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 shells of
glacial period--Drift of eastern Norfolk--Associated freshwater
deposit--Bent and folded strata lying on undisturbed beds--Shells 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 mass 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.
[Illustration: Fig. 110. Limestone polished, furrowed, and scratched by the
glacier of Rosenlaui, in Switzerland. (Agassiz.)
_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 mass of ice, and
pushed along under the pressure of that mass, 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 glass. 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 latitudes 69 deg. and 70 deg.
N., which rose above the surface from 100 to 200 feet, and measured from a
few yards to a mile in circumference. 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
passages 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
masses 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 accumulation throughout this area took place in part
during the post-pliocene period is proved by its superposition at several
points to strata containing recent shells. 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 shells 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, cockle, and other marine shells, intermixed with some
of freshwater species. The marine shells 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 assumed 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 shells have been found in stratified beds, closely
associated 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 mass 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 pass 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 latitudes 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 shells have been
discovered. The same shells have also been met with in the north, at
Wick in Caithness, and on the shores of the Moray Frith. The principal
deposit on the Clyde occurs at the height of about 70 feet, but a few
shells 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 shells
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 shells of more arctic
latitudes 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, Essex, and Middlesex, 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 Sussex.
_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 substituted 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 accumulation; but at some points it overlies
a freshwater formation containing recent shells, and at others it is
blended with the same in such a manner as to force us to conclude that
both were contemporaneously deposited.
[Illustration: 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 mass
indicating the position of the freshwater beds, which contain much
vegetable matter, and are divided into thin layers. The imbedded shells
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.)
[Illustration: 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 shells; but the species
are not considered by M. Agassiz to be identical with known British
or European kinds.
[Illustration: 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.
[Illustration: Fig. 114. Cliff 50 feet high between Bacton
Gap and Mundesley.]
[Illustration: Fig. 115. Folding of the strata between East
and West Runton.]
[Illustration: 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
mass 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 mass 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 illustrations 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.)
[Illustration: 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 shells
of recent species, cemented by oxide of iron.]
This chalky fragment is only one of many detached masses 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 incumbent drift has been exposed.[129-A]
We are called upon, then, to explain how any force can have been exerted
against the upper masses, so as to produce movements in which the
subjacent strata have not participated. 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 masses of rock fallen from
cliffs, may have become interstratified. Ice-islands so constituted
often capsize when afloat, and gravel once horizontal may have assumed,
before the associated 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 assume 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 mass 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 shells of recent species, rising to various heights, from 100
to 350 feet above the sea. The erratics have come partly from the mountains
of Cumberland, 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 altitude.
On Moel Tryfane, near the Menai Straits, Mr. Trimmer met with shells of
the species commonly found in the drift at the height of 1392 feet above
the level of the sea.
It is remarkable that in the same neighbourhood where there is evidence of
so great a submergence of the land during part of the glacial period, we
have also the most decisive proofs yet discovered in the British Isles of
subaerial glaciers. Dr. Buckland published in 1842 his reasons for
believing that the Snowdonian mountains in Caernarvonshire were formerly
covered with glaciers, which radiated from the central heights through the
seven principal valleys of that chain, where striae and flutings are seen
on the polished rocks directed towards as many different points of the
compass. He also described the "moraines" of the ancient glaciers, and the
rounded "bosses" or small flattened domes of polished rock, such as the
action of moving glaciers is known to produce in Switzerland, when gravel,
sand, and boulders, underlying the ice, are forced along over a foundation
of hard stone. Mr. Darwin, and subsequently Prof. Ramsay, have confirmed
Dr. Buckland's views in regard to these Welsh glaciers. Nor indeed was it
to be expected that geologists should discover proofs of icebergs having
abounded in the area now occupied by the British Isles in the Pleistocene
period without sometimes meeting with the signs of contemporaneous glaciers
which covered hills even of moderate elevation between the 50th and 60th
degrees of latitude.
In Ireland the "drift" exhibits the same general characters and fossil
remains as in Scotland and England; but in the southern part of that
island, Prof. E. Forbes and Capt. James found in it some shells which show
that the glacial sea communicated with one inhabited by a more southern
fauna. Among other species in the south, they mention at Wexford and
elsewhere the occurrence of _Nucula Cobboldiae_ (see fig. 120. p. 149.) and
_Turritella incrassata_ (a crag fossil); also a southern form of _Fusus_,
and a _Mitra_ allied to a Spanish species.[131-A]
FOOTNOTES:
[122-A] Chap. xvi. and the references there given.
[122-B] Voyage in 1822, p. 233.
[123-A] T. L. Hayes, Boston Journ. Nat. Hist. 1844.
[124-A] See paper by the author, Phil. Trans. 1835, p. 15.
[125-A] See above, section, p. 48.
[125-B] Geol. of Fife, &c. p. 220.
[129-A] For a full account of the drift of East Norfolk, see a paper by the
author, Phil. Mag. No. 104. May, 1840.
[130-A] Quart. Journ. Geol. Soc. vol. vii. p. 22.
[131-A] Forbes, Memoirs of Geol. Survey of Great Britain, vol. i. p. 377.