The Student's Elements of Geology Part 20

But to whatever combination of causes we attribute the great Alpine lakes one thing is clear, namely, that they are, geologically speaking, of modern origin.

Every one must admit that the upper valley of the Rhone has been chiefly caused by fluviatile denudation, and it is obvious that the quant.i.ty of matter removed from that valley previous to the glacial period would have been amply sufficient to fill up with sediment the basin of the Lake of Geneva, supposing it to have been in existence, even if its capacity had been many times greater than it is now. (See Principles volume 1 page 420 10th edition 1867.)

On the whole, it appears to me, in accordance with the views of Professor Ramsay, M. Mortillet, Mr. Geikie, and others, that the abrading action of ice has formed some mountain tarns and many morainic lakes; but when it is a question of the origin of larger and deeper lakes, like those of Switzerland or the north of Italy, or inland fresh-water seas, like those of Canada, it will probably be found that ice has played a subordinate part in comparison with those movements by which changes of level in the earth's crust are gradually brought about.

TERTIARY OR CAINOZOIC PERIOD.

CHAPTER XIII.

PLIOCENE PERIOD.

Glacial Formations of Pliocene Age.

Bridlington Beds.

Glacial Drifts of Ireland.

Drift of Norfolk Cliffs.

Cromer Forest-bed.

Aldeby and Chillesford Beds.

Norwich Crag.

Older Pliocene Strata.

Red Crag of Suffolk.

Coprolitic Bed of Red Crag.

White or Coralline Crag.

Relative Age, Origin, and Climate of the Crag Deposits.

Antwerp Crag.

Newer Pliocene Strata of Sicily.

Newer Pliocene Strata of the Upper Val d'Arno.

Older Pliocene of Italy.

Subapennine Strata.

Older Pliocene Flora of Italy.

It will be seen in the description given in the last chapter of the Post- pliocene formations of the British Isles that they comprise a large proportion of those commonly termed glacial, characterised by sh.e.l.ls which, although referable to living species, usually indicate a colder climate than that now belonging to the lat.i.tudes where they occur fossil. But in parts of England, more especially in Yorks.h.i.+re, Norfolk, and Suffolk, there are superficial formations of clay with glaciated boulders, and of sand and pebbles, containing occasional, though rare, patches of sh.e.l.ls, in which the marine fauna begins to depart from that now inhabiting the neighbouring sea, and comprises some species of mollusca not yet known as living, as well as extinct varieties of others, ent.i.tling us to cla.s.s them as Newer Pliocene, although belonging to the close of that period and chronologically on the verge of the later or Post-pliocene epoch.

BRIDLINGTON DRIFT.

To this era belongs the well-known locality of Bridlington, near the mouth of the Humber, in Yorks.h.i.+re, where about seventy species or well-marked varieties of sh.e.l.ls have been found on the coast, near the sea-level, in a bed of sand several feet thick resting on glacial clay with much chalk debris, and covered by a deposit of purple clay with glaciated boulders. More than a third of the species in this drift are now inhabitants of arctic regions, none of them extending southward to the British seas; which is the more remarkable as Bridlington is situated in lat.i.tude 54 degrees north. Fifteen species are British and Arctic, a very few belong to those species which range south of our British seas. Five species or well-marked varieties are not known living, namely, the variety of Astarte borealis (called A. Withami); A. mutabilis; the sinistral form of Tritonium carinatum, Cardita a.n.a.lis, and Tellina obliqua, Figure 120. Mr. Searles Wood also inclines to consider Nucula Cobboldiae, Figure 119, now absent from the European seas and the Atlantic, as specifically distinct from a closely-allied sh.e.l.l now living in the seas surrounding Vancouver's Island, which some conchologists regard as a variety. Tellina obliqua also approaches very near to a sh.e.l.l now living in j.a.pan.

GLACIAL DRIFT OF IRELAND.

Marine drift containing the last-mentioned Nucula and other glacial sh.e.l.ls reaches a height of from 1000 to 1200 feet in the county of Wexford, south of Dublin. More than eighty species have already been obtained from this formation, of which two, Conovulus pyramidalis and Na.s.sa monensis, are not known as living; while Turritella incra.s.sata and Cypraea lucida no longer inhabit the British seas, but occur in the Mediterranean. The great elevation of these sh.e.l.ls, and the still greater height to which the surface of the rocks in the mountainous regions of Ireland have been smoothed and striated by ice-action, has led geologists to the opinion that that island, like the greater part of England and Scotland, after having been united with the continent of Europe, from whence it received the plants and animals now inhabiting it, was in great part submerged.

The conversion of this and other parts of Great Britain into an archipelago was followed by a re-elevation of land and a second continental period. After all these changes the final separation of Ireland from Great Britain took place, and this event has been supposed to have preceded the opening of the straits of Dover. (See Antiquity of Man chapter 14.)

DRIFT OF NORFOLK CLIFFS.

(FIGURE 116. Tellina balthica (T. solidula).)

There are deposits of boulder clay and till in the Norfolk cliffs princ.i.p.ally made up of the waste of white chalk and flints which, in the opinion of Mr.

Searles Wood, jun., and others, are older than the Bridlington drift, and contain a larger proportion of sh.e.l.ls common to the Norwich and Red Crag, including a certain number of extinct forms, but also abounding in Tellina balthica (T. solidula, Figure 116), which is found fossil at Bridlington, and living in our British seas, but wanting in all the formations, even the newest, afterwards to be described as Crag. As the greater part of these drifts are barren of organic remains, their cla.s.sification is at present a matter of great uncertainty.

They can nowhere be so advantageously studied as on the coast between Happisburgh and Cromer. Here we may see vertical cliffs, sometimes 300 feet and more in height, exposed for a distance of fifty miles, at the base of which the chalk with flints crops out in nearly horizontal strata. Beds of gravel and sand repose on this undisturbed chalk. They are often strangely contorted, and envelop huge ma.s.ses or erratics of chalk with layers of vertical flint. I measured one of these fragments in 1839 at Sherringham, and found it to be eighty feet in its longest diameter. It has been since entirely removed by the waves of the sea. In the floor of the chalk beneath it the layers of flint were horizontal. Such erratics have evidently been moved bodily from their original site, probably by the same glacial action which has polished and striated some of the accompanying granitic and other boulders, occasionally six feet in diameter, which are imbedded in the drift.

CROMER FOREST-BED.

Intervening between these glacial formations and the subjacent chalk lies what has been called the Cromer Forest-bed. This buried forest has been traced from Cromer to near Kessingland, a distance of more than forty miles, being exposed at certain seasons between high and low water mark. It is the remains of an old land and estuarine deposit, containing the submerged stumps of trees standing erect with their roots in the ancient soil. a.s.sociated with the stumps and overlying them, are lignite beds with fresh-water sh.e.l.ls of recent species, and laminated clay without fossils. Through the lignite and forest-bed are scattered cones of the Scotch and spruce firs with the seeds of recent plants, and the bones of at least twenty species of terrestrial mammalia. Among these are two species of elephant, E. meridionalis, Nesti, and E. antiquus, the former found in the Newer Pliocene beds of the Val d'Arno, near Florence. In the same bed occur Hippopotamus major, Rhinoceros etruscus, both of them also Val d'Arno species, many species of deer considered by Mr. Boyd Dawkins to be characteristic of warmer countries, and also a horse, beaver, and field-mouse.

Half of these mammalia are extinct, and the rest still survive in Europe. The vegetation taken alone does not imply a temperature higher than that now prevailing in the British Isles. There must have been a subsidence of the forest to the amount of 400 or 500 feet, and a re-elevation of the same to an equal extent in order to allow the ancient surface of the chalk or covering of soil, on which the forest grew, to be first covered with several hundred feet of drift, and then upheaved so that the trees should reach their present level.

Although the relative antiquity of the forest-bed to the overlying glacial till is clear, there is some difference of opinion as to its relation to the crag presently to be described.

CHILLESFORD AND ALDEBY BEDS.

(FIGURE 117. Natica helicoides, Johnson.)

It is in the counties of Norfolk, Suffolk, and Ess.e.x, that we obtain our most valuable information respecting the British Pliocene strata, whether newer or older. They have obtained in those counties the provincial name of "Crag,"

applied particularly to ma.s.ses of sh.e.l.ly sand which have long been used in agriculture to fertilise soils deficient in calcareous matter. At Chillesford, between Woodbridge and Aldborough in Suffolk, and Aldeby, near Beccles, in the same county, there occur stratified deposits, apparently older than any of the preceding drifts of Yorks.h.i.+re, Norfolk, and Suffolk. They are composed at Chillesford of yellow sands and clays, with much mica, forming horizontal beds about twenty feet thick. Messrs. Prestwich and Searles Wood, senior, who first described these beds, point out that the sh.e.l.ls indicate on the whole a colder climate than the Red Crag; two-thirds of them being characteristic of high lat.i.tudes. Among these are Cardium Groenlandic.u.m, Leda limatula, Tritonium carinatum, and Scalaria Groenlandica. In the upper part of the laminated clays a skeleton of a whale was found a.s.sociated with casts of the characteristic sh.e.l.ls, Nucula Cobboldiae and Tellina obliqua, already referred to as no longer inhabiting our seas, and as being extinct varieties if not species. The same sh.e.l.ls occur in a perfect state in the lower part of the formation. Natica helicoides (Figure 117) is an example of a species formerly known only as fossil, but which has now been found living in our seas.

At Aldeby, where beds occur decidedly similar in mineral character as well as fossil remains, Messrs. Crowfoot and Dowson have now obtained sixty-six species of mollusca, comprising the Chillesford species and some others. Of these about nine-tenths are recent. They are in a perfect state, clearly indicating a cold climate; as two-thirds of them are now met with in arctic regions. As a rule, the lamellibranchiate molluscs have both valves united, and many of them, such as Mya arenaria, stand with the siphonal end upward, as when in a living state.

Tellina balthica, before mentioned (Figure 116) as so characteristic of the glacial beds, including the drift of Bridlington, has not yet been found in deposits of Chillesford and Aldeby age, whether at Sudbourn, East Bavent, Horstead, Coltishall, Burgh, or in the highest beds overlying the Norwich Crag proper at Bramerton and Thorpe.

NORWICH OR FLUVIO-MARINE CRAG.

(FIGURE 118. Mastodon arvernensis, third milk molar, left side, upper jaw; grinding surface, natural size. Norwich Crag, Postwick, also found in Red Crag, see below.)

The beds above alluded to ought, perhaps, to be regarded as beds of pa.s.sage between the glacial formations and those called from a provincial name "Crag,"

the newest member of which has been commonly called the "Norwich Crag." It is chiefly seen in the neighbourhood of Norwich, and consists of beds of incoherent sand, loam, and gravel, which are exposed to view on both banks of the Yare, as at Bramerton and Thorpe. As they contain a mixture of marine, land, and fresh- water sh.e.l.ls, with bones of fish and mammalia, it is clear that these beds have been acc.u.mulated at the bottom of a sea near the mouth of a river. They form patches rarely exceeding twenty feet in thickness, resting on white chalk. At their junction with the chalk there invariably intervenes a bed called the "Stone-bed," composed of unrolled chalk-flints, commonly of large size, mingled with the remains of a land fauna comprising Mastodon arvernensis, Elephas meridionalis, and an extinct species of deer. The mastodon, which is a species characteristic of the Pliocene strata of Italy and France, is the most abundant fossil, and one not found in the Cromer forest before mentioned. When these flints, probably long exposed in the atmosphere, became submerged, they were covered with barnacles, and the surface of the chalk became perforated by the Pholas crispata, each fossil sh.e.l.l still remaining at the bottom of its cylindrical cavity, now filled up with loose sand from the inc.u.mbent crag. This species of Pholas still exists, and drills the rocks between high and low water on the British coast. The name of "Fluvio-marine" has often been given to this formation, as no less than twenty species of land and fresh-water sh.e.l.ls have been found in it. They are all of living species; at least only one univalve, Paludina lenta, has any, and that a very doubtful, claim to be regarded as extinct.

(FIGURE 119. Nucula Cobboldiae.)

(FIGURE 120. Tellina obliqua.)

Of the marine sh.e.l.ls, 124 in number, about 18 per cent are extinct, according to the latest estimate given me by Mr. Searles Wood; but, for reasons presently to be mentioned, this percentage must be only regarded as provisional. It must also be borne in mind that the proportion of recent sh.e.l.ls would be augmented if the uppermost beds at Bramerton, near Norwich, which belong to the most modern or Chillesford division of the Crag, had been included, as they were formerly, by Mr. Woodward and myself, in the Norwich series. Arctic sh.e.l.ls, which formed so large a proportion in the Chillesford and Aldeby beds, are more rare in the Norwich Crag, though many northern species-- such as Rhynchonella psittacea, Scalaria Groenlandica, Astarte borealis, Panopaea Norvegia, and others-- still occur. The Nucula Cobboldiae and Tellina obliqua, Figures 119 and 120, before mentioned, are frequent in these beds, as are also Littorina littorea, Cardium edule, and Turritella communis, of our seas, proving the littoral origin of the beds.

OLDER PLIOCENE STRATA.

RED CRAG.

(FIGURE 121. Section through (left) sea, Red Crag, London Clay and Chalk (right).)

Among the English Pliocene beds the next in antiquity is the Red Crag, which often rests immediately on the London Clay, as in the county of Ess.e.x, ill.u.s.trated in Figure 121.

It is chiefly in the county of Suffolk that it is found, rarely exceeding twenty feet in thickness, and sometimes overlying another Pliocene deposit, the Coralline Crag, to be mentioned in the sequel. It has yielded-- exclusive of 25 species regarded by Mr. Wood as derivative-- 256 species of mollusca, of which 65, or 25 per cent, are extinct. Thus, apart from its order of superposition, its greater antiquity than the Norwich and glacial beds, already described, is proved by the greater departure from the fauna of our seas. It may also be observed that in most of the deposits of this Red Crag, the northern forms of the Norwich Crag, and of such glacial formations as Bridlington, are less numerous, while those having a more southern aspect begin to make their appearance. Both the quartzose sand, of which it chiefly consists, and the included sh.e.l.ls, are most commonly distinguished by a deep ferruginous or ochreous colour, whence its name. The sh.e.l.ls are often rolled, sometimes comminuted, and the beds have much the appearance of having been s.h.i.+fting sand- banks, like those now forming on the Dogger-bank, in the sea, sixty miles east of the coast of Northumberland. Cross stratification is almost always present, the planes of the strata being sometimes directed towards one point of the compa.s.s, sometimes to the opposite, in beds immediately overlying. That such a structure is not deceptive or due to any subsequent concretionary rearrangement of particles, or to mere bands of colour produced by the iron, is proved by each bed being made up of flat pieces of sh.e.l.l which lie parallel to the planes of the smaller strata.

(FIGURE 122. Purpura tetragona, Sowerby; natural size.)

(FIGURE 123. Voluta Lamberti, Sowerby. Variety characteristic of Suffolk Crag.

Pliocene.)

(FIGURE 124. Voluta Lamberti, young individual, Cor. and Red Crag.)

It has long been suspected that the different patches of Red Crag are not all of the same age, although their chronological relation can not be decided by superposition. Separate ma.s.ses are characterised by sh.e.l.ls specifically distinct or greatly varying in relative abundance, in a manner implying that the deposits containing them were separated by intervals of time. At Butley, Tunstall, Sudbourn, and in the Red Crag of Chillesford, the mollusca appear to a.s.sume their most modern aspect when the climate was colder than when the earliest deposits of the same period were formed. At Butley, Nucula Cobboldiae, so common in the Norwich and certain glacial beds, is found, and Purpura tetragona (Figure 122) is very abundant. On the other hand, at Walton-on-the-Naze, in Ess.e.x, we seem to have an exhibition of the oldest phase of the Red Crag; and a warmer climate seems indicated, not only by the absence of many northern forms, but also by the abundance of some now living in the British seas and the Mediterranean. Voluta Lamberti (see Figures 123 and 124), an extinct form, which seems to have flourished chiefly in the antecedent Coralline Crag period, is still represented here by individuals of every age.