin away and disappear in the direction of mid-ocean. The converse of this is (within certain limits) equally true, namely, that the sediment will tend to increase in thickness in the direction of its source ; so that we have here a most valuable guide for the determination of the directions towards which we are to look for the lands which were the sources of strata during any special geological period.

If, in the case of any formation composed of strata formed under water — such as sandstone, shale, and their varieties — we find that it gradually becomes attenuated towards the south, or, conversely, that it swells out in vertical dimensions in a northerly direction, we have some evidence at least (if not always conclusive) that the land of the period from which the sediment was primarily derived lay to the north of its position. The above is a principle of very wide application, * First shown by Dr. Wallich from the soundings made in the Atlantic preparatory to laying the Atlantic telegraph cable; more recently by those of the Challenger expedition under Captain Nares. -f* In some cases the distance is much greater, as, for instance, in that of the Amazon, the muddy water from which discolours the ocean at the surface for several hundred miles, and by the time the mud subsides, it must reach to much greater distances. and we shall find when dealing with special formations over the area of the British Isles that it has numerous illustrations.

It has been ascertained, for instance, that some formations, such as the Upper and Middle Carboniferous, persistently become attenuated in certain definite directions, and tend to swell out in those opposite thereto ; thus pointing in a manner not to be mistaken to the position of the old Carboni ferous lands. (J>.) Changes in the characters of strata. — In addition to strata of strictly sedimentary origin, such as sandstones, slates, shales, clays, and their varieties, derived by mechanical processes from the waste of land, geological formations are often composed entirely or in part of limestones, the origin of which is widely different from that of the former class of rocks. In general a close examination of specimens, especially if aided by the microscope, will suffice to convince the observer that both the material of these rocks and their structure is essentially different from those previously described.

They are found to be com posed of carbonate of lime, sometimes in combination with carbonate of magnesia, and other ingredients not derivative in the form of sediment from the waste of lands. Further examination will in most cases tend to convince him that they are essentially organic in structure.* They show structures corresponding in form to those of shells and skeletons of * Except in the case of metamorphic limestones, which have undergone a process of transformation, owing to which the original organic structure has been changed into one more or less crystalline. The marbles of Carrara, Pentelicus, &c, are illustrations of the kind. molluscs, corals, crinoids, star-fishes, echini, foraminifera, &c, such as inhabit the waters of the sea or ocean, and often form the material of its bed over the central portions, far out of reach of muddy or sandy sediment.

The examination of thin trans lucent sections of the limestones under the microscope will often disclose organic structure where none is apparent to the eye even when aided by the lens ; and the most dense and compact portions will be found to display the forms of foraminifera, together with fragments of shells, corals, or sponges ; the cementing material being impalpable calcareous mud derived from the decay, or attrition, of similar bodies.* Viewed as a whole, it becomes evident that such limestone formations must have been built up in a manner altogether different from that under which the sedimentary strata were accumulated. They are essentially organic in structure, and must therefore have been constructed through organic agency. They are more or less free from sediment — sometimes absolutely so — from which we infer that they have been deposited far from land, and in pure transparent waters.

They resemble the calcareous ooze of the mid-Atlantic ocean, or the material in course of formation inside the walls of an atoll, or of a barrier reef; allowance being made for the differences in the forms of the animal structures belonging to past geological times as compared with those now living in the sea or ocean. Wherever, therefore, we find * Spicules of sponges, shields of Polycistineæ, and other siliceous forms are frequently associated with those of calcareous composition. great beds of limestone of hundreds or thousands of 'feet in thickness, not associated with beds of sand stone, shale, or clay, we may feel sure that they were formed far out from the lands of the period to which they belong, and are more or less deep-sea deposits. (c.) Interchange of deposits of organic and mechanical origin. — If we confine our attention for a moment to any considerable oceanic region, such as that of the North Atlantic, it will be evident on consideration that very dissimilar deposits are being contemporaneously laid down over different parts of its bed.

Thus in the central portions calcareous mud formed of the shells and skeletons of marine animals, chiefly foraminiferal, is in course of deposition ; while round the coasts and opposite the mouths of rivers, muddy and sandy sediment passing landwards into shingle is being laid down. But between these two regions, the central calcareous and the marginal sedimentary, there lies a "debatable land," so to speak, of varied materials constantly mingling or inosculating, and probably of considerable breadth in some places. Considered in relation to vital action, a perpetual struggle between the organic and inorganic agencies is in progress over this area, arising from the in jurious effects of sandy or muddy sediment on the vitality of many of the forms which may be called " limestone builders." It is known from the observations of Ehrenberg, Darwin, and Jukes, that most of the existing polyps are destroyed by tbe presence of sediment in the sea, and that it is only in clear uncontaminated waters that they are capable of carrying on their vital functions.

The same obser vation applies with more or less force to other forms of marine life, such as crinoids and foraminifera, and in a still less degree to some classes of the mollusca.* Hence we must suppose that in the border zone between the regions of calcareous and sedimentary deposits, there is a constant advance and retreat on the part of the limestone builders according as the waters of the sea become more or less pure. Now, such changes in the character of the waters over the tract referred to, must from time to time recur in consequence of changes in the physical geography of the adjoining land and sea areas. In some places the land is gaining on the sea, and the areas of drainage are being proportionately extended ; in such cases the amount of sediment carried down into the sea will tend to augment, and will influence the character of the waters, which in turn will have a reflex action on the deposits going on over the sea bed.

Other causes, such as the temporary or per manent deflection of tides and currents, will bring about similar results, and according as the sea water so affected becomes favourable or otherwise, organic action will be quickened or retarded. Upon such principles we may explain the occurrence of alternate beds of limestone or shale and sandstone amongst some formations, or the entire * It is true that foraminifera and crinoids are found alive near our coasts, but the extent to which they contributed to the structure of the great and pure limestone masses — such as those of the Carboniferous, Cretaceous, and Tertiary periods— show that they flourished only where the waters of the sea were free from sediment. replacement of strata of one kind by those of another ; as was the case when the Carboniferous limestone gave place to the shales and sandstones of the Middle Carboniferous group.

Our special purpose here is to note the bearing of these changes on the question of the relative positions of land and sea. It will be clear from what has been said — that in the direction where a limestone formation becomes more or less intermingled with beds of sandstone or shale, or is entirely replaced by them, in that direction may we look for the position of the land whence these materials were derived ; while, on the other hand, where the formation is free from such ingredients we may assume the area of deposition to have been remote from land, or at least free from the presence of mechanical impurities. On such grounds we have reason for believing that during the epoch of the Carboniferous limestone considerable tracts of land existed to the north and west of the British Isles.* {d.) Shelving sea-bottoms and coasts. — On the other hand, it must not be forgotten that all formations tend rapidly to thin away in the direction of any tract of contemporaneous land, and its bordering shelving sea-bottom, though not of sufficient extent to be in itself a source of much, if any, sedimentary materials.

A rock, or island, rising out of deep water, and around which calcareous or sedi mentary strata are being deposited, will necessitate the wedging out of such strata, as the water becomes shallower on approaching its margin ; and if the * Plate Vs. rock or island be of small area, very little alteration will take place in the nature of the deposit up to within a short distance of the shore. Of this we have a good illustration in the case of the old ridges of Lower Palæozoic rocks which stretched across the centre of England during Carboniferous and Permian times, and separated the strata of these periods into two areas of deposition ;* and in the concealed ridge of Palæozoic rocks which occupied the region of the Thames valley and the south-eastern counties of England during the Jurassic period.t Against the shelving shores of these ridges the newer formations successively wedge out, with but very little change in their composition.} (e.) Shore beds. — The presence of beds of shingle or conglomerate generally indicate marginal con ditions.

These are the materials laid down at the mouths of rivers, or broken off, rolled about, and rounded by wave action along the coast of the adjoining land, and strewn over the sea bed to greater or less distances from the shore. Such shore beds are seldom found unless in cases where strata are being formed in the vicinity of rocks which have been tilted and upraised somewhat abruptly in rela tion to the adjoining sea bed. The British formations afford numerous illustrations of such littoral deposits. * Plates VI. and VII. t Plate X. X Thus the Carboniferous limestone of Breedon Hill and Ticknall, near Ashby de la Zouch, preserves its usual character, though only a short distance from the margin of the very old rocks of Charnwood Forest, whicli must have formed a ridge of unsubmerged land, or at least a series of rocky islets, a short distance to the southwards.