James Croll.

Climate and time in their geological relations: a theory of secular changes ... online

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The Proposed Theory. — ^It may now be regarded as an es-
tablished fact that, during the severer part of the glacial
period, Scotland was covered with one continuous mantle of
ice, so thick as t<ibury under it the Ochil, Sidlaw, Pentland,
Campsie, and other moderately high mountain ranges. For
example, Mr. J. Geikie and Mr. B. N. Peach found that the
great masses of the ice from the North-west Highlands, came
straight over the Ochils of Perthshire and the Lomonds of Fife.
In fact, these moimtain ridges were not sufficiently high to deflect
the icy stream either to the right hand or to the left ; and the
flattened and rounded tops of the Campsie, Pentland, and
Lammermoor ranges bear ample testimony to the denuding
power of ice.

Further, to quote from Mr. Jamieson, " the detached moun-
tain of Schehallion in Perthshire, 3,500 feet high, is marked
near the top as well as on its flanks, and this not by ice flowing
down the sides of the hill itself, but by ice pressing over it
from the north. On the top of another isolated hill, called


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Morven, about 3,000 feet high, and situated a few miles to the
north of the village of Ballater, in the county of Aberdeen, I
found granite boulders unlike the rock of the hill, and appa-
rently derived from the mountains to the west. Again, on the
highest water-sheds of the Ochils, at altitudes of about 2,000
feet, I foimd this summer (1864) pieces of mica schist full of
garnets, which seem to have come from the Grampian Hills to
the north-west, showing that the transporting agent had over-
flowed even the highest parts of the Ochil ridge. And on the
West Lomonds, in Fifeshire, at Clattering-well Quarry, 1,450
feet high, I found ice- worn pebbles of Red Sandstone and
porphyry in the dibris covering the Carboniferous Limestone
of the top of the Bishop Hill. Facts like these meet us every-
where. Thus on the Perthshire Hills, between Blair Athol
and Dunkeld, I foimd ice- worn surfaces of rocks on the tops of
hills, at elevations of 2,200 feet, as if caused by ice pressing
over them from the north-west, and transporting boulders at
even greater heights." *

Facts still more important, however, in their bearing on the
question before us were observed on the Pentland range by Mr.
Bennie and myself during the summer of 1870. On ascending
AUermuir, one of the hills forming the nortkem termination of
the Pentland range, we were not a little surprised to find its
summit ice- worn and striated. The top of the hill is composed
of a compact porphyritic felstone, which is very much broken
up ; but wherever any remains of the original surface could be
seen, it was foimd to be polished and striated in a most decided
manner. These strisB are all in one uniform direction, nearly
east and west ; and on minutely examining them with a l^is
we had no difficulty whatever in determining that the ice
which efiected them came from the west and not from the east,
a fact which clearly shows that they must have been made at
the time when, as is well known, the entire Midland valley waa
filled with ice, coming from the North-west Highlands. On
the simimit of the hill we also foimd patches of boulder clay in

♦ Joum. Geol. Soo., vol. xxi., p. 166.


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hollow basins of the rock. At one spot it was upwards of a
foot in depth, and rested on the ice-polished surface. The
clay was somewhat loose and sandy, a,p might be expected of a
layer so thin, exposed to rain, frost, and snow, during the long
course of ages which must have elapsed since it was deported
there. Of 100 pebbles collected from the clay, just as they
turned up, every one, with the exception of three or four
composed of hard quartz, presented a flattened and ice-worn
surface; and forty-four were distinctly striated: in short,
every stone which was capable of receiving and retaining
scratches was striated. A number of these stones must have
come from the Highlands to the north-west.*

The height of Allermuir is 1,617 feet, and, from its position,
it is impossible that the ice could have gone over its summit,
unless the entire Midland valley, at this place, had been filled
with ice to the depth of more than 1,600 feet. The hill is
situated about four or five miles to the south of Edinburgh,
and forms, as has already been stated, the northern termination
of the Pentland range. Immediately to the north lies the
broad valley of the Firth of Forth, more than twelve miles
across, oflering a most free and unobstructed outlet for the
great mass of ice coming along the Midland valley from the
west. Now, when we reflect how easily ice can accommodate
itself to the inequalities of the channel along which it moves,
how it can turn to the right hand or to the left, so as to find
for itself the path of least resistance, it becomes obvious that
the ice never would have gone over Allermuir, imless not only
the Midland vaUey at this point, but also the whole sur-
rounding coimtry had been covered with one continuous mass
of ice to a depth of more than 1,600 feet. But it must not be
supposed that the height of Allermuir represents the thickness
of the ice ; for on ascending Scald Law, a hill four miles to the
south-west of Allermuir, and the highest of the Pentland
range, we found, in the debris covering its summit, hundreds

* Specimens of tlie striated summit and boulder clay stones are to be seen in
the Edinburgh Museum of Science and Art.


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of transported stones of all sizes^ from one to eighteen indies
in diameter. We also dug up a Greenstone boulder aboat
eighteen inches in dian^eter^ which was finely polished and
striated. As the height of this hill is 1^898 feet, the mass of
ice covering the surroimding country must haye been at least
1^900 feet deep. But this is not alL Directly to the north of
the Pentlands, in a line nearly parallel with the east coadt^ and
at right angles to the path of ice from the interior^ there is not,
with the exception of the solitary peak of East Lomond, and a
low hill or two of the Sidlaw range, an eminence worthy of the
name of a hill nearer than the Qrampians in the north of
Forfarshire, distant upwards of sixty miles. This broad plain,
extending from almost the Southern to the Northern Highlands,
was the great channel through which the ice of the interior of
Scotland found an outlet into the North Sea. If the depth of
the ice in the Firth of Forth, which forms the southern side of
this broad hollow, was at least 1,900 feet, it is not at all
probable that its depth in the northern side, formed by the
Valley of Strathmore and the Firth of Tay, which lay more
directly in the path of the ice from the North Highlands, oould
have been less. Here we have one vast glacier, more than
sixty miles broad and 1,900 feet thick, coming from the interior
of the country.

It is, therefore, evident that the great mass of ice entering
the North Sea to the east of Scotland, especially about the
Firths of Forth and Tay, coxdd not have been less, and was
probably much more, than from 1,000 to 2,000 feet in thick-
ness. The grand question now to be considered is. What
became of the huge sheet of ice after it entered the North Sea ?
Did it break up and float away as icebergs P This appears to
have been hitherto taken for granted ; but the shallowness of
the North Sea shows such a process to have been utterly im-
possible. The depth of the sea in the English Channel is only
about twenty fathoms, and although it gradually increases to
about forty fethoms at the Moray Firth, yet we must go to the
north and west of the Orkney and Shetland Islands ere we


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reach the 100 fathom line. Thus the average depth of the
entire North Sea is not over forty fathoms, which is even in-
8ii£Scient to float an iceberg 300 feet thick.

No doubt the North Sea, for two reasons, is now much
shallower than it was during the period in question. (1.)
There would, at the time of the great extension of the ice on
the northern hemisphere, be a considerable submergence, result-
ing from the displacement of the earth's centre of gravity.*
(2.) The sea-bed is now probably filled up to a larger extent
with drift deposits than it was at the ice period. But, after
making the most extravagant allowance for the additional depth
gained on this account, still there could not jwssibly hdve been
water sufficiently deep to float a glacier of 1,000 or 2,000 feet
in thickness. Indeed, the North Sea would have required to
be nearly ten times deeper than it is at present to have floated
the ice of the glacial period. We may, therefore, conclude with
the most perfect certainty that the ice-sheet of Scotland could
not possibly have broken up into icebergs in such a channel,
but must have moved along on the bed of the sea in one un-
broken mass, and must have found its way to the deep trough
of the Atlantic, west of the Orkney and Shetland Islands, ere
it broke up and floated away in the iceberg form.

It is hardly necessary to remark that the waters of the
North Sea would have but little effect in melting the ice. A
shallow sea like this, into which large masses of ice were
entering, would be kept constantly about the freezing-point,
and water of this temperature has but little melting power,
for it takes 142 lbs. of water, at 33°, to melt one poimd of ice.
In fact, an icy sea tends rather to protect the ice entering it
from being melted than otherwise. And besides, owing to
fresh acquisitions of snow, the ice-sheet would be accimiulating
more rapidly upon its upper surface than it would be melting at
its lower surface, supposing there were sea- water under that
surface. The ice of Scotland during the glacial period must,
of necessity, have foimd its way into warmer water than that of
• PhiL Mag. for April, 1866.

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the Nortli Sea before it could have been meltecL But ibis it
could not do without reaching the Atlantic^ and in getting
there it woxdd have to pass round by the Orkney IsIandB,
along the bed of the North Sea, as land-ice.

This will explain how the Orkney Islands may have been
glaciated by land-ice ; but it does not, howeyer, explain how
Caithness should have been glaciated by that means. These
islands lay in the very track of the ice on its way to die
Atlantic, alid could hardly escape being overridden; but
Caithness lay considerably to the left of the path which we
should expect the ice to have taken. The ice would not leare
its channel, turn to the left, and ascend upon Caithness, unless
it were forced to do so. What, then, compelled the ice to pass
over Caithness ?

Path of the Scandinavian Ice. — ^We must consider that the ice
from Scotland and England was but a fraction of that which
entered the North Sea. The greater part of the ice of
Scandinavia must have gone into this sea, and if the ice of our
island could not find water sufficiently deep in which to float,
far less would the much thicker ice of Scandinavia do so. The
Scandinavian ice, before it could break up, would thus, like the
Scottish ice, have to cross the bed of the North Sea and pass
into the Atlantic. It could not pass to the north, or to the
north-west, for the ocean in these directions would be blocked
up by the polar ice. It is true that along the southern shore
of Norway there extends a comparatively deep trough of from
one to two hundred &thoms. But this is evidently not deep
enough to have floated the Scandinavian ice-sheet ; and even
supposing it had been sufficiently deep, the floating ice must have
found its way to the Atlantic, and this it could not have done
without passing along the coast. Now, its passage would not
only be obstructed by the mass of ice continually protruding
into the sea directly at right angles to its course, but it would
be met by the still more enormous masses of ice coming off
the entire Norwegian coast-line. And, besides this^ the ice
entering the Arctic Ocean from Lapland and the nortiiem


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parts of Siberia, except the very small portion which might
find an outlet into tiie Pacific through Behring's Straits,
would have to pass along the Scandinavian coast in its way to
the Atlantic. No matter, then, what the depth of this trough
may have been, if the ice from the Lmd, after entering it, coxdd
not make its escape, it would continue to accumulate till the
trough became blocked up ; and after this, the great mass
from the land woxdd move forward as though the trough had
no existence. Thus, the only path for the ice woxdd be by the
Orkney and Shetland Islands. Its more direct and natural
path would, no doubt, be to the south-west, in the direction of
our shores ; and in all probability, had Scotland been a low flat
island, instead of being a high and mountainous one, the ice
would have passed completely over it. But its mountainous
character, and the enormous masses of ice at the time pro-
ceeding from its interior, would efiectually prevent this, so
that the ice of Scandinavia would be compelled to move round
by the Orkney Islands. Consequently, these two huge masses
of moving ice — the one firom Scotland and the much greater
one from Scandinavia — ^would meet in the North Sea, probably
not far from our shores, and would move, as represented in the
diagram, side by side northwards into the Atlantic as one
gigantic glacier.

Nor can this be regarded as an anomalous state of things ;
for in Greenland and the antarctic continent the ice does not
break up into icebergs on reaching the sea, but moves along the
sea-bottom in a continuous mass until it reaches water suffi-
ciently deep to float it. It is quite possible that the ice at the
present day may nowhere traverse a distance of three or four
hundred miles of sea-bottom, but this is wholly owing to the
fact that it finds water sufficiently deep to float it before having
travelled so far. Were Baffin's Bay and Davis's Straits, for
example, as shallow as the North Sea, the ice of Greenland
would not break up into icebergs in these seas, but cross in one
continuous mass to and over the American continent.

The median line of the Scandinavian and Scottish ice* sheets


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would be eitoated not far from the east coaat of Scotland. The
Scandinavian ice would press up as near to our coast as the
resistance of the ice from this side permitted. The enormous
mass of ice from Seotland, pressing out into the North Sea, would
compel the Scandinavian. ice to move round by the Orkneys,
and would also keep it at some little distance from Scotland.
Where, on the other hand, there was but little resistance offered
by ice from the interior of this country (and this might be the
case along many parts of the English coast), the Scandinavian
ice might reach the shores, and even ovemm the country for
some distance inland.

We have hitherto confined our attention to the action of ice
proceeding from Norway ; but if we now consider what took
place in Sweden and the Baltic, we shall find more conclusive
proof of the downward pressure of Scandinavian ice on our own
shores. The western half of Gothland is striated in the direc-
tion of N.E. and S.W., and that this has been effected by a
huge mass of ice covering the country, and not by local
glaciers, is apparent from the fact observed by Robert Cham-
bers,* and officers of the Swedish Geological Survey, that the
general direction of the groovings and striae on the rocks bears
little or no relation to the conformation of the surface, showing
that the ice was of sufficient thickness to move straight forward,
regardless of the inequalities of the ground.

At Gottenburg, on the shores of the Cattegat, and all around
Lake Wener and Lake Wetter, the ice-markings are of the
most remarkable character, indicating, in the most decided
manner, that the ice came from the interior of the country to
the north-east in one vast mass. All this mass of ice must have
gone into the shallow Cattegat, a sea not sufficiently deep to
float even an ordinary glacier. The ice coming off Qt)thland
would therefore cross the Cattegat, and thence pass over Jut-
land into the North Sea. After entering the North Sea, it
would be obliged to keep between our shores and the ice coming
direct from the western side of Scandinavia.

• "Tracings of the North of Europe,'* 1850, pp. 48—61.

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But this is not alL A very large proportion of the Scandi-
navian ice would pass into the Ghilf of Bothnia, where it coxdd
not possibly float. It would then move south into the Baltic
as land-ice. After passing down the Baltic, a portion of the ice
would probably move south into the flat plains in the north of
Germany, but the greater portion would keep in the bed of the
Baltic, and of course turn to the right roimd the south end of
Gothland, and thence cross over Denmark into the North Sea.
That this must have been the path of the ice is, I think, obvious
from the observations of Murchison, Chambers, Horbye, and
other geologists. Sir Boderick Murchison found — though he
does not attribute it to land- ice — that the Aland Islands, which
lie between the Gulf of Bothnia and the Baltic, are all striated
in a north and south direction.*

Upsala and Stockholm, a tract of flat coimtry projecting for
some distance into the Baltic, is also grooved and striated, not
in the direction that would be efiected by ice coming &om the
interior of Scandinavia, but north and south, in a direction
parallel to what must have been the course of the ice moving
down the Baltic, t This part of the country must have been
striated by a mass of ice coming from the direction of the Gulf
of Bothnia. And that this mass must have been great is appa-
rent from the fact that Lake Malar, which crosses the country
from east to west, at right angles to the path of the ice, does
not seem to have had any influence in deflecting the icy stream.
That the ice came from the north and not from the south is
also evident from the fact that the northern sides of rocky
eminences are polished, rounded, and ice-worn, while the
southern sides are comparatively rough. The northern banks
of Lake Malar, for example, which, of course, face the south,
are rough, while the southern banks, which must have offered
opposition to the advance of the ice, are smoothed and rounded
in a most singular manner.

♦ Quart. Jonm. Geol. Soc., vol. ii., p. 864.

t "Tracings of the North of Europe," by Bohert Chambers, pp. 259, 285.
" Observations sur les Ph^nom^nes d'Erosion en Norv^ge," by M. Horbye, 1867.
See also Professor Erdman^'s '* Formations Quatemairos de la SuMe."


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Again, tliat the ice, after passing down the Baltic, turned to
the right along the southern end of Gothland, is shown by the
direction of the strise and ice-groovings observed on such
islands as Gothland, Oland, and Bomholm. Sir B. Murchison
found that the island of Gothland is grooved and striated in
one uniform direction from N.E. to S. W. " These groovings,"
says Sir Boderick, " so perfectly resemble the flutings and strias
produced in the Alps by the actual movement of glaciers, that
neither M. Agassiz nor any one of his supporters could detect a
diflferenc^/' He concludes, however, that the markings could
not have been made by land-ice, because Gothland is not only
a low, flat island in the middle of the Baltic, but is " at least
400 miles distant from any elevation to which the term of
moimtain can be applied.'* This, of course, is conclusive
against the hypothesis that Gothland and the other islands of
the Baltic could have been glaciated by ordinary glaciers ; but
it is quite in harmony with the theory that the Gxdf of Bothnia
and the entire Baltic were filled with one continuous mass of
land-ice, derived from the drainage of the greater part of
Sweden, Lapland, and : Finland. In fact, the whole glacial
phenomena of Scandinavia are inexplicable on the hypothesis
of local glaciers.

That the Baltic was completely filled by a mass of ice moving
from the north is further evidenced by the fact that the main-
land, not only at TJpsala, but at several places along the coast
of Gothland, is grooved and striated parallel to the shore, and
often at right angles to the markings of the ice from the
interior, showing that the present bed of the Baltic was not
large enough to contain the icy stream. For example, along
the shores between Kalmar and Karlskrona, as described by Sir
Roderick Murchison and by M. Horbye, the striations are
parallel to the shore. Perhaps the slight obstruction offered
by the island of Oland, situated so close to the shore, would
deflect the edge of the stream .at this point over on the land.
The icy stream, after passing £arlskrona, bent round to the
west along the present entrance to the Baltic, and again


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JL-.A^%«Mw— •♦^f^



invaded the mainland, and crossed over the low headland of

Christianstadt, and thence passed westward in the direction of


/ This immense Baltic glacier would in all probability pass

-jX\ over Denmark, and enter the North Sea somewhere to the

^Ar-\ north of the River Elbe, and would then have to find an outlet

jj^ to the Atlantic through the English Channel, or pass in

^ between our eastern shores and the mass from Gothland and

^^^ . the north-western shores of Europe. The entire probable path

^ of the ice majube seen by a reference to the accompanying

^-^ chart (Plate V.) That the ice crossed over Denmark is evident

*^^^/^ from the &ct that the surface of that country is strewn with

^ debris derived from the Scandinavian peninsula.

Taking all these various considerations into account, the con-
clusion is inevitable that the great masses of ice from Scotland
would be obliged to turn abruptly to the north, as represented
in the diagram, and pass round into the Atlantic in the direc-
tion of Caithness and tho Orkney Islands.

If the foregoing be a feir representation of the state of
matters, it is physically impossible that Caithness could have
escaped being overridden by the land-ice of the North Sea.
Caithness, as is well known, is not only a low, flat tract of land,
little elevated above the sea-level, and consequently incapable
of supporting large glaciers; but, in addition, it projects in the
form of a headland across the very path of the ice. Unless
Caithness could have protected itself by pushing into the sea
glaciers of one or two thousand feet in thickness, it could not
possibly have escaped the inroads of the ice of the North Sea.
But Caithness itself could not have supported glaciers of this
magnitude, neither could it have derived them from the adjoin-
ing mountainous regions of Sutherland, for the ice of this county
found a more direct outlet than along the flat plains of Caith-

The shells which the boulder clay of Caithness contains have
thus evidently been pushed out of the bed of the North Sea by
the land-ice, which formed the clay itself.


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The fact that tliese shells are not so intensely arctic as those
found in some other quarters of Scotland, is no evidence that
the clay was not formed during the most severe part of the
glacial epoch, for the shells did not live in the North Sea at the
time that it was filled with land-ice. The shells must have
heloDged to a period prior to the invasion of the ice, and con-
sequently before the cold had reached its greatest intensity.
Neither is there any necessity for supposing the shells to be
pre-glacial, for these shells may have belonged to an inter-
glacial period. In so far as Scotland is concerned, it would be
hazardous to conclude that a plant or an animal is either pre-
glacial or post-glacial simply because it may happen not to be
of an arctic or of a boreal type.

The same remarks which apply to Caithness apply to a
certain extent to the headland at Fraserburgh. It, too, lay in
the path of the ice, and from the direction of the strise on the
rocks, and the presence of shells in the clay, as described by
Mr. Jamieson, it bears evidence also of having been overridden
by the land-ice of the North Sea. In fact, we have, in the

Online LibraryJames CrollClimate and time in their geological relations: a theory of secular changes ... → online text (page 40 of 54)