A Manual of Elementary Geology Part 20

CHAPTER XI.

NEWER PLIOCENE PERIOD.--BOULDER FORMATION.

Drift of Scandinavia, northern Germany, and Russia--Its northern origin--Not all of the same age--Fundamental rocks polished, grooved, and scratched--Action of glaciers and icebergs--Fossil sh.e.l.ls of glacial period--Drift of eastern Norfolk--a.s.sociated freshwater deposit--Bent and folded strata lying on undisturbed beds--Sh.e.l.ls on Moel Tryfane--Ancient glaciers of North Wales--Irish drift.

Among the different kinds of alluvium described in the seventh chapter, mention was made of the boulder formation in the north of Europe, the peculiar characters of which may now be considered, as it belongs in part to the post-pliocene, and partly to the newer pliocene, period. I shall first allude briefly to that portion of it which extends from Finland and the Scandinavian mountains to the north of Russia, and the low countries bordering the Baltic, and which has been traced southwards as far as the eastern coast of England. This formation consists of mud, sand, and clay, sometimes stratified, but often wholly devoid of stratification, for a depth of more than a hundred feet. To this unstratified form of the deposit, the name of _till_ has been applied in Scotland. It generally contains numerous fragments of rocks, some angular and others rounded, which have been derived from formations of all ages, both fossiliferous, volcanic, and hypogene, and which have often been brought from great distances. Some of the travelled blocks are of enormous size, several feet or yards in diameter; their average dimensions increasing as we advance northwards. The till is almost everywhere devoid of organic remains, unless where these have been washed into it from older formations; so that it is chiefly from relative position that we must hope to derive a knowledge of its age.

Although a large proportion of the boulder deposit, or "northern drift," as it has sometimes been called, is made up of fragments brought from a distance, and which have sometimes travelled many hundred miles, the bulk of the ma.s.s in each locality consists of the ruins of subjacent or neighbouring rocks; so that it is red in a region of red sandstone, white in a chalk country, and grey or black in a district of coal and coal-shale.

The fundamental rock on which the boulder formation reposes, if it consist of granite, gneiss, marble, or other hard stone capable of permanently retaining any superficial markings which may have been imprinted upon it, is smoothed or polished, and usually exhibits parallel striae and furrows having a determinate direction. This direction, both in Europe and North America, is evidently connected with the course taken by the erratic blocks in the same district being north or south, or 20 or 30 degrees to the east or west of north, according as the large angular and rounded stones have travelled. These stones themselves also are often furrowed and scratched on more than one side.

[Ill.u.s.tration: Fig. 110. Limestone polished, furrowed, and scratched by the glacier of Rosenlaui, in Switzerland. (Aga.s.siz.)

_a a._ White streaks or scratches, caused by small grains of flint frozen into the ice.

_b b._ Furrows.]

In explanation of such phenomena I may refer the student to what was said of the action of glaciers and icebergs in the Principles of Geology.[122-A]

It is ascertained that hard stones, frozen into a moving ma.s.s of ice, and pushed along under the pressure of that ma.s.s, scoop out long rectilinear furrows or grooves parallel to each other on the subjacent solid rock. (See fig. 110.) Smaller scratches and striae are made on the polished surface by crystals or projecting edges of the hardest minerals, just as a diamond cuts gla.s.s. The recent polishing and striation of limestone by coast-ice carrying boulders even as far south as the coast of Denmark, has been observed by Dr. Forchhammer, and helps us to conceive how large icebergs, running aground on the bed of the sea, may produce similar furrows on a grander scale. An account was given so long ago as the year 1822, by Scoresby, of icebergs seen by him drifting along in lat.i.tudes 69 and 70 N., which rose above the surface from 100 to 200 feet, and measured from a few yards to a mile in circ.u.mference. Many of them were loaded with beds of earth and rock, of such thickness that the weight was conjectured to be from 50,000 to 100,000 tons.[122-B] A similar transportation of rocks is known to be in progress in the southern hemisphere, where boulders included in ice are far more frequent than in the north. One of these icebergs was encountered in 1839, in mid-ocean, in the antarctic regions, many hundred miles from any known land, sailing northwards, with a large erratic block firmly frozen into it. In order to understand in what manner long and straight grooves may be cut by such agency, we must remember that these floating islands of ice have a singular steadiness of motion, in consequence of the larger portion of their bulk being sunk deep under water, so that they are not perceptibly moved by the winds and waves even in the strongest gales. Many had supposed that the magnitude commonly attributed to icebergs by unscientific navigators was exaggerated, but now it appears that the popular estimate of their dimensions has rather fallen within than beyond the truth. Many of them, carefully measured by the officers of the French exploring expedition of the Astrolabe, were between 100 and 225 feet high above water, and from 2 to 5 miles in length. Captain d'Urville ascertained one of them which he saw floating in the Southern Ocean to be 13 miles long and 100 feet high, with walls perfectly vertical.

The submerged portions of such islands must, according to the weight of ice relatively to sea-water, be from six to eight times more considerable than the part which is visible, so that the mechanical power they might exert when fairly set in motion must be prodigious.[123-A]

Glaciers formed in mountainous regions become laden with mud and stones, and if they melt away at their lower extremity before they reach the sea, they leave wherever they terminate a confused heap of unstratified rubbish, called "a moraine," composed of mud and pieces of all the rocks with which they were loaded. We may expect, therefore, to find a formation of the same kind, resulting from the liquefaction of icebergs, in tranquil water. But, should the action of a current intervene at certain points or at certain seasons, then the materials will be sorted as they fall, and arranged in layers according to their relative weight and size. Hence there will be pa.s.sages from _till_, as it is called in Scotland, to stratified clay, gravel, and sand, and intercalations of one in the other.

I have yet to mention another appearance connected with the boulder formation, which has justly attracted much attention in Norway and other parts of Europe. Abrupt pinnacles and outstanding ridges of rock are often observed to be polished and furrowed on the north, or "strike" side as it is called, or on the side facing the region from which the erratics have come; while, on the other side, which is usually steeper and often perpendicular, called the "lee-side," such superficial markings are wanting. There is usually a collection on this lee-side of boulders and gravel, or of large angular fragments. In explanation we may suppose that the north side was exposed, when still submerged, to the action of icebergs, and afterwards, when the land was upheaved, of coast-ice, which ran aground upon shoals, or was _packed_ on the beach; so that there would be great wear and tear on the seaward slope, while, on the other, gravel and boulders might be heaped up in a sheltered position.

_Northern origin of erratics._--That the erratics of northern Europe have been carried southward cannot be doubted; those of granite, for example, scattered over large districts of Russia and Poland, agree precisely in character with rocks of the mountains of Lapland and Finland; while the ma.s.ses of gneiss, syenite, porphyry, and trap, strewed over the low sandy countries of Pomerania, Holstein, and Denmark, are identical in mineral characters with the mountains of Norway and Sweden.

It is found to be a general rule in Russia, that the smaller blocks are carried to greater distances from their point of departure than the larger; the distance being sometimes 800 and even 1000 miles from the nearest rocks from which they were broken off; the direction having been from N.W. to S.E., or from the Scandinavian mountains over the seas and low lands to the south-east. That its acc.u.mulation throughout this area took place in part during the post-pliocene period is proved by its superposition at several points to strata containing recent sh.e.l.ls. Thus, for example, in European Russia, MM. Murchison and De Verneuil found in 1840, that the flat country between St. Petersburg and Archangel, for a distance of 600 miles, consisted of horizontal strata, full of sh.e.l.ls similar to those now inhabiting the arctic sea, on which rested the boulder formation, containing large erratics.

In Sweden, in the immediate neighbourhood of Upsala, I observed, in 1834, a ridge of stratified sand and gravel, in the midst of which is a layer of marl, evidently formed originally at the bottom of the Baltic, by the slow growth of the mussel, c.o.c.kle, and other marine sh.e.l.ls, intermixed with some of freshwater species. The marine sh.e.l.ls are all of dwarfish size, like those now inhabiting the brackish waters of the Baltic; and the marl, in which myriads of them are imbedded, is now raised more than 100 feet above the level of the Gulf of Bothnia. Upon the top of this ridge repose several huge erratics, consisting of gneiss for the most part unrounded, from 9 to 16 feet in diameter, and which must have been brought into their present position since the time when the neighbouring gulf was already characterized by its peculiar fauna.[124-A] Here, therefore, we have proof that the transport of erratics continued to take place, not merely when the sea was inhabited by the existing testacea, but when the north of Europe had already a.s.sumed that remarkable feature of its physical geography, which separates the Baltic from the North Sea, and causes the Gulf of Bothnia to have only one fourth of the saltness belonging to the ocean. In Denmark, also, recent sh.e.l.ls have been found in stratified beds, closely a.s.sociated with the boulder clay.

It was stated that in Russia the erratics diminished generally in size in proportion as they are traced farther from their source. The same observation holds true in regard to the average bulk of the Scandinavian boulders, when we pursue them southwards, from the south of Norway and Sweden through Denmark and Westphalia. This phenomenon is in perfect harmony with the theory of ice-islands floating in a sea of variable depth; for the heavier erratics require icebergs of a larger size to buoy them up; and, even when there are no stones frozen in, more than seven eighths, and often nine tenths, of a ma.s.s of drift ice is under water. The greater, therefore, the volume of the iceberg, the sooner would it impinge on some shallower part of the sea; while the smaller and lighter floes, laden with finer mud and gravel, may pa.s.s freely over the same banks, and be carried to much greater distances. In those places, also, where in the course of centuries blocks have been carried southwards by coast-ice, having been often stranded and again set afloat in the direction of a prevailing current, the blocks will be worn and diminish in size the farther they travel from their point of departure.

The "northern drift" of the most southern lat.i.tudes is usually of the highest antiquity. In Scotland it rests immediately on the older rocks, and is covered by stratified sand and clay, usually devoid of fossils, but in which, at certain points near the east and west coast, as, for example, in the estuaries of the Tay and Clyde, marine sh.e.l.ls have been discovered. The same sh.e.l.ls have also been met with in the north, at Wick in Caithness, and on the sh.o.r.es of the Moray Frith. The princ.i.p.al deposit on the Clyde occurs at the height of about 70 feet, but a few sh.e.l.ls have been traced in it as high as 554 feet above the sea.

Although a proportion of between 85 or 90 in 100 of the imbedded sh.e.l.ls are of recent species, the remainder are unknown; and even many which are recent now inhabit more northern seas, where we may, perhaps, hereafter find living representatives of some of the unknown fossils.

The distance to which erratic blocks have been carried southwards in Scotland, and the course they have taken, which is often wholly independent of the present position of hill and valley, favours the idea that ice-rafts rather than glaciers were in general the transporting agents. The Grampians in Forfarshire and in Perthshire are from 3000 to 4000 feet high. To the southward lies the broad and deep valley of Strathmore, and to the south of this again rise the Sidlaw Hills[125-A]

to the height of 1500 feet and upwards. On the highest summits of this chain, formed of sandstone and shale, and at various elevations, are found huge angular fragments of mica schist, some 3 and others 15 feet in diameter, which have been conveyed for a distance of at least 15 miles from the nearest Grampian rocks from which they could have been detached. Others have been left strewed over the bottom of the large intervening vale of Strathmore.

Still farther south on the Pentland Hills, at the height of 1100 feet above the sea, Mr. Maclaren has observed a fragment of mica-schist weighing from 8 to 10 tons, the nearest mountain composed of this formation being 50 miles distant.[125-B]

The testaceous fauna of the boulder period, in Scotland, England, and Ireland, has been shown by Prof. E. Forbes to contain a much smaller number of species than that now belonging to the British seas, and to have been also much less rich in species than the Older Pliocene fauna of the crag which preceded it. Yet the species are nearly all of them now living either in the British or more northern seas, the sh.e.l.ls of more arctic lat.i.tudes being the most abundant and the most wide spread throughout the entire area of the drift from north to south.

This extensive range of the fossils can by no means be explained by imagining the mollusca of the drift to have been inhabitants of a deep sea, where a more uniform temperature prevailed. On the contrary, many species were littoral, and others belonged to a shallow sea, not above 100 feet deep, and very few of them lived, according to Prof. E. Forbes, at greater depths than 300 feet.

From what was before stated it will appear that the boulder formation displays almost everywhere, in its mineral ingredients, a strange heterogeneous mixture of the ruins of adjacent lands, with stones both angular and rounded, which have come from points often very remote. Thus we find it in our eastern counties, as in Norfolk, Suffolk, Cambridge, Huntingdon, Bedford, Hertford, Ess.e.x, and Middles.e.x, containing stones from the Silurian and Carboniferous strata, and from the lias, oolite, and chalk, all with their peculiar fossils, together with trap, syenite, mica-schist, granite, and other crystalline rocks. A fine example of this singular mixture extends to the very suburbs of London, being seen on the summit of Muswell Hill, Highgate. But south of London the northern drift is wanting, as, for example, in the Wealds of Surrey, Kent, and Suss.e.x.

_Norfolk drift._--The drift can nowhere be studied more advantageously in England than in the cliffs of the Norfolk coast between Happisburgh and Cromer. Vertical sections, having an ordinary height of from 50 to 70 feet, are there exposed to view for a distance of about 20 miles. The name of diluvium was formerly given to it by those who supposed it to have been produced by the violent action of a sudden and transient deluge, but the term drift has been subst.i.tuted by those who reject this hypothesis. Here, as elsewhere, it consists for the most part of clay, loam, and sand, in part stratified, in part devoid of stratification. Pebbles, together with some large boulders of granite, porphyry, greenstone, lias, chalk, and other transported rocks, are interspersed, especially through the till.

That some of the granitic and other fragments came from Scandinavia I have no doubt, after having myself traced the course of the continuous stream of blocks from Norway and Sweden to Denmark, and across the Elbe, through Westphalia, to the borders of Holland. We need not be surprised to find them reappear on our eastern coast, between the Tweed and the Thames, regions not half so remote from parts of Norway as are many Russian erratics from the sources whence they came.

White chalk rubble, unmixed with foreign matter, and even huge fragments of solid chalk, also occur in many localities in these Norfolk cliffs.

No fossils have been detected in this drift, which can positively be referred to the era of its acc.u.mulation; but at some points it overlies a freshwater formation containing recent sh.e.l.ls, and at others it is blended with the same in such a manner as to force us to conclude that both were contemporaneously deposited.

[Ill.u.s.tration: Fig. 111. The shaded portion consists of Freshwater beds. Intercalation of freshwater beds and of boulder clay and sand at Mundesley.]

This interstratification is expressed in the annexed figure, the dark ma.s.s indicating the position of the freshwater beds, which contain much vegetable matter, and are divided into thin layers. The imbedded sh.e.l.ls belong to the genera _Planorbis_, _Lymnea_, _Paludina_, _Unio_, _Cyclas_, and others, all of British species, except a minute _Paludina_ now inhabiting France. (See fig. 112.)

[Ill.u.s.tration: Fig. 112. _Paludina marginata_, Michaud. (_P. minuta_, Strickland.) The middle figure is of the natural size.]

The _Cyclas_ (fig. 113.) is merely a remarkable variety of the common English species. The scales and teeth of fish of the genera Pike, Perch, Roach, and others, accompany these sh.e.l.ls; but the species are not considered by M. Aga.s.siz to be identical with known British or European kinds.

[Ill.u.s.tration: Fig. 113. _Cyclas_ (_Pisidium_) _amnica_, var.? The two middle figures are of the natural size.]

The series of formations in the cliffs of eastern Norfolk, now under consideration, beginning with the lowest, is as follows:--First, chalk; secondly, patches of a marine tertiary formation, called the Norwich Crag, hereafter to be described; thirdly, the freshwater beds already mentioned; and lastly, the drift. Immediately above the chalk, or crag, when that is present, is found here and there a buried forest, or a stratum in which the stools and roots of trees stand in their natural position, the trunks having been broken short off and imbedded with their branches and leaves. It is very remarkable that the strata of the overlying boulder formation have often undergone great derangement at points where the subjacent forest bed and chalk remain undisturbed.

There are also cases where the upper portion of the boulder deposit has been greatly deranged, while the lower beds of the same have continued horizontal. Thus the annexed section (fig. 114.) represents a cliff about 50 feet high, at the bottom of which is _till_, or unstratified clay, containing boulders, having an even horizontal surface, on which repose conformably beds of laminated clay and sand about 5 feet thick, which, in their turn, are succeeded by vertical, bent, and contorted layers of sand and loam 20 feet thick, the whole being covered by flint gravel. Now the curves of the variously coloured beds of loose sand, loam, and pebbles are so complicated that not only may we sometimes find portions of them which maintain their verticality to a height of 10 or 15 feet, but they have also been folded upon themselves in such a manner that continuous layers might be thrice pierced in one perpendicular boring.

[Ill.u.s.tration: Fig. 114. Cliff 50 feet high between Bacton Gap and Mundesley.]

[Ill.u.s.tration: Fig. 115. Folding of the strata between East and West Runton.]

[Ill.u.s.tration: Fig. 116. Section of concentric beds west of Cromer.

1. Blue clay.

2. White sand.

3. Yellow Sand.

4. Striped loam and clay.

5. Laminated blue clay.]

At some points there is an apparent folding of the beds round a central nucleus, as at _a_, fig. 115., where the strata seem bent round a small ma.s.s of chalk; or, as in fig. 116., where the blue clay, No. 1., is in the centre; and where the other strata, 2, 3, 4, 5, are coiled round it; the entire ma.s.s being 20 feet in perpendicular height. This appearance of concentric arrangement around a nucleus is, nevertheless, delusive, being produced by the intersection of beds bent into a convex shape; and that which seems the nucleus being, in fact, the innermost bed of the series, which has become partially visible by the removal of the protuberant portions of the outer layers.

To the north of Cromer are other fine ill.u.s.trations of contorted drift reposing on a floor of chalk horizontally stratified and having a level surface. These phenomena, in themselves sufficiently difficult of explanation, are rendered still more anomalous by the occasional inclosure in the drift of huge fragments of chalk many yards in diameter. One striking instance occurs west of Sherringham, where an enormous pinnacle of chalk, between 70 and 80 feet in height, is flanked on both sides by vertical layers of loam, clay, and gravel. (Fig. 117.)

[Ill.u.s.tration: Fig. 117. Included pinnacle of chalk at Old Hythe point, west of Sherringham.

_d._ Chalk with regular layers of chalk flints.

_c._ Layer called "the pan," of loose chalk, flints, and marine sh.e.l.ls of recent species, cemented by oxide of iron.]

This chalky fragment is only one of many detached ma.s.ses which have been included in the drift, and forced along with it into their present position. The level surface of the chalk _in situ_ (_d_) may be traced for miles along the coast, where it has escaped the violent movements to which the inc.u.mbent drift has been exposed.[129-A]

We are called upon, then, to explain how any force can have been exerted against the upper ma.s.ses, so as to produce movements in which the subjacent strata have not partic.i.p.ated. It may be answered that, if we conceive the _till_ and its boulders to have been drifted to their present place by ice, the lateral pressure may have been supplied by the stranding of ice-islands. We learn, from the observations of Messrs.

Dease and Simpson in the polar regions, that such islands, when they run aground, push before them large mounds of shingle and sand. It is therefore probable that they often cause great alterations in the arrangement of pliant and incoherent strata forming the upper part of shoals or submerged banks, the inferior portions of the same remaining unmoved. Or many of the complicated curvatures of these layers of loose sand and gravel may have been due to another cause, the melting on the spot of icebergs and coast ice in which successive deposits of pebbles, sand, ice, snow, and mud, together with huge ma.s.ses of rock fallen from cliffs, may have become interstratified. Ice-islands so const.i.tuted often capsize when afloat, and gravel once horizontal may have a.s.sumed, before the a.s.sociated ice was melted, an inclined or vertical position.

The packing of ice forced up on a coast may lead to similar derangement in a frozen conglomerate of sand or shingle, and, as Mr. Trimmer has suggested[130-A], alternate layers of earthy matter may have sunk down slowly during the liquefaction of the intercalated ice, so as to a.s.sume the most fantastic and anomalous positions, while the aqueous strata below, and those afterwards thrown down above, may be perfectly horizontal.

A buried forest has been adverted to as underlying the drift on the coast of Norfolk. At the time when the trees grew there must have been dry land over a large area, which was afterwards submerged, so as to allow a ma.s.s of stratified and unstratified drift, 200 feet and more in thickness, to be superimposed. The undermining of the cliffs by the sea in modern times has enabled us to demonstrate, beyond all doubt, the fact of this superposition, and that the forest was not formed along the present coast-line. Its situation implies a subsidence of several hundred feet since the commencement of the drift period, after which there must have been an upheaval of the same ground; for the forest bed of Norfolk is now again so high as to be exposed to view at many points at low water; and this same upward movement may explain why the _till_, which is conceived to have been of submarine origin, is now met with far inland, and on the summit of hills.

The boulder formation of the west of England, observed in Lancashire, Cheshire, Shropshire, Staffordshire, and Worcestershire, contains in some places marine sh.e.l.ls of recent species, rising to various heights, from 100 to 350 feet above the sea. The erratics have come partly from the mountains of c.u.mberland, and partly from those of Scotland.

But it is on the mountains of North Wales that the "Northern drift,"

with its characteristic marine fossils, reaches its greatest alt.i.tude.