country which have puzzled geologists there is none more remarkable
than the "intercrossing of erratics." The fact that such wandered
blocks have apparently crossed each other's tracks in their journeys
appears at first sight inexplicable on the assumption that their
transport has been effected by land-ice. The phenomena in question,
therefore, have always been appealed to by those who uphold the
iceberg origin of our boulder-clays, etc., as evidence decisively in
favour of their views. No one can deny that any degree and amount of
intercrossing might take place in the case of icebergs. We can readily
conceive how floating ice, detached from a long line of coast, might be
compelled by shifting winds and changing currents to tack about again
and again, so as to pursue the most devious course, and scatter their
stony burdens in the most erratic manner over the sea-bottom; while,
on the other hand, it is quite impossible to understand how a similar
irregular distribution of erratics could take place under one and the
same glacier flowing in a determinate direction. It is little wonder,
then, that the curious phenomena of the intercrossing of erratics
should have had much importance attached to it by the upholders of
the iceberg theory, seeing that all the other proofs which have been
adduced in favour of this theory have only served to demonstrate its
insufficiency. Upon the facts connected with the intercrossing of
erratics, the supporters of this time-honoured theory are now making
what I must believe is their last stand. I purpose therefore, in this
paper, to give a short outline of those facts, with the view of showing
that so far from being antagonistic to the land-ice theory, they are in
complete harmony with it; and indeed must be considered as affording an
additional demonstration of its truth.
Some years ago I called attention to the fact that in the middle
districts of Scotland the boulder-clay not infrequently contains a
curious commingling of northern and southern erratics.[R] I showed
that this was the case throughout a belt of country extending from the
sea-coast near Ayr, north-east to the valley of the Irvine, and thence
across the watershed into the Avon, and east to Lesmahagow, then down
the valley of the Clyde to Carluke, stretching away to the east by
Wilsontown, and thereafter continuing along the crest of the Pentlands
and the northern slopes of the Lammermuir Hills, by Reston and Ayton,
to the sea. "All along this line," I remarked, "we have a 'debatable
ground' of variable breadth, throughout which we find a commingling in
the till of stones which have come from north and from south. South of
it, characteristic Highland stones do not occur, and north of it stones
derived from the south are similarly absent." The explanation of these
facts is obvious. The belt of ground referred to was evidently the
meeting-place of the Highland and southern _mers de glace_. Here the
two opposing ice-flows coalesced and became deflected by their mutual
pressure to right and left--one great current going east and another
west. It is evident that the line of junction between the two _mers de
glace_ could not be rigorously maintained in one and the same position
during a period of glaciation, but would tend to oscillate backwards
and forwards, according as one or the other ice-sheet prevailed.
Sometimes the southern ice-sheet would be enabled to push back the
northern _mer de glace_, while at other times the converse would take
place. Nor is it necessary to suppose that the advance of one ice-sheet
was general along the whole line. On the contrary, it is most likely
that the movement was quite irregular--an ice-sheet advancing in some
places, while at other points its line of junction with the opposing
ice-sheet remained stationary, or even retrograded. Such movements
would obviously give rise to oscillations in the sub-glacial débris of
clay and stones; and thus we have a simple and natural explanation of
those intercrossings of erratics which are so characteristic of that
region which I have termed the "debatable ground." And this conclusion
is borne out by the fact that the glacial striæ of the same "debatable
ground" afford like evidence of oscillation in the trend of the
ice-flow.
[R] _Great Ice Age_, 2nd edit., p. 609.
Along the base of the Highland mountains in Forfarshire, etc., we
meet with similar intercrossings of erratics. Thus we occasionally
encounter in the boulder-clays overlying the Silurian regions erratics
of Old Red Sandstone rocks which have come from the east or south-east;
while the abundant presence of erratics of Silurian origin, on the
other hand, bespeak an ice-flow from the west towards the low-grounds.
In some places within the Silurian area we encounter a greyish-blue
boulder-clay containing Silurian fragments only, while in other places
within the same area the boulder-clay becomes reddish, and is charged
with many boulders of Old Red Sandstone rocks. Now the greyish-blue
till could only have been laid down by glacier-ice descending from
the Silurian high-grounds to Strathmore, while the red boulder-clay
points to a partial invasion of the Silurian regions by land-ice,
which had previously traversed the lower-lying Old Red Sandstone
areas. These apparently contradictory movements are readily accounted
for by the former presence in the area of the North Sea of the great
Scandinavian _mer de glace_. Dr. James Croll was the first to point out
that the glacial phenomena of Caithness and the Shetlands could only
be accounted for by the advance of the Scandinavian ice-sheet towards
our coasts, where it encountered and deflected the Scottish ice-sheet
out of its normal course--a sagacious induction, which the admirable
and exhaustive researches of my colleagues, Messrs. B. N. Peach and
J. Horne, have now firmly established. The lower blue boulder-clay
was evidently accumulated at a time when the Scottish ice was able
to flow more or less directly east or south-east towards what is now
the coast-line; while the overlying red boulder-clay points to a
subsequent period when the presence of the Scandinavian _mer de glace_
was sufficiently great to compel the Scottish ice out of its normal
course, and cause it to flow in a north-easterly direction. In doing
so it now and again passed from tracts of Old Red Sandstone to invade
the Silurian area, and thus an overlying red boulder-clay was here and
there accumulated upon the surface of a greyish-blue till in which not
a single fragment of any Old Red Sandstone rock occurs.
Recently Messrs. B. N. Peach and J. Horne, in a most instructive paper
on the "Glaciation of Caithness,"[S] have described some remarkable
comminglings of material which occur in a region where the glacial
striæ afford equally striking evidence of conflicting ice-movements.
These phenomena are developed here and there along a line which
indicates the meeting-place of two rival ice-streams, on each side
of which the boulder-clay presents different characteristics--the
one boulder-clay being the _moraine profonde_ of the ice that flowed
ENE. and NNE. towards the Caithness plain, while the other is an
accumulation formed underneath the ice that streamed across that plain
from SE. to NW. These phenomena are thus, as my colleagues remark,
quite analogous to those met with in the middle districts of Scotland,
as described by me, and referred to in a preceding paragraph. Now it
is obvious that while these examples of "intercrossings" of erratics
and "cross-hatching" of striæ all go strongly to support the land-ice
theory of the glacial phenomena, they at the same time negative the
notion of floating-ice having had anything to do with the production of
the phenomena under review.
[S] _Proceedings Royal Physical Society_, Edinburgh, 1881.
Before considering the evidence adduced by Mr. Mackintosh and others
as to the intercrossings of erratics in the drift-deposits of England,
I shall mention some of the more remarkable examples of the same
phenomena which have been noticed by continental geologists. The first
cases I shall cite are those which have been observed in the glacial
accumulations of the Rhone valley in eastern France. The land-ice
origin of these accumulations has never been called in question, and
as the intercrossings of erratics in that region are not only more
common, but much more striking and apparently inexplicable than any
which have been noticed elsewhere, it will be admitted that they of
themselves afford a strong presumption that the conflicting courses
followed by the erratics in certain regions of our own country are
the result rather of oscillations in the flow of land-ice than of the
random and eccentric action of icebergs. The researches of Swiss and
French glacialists have proved that during the climax of the Glacial
period an enormous area in the low-grounds of eastern France was
covered with a huge _mer de glace_, formed by the union of the great
Rhone glacier with the glaciers descending from the mountains of Savoy
and Dauphiny. A line drawn from Bourg by way of Chatillon, Villeneuve,
Trévoux, and Lyons to Vienne, and thence south-east by Beaurepaire to
the valley of the Isère, a few miles above St. Marcellin, indicates
roughly the furthest limits reached by the _mer de glace_. Over all
the low-grounds between that terminal line and the mountains are found
widespread sheets of boulder-clay and sand and gravel, together with
loose erratics. Now and again, too, well-marked terminal moraines make
their appearance, while the rock-surfaces, when these are visible and
capable of bearing and retaining glacial markings, present the usual
aspect of _roches moutonnées_. The same kinds of morainic materials
and ice-markings may of course be followed up into the valleys not
only of the Alps properly so-called, but also into those of the hills
of Bugey and the secondary mountain-chain of Savoy and Dauphiny. It
has indeed long been known that local glaciers formerly occupied the
mountain-valleys of Bugey. For example, a number of small glaciers have
descended from the slopes of the mountains west of Belley (such as Bois
de la Morgue, Bois de Lind, etc.) to the Rhone, and again from Mont du
Chat to the north-west. These glaciers were quite independent of the
greater ice-streams of the neighbouring Alps of Savoy, and the same was
the case with the glaciers of that mountainous tract which extends from
Nantua south to Culoz, between the valleys of the Ain and the Rhone.
From this elevated region many local glaciers descended, such as that
of the Valromey, which flowed for a distance of some twenty miles from
north to south. Again, similar local glaciers have left abundant traces
of their former presence throughout the mountainous belt of land that
stretches between Chambery and Grenoble to the west of the valley of
the Isère. The moraines of all those local glaciers, charged as they
are with the débris of the neighbouring heights, clearly indicate that
the local glaciers flowed each down its own particular valley. There
are certain other appearances, however, which seem at first sight to
contradict this view. Sometimes, for example, we encounter in the same
valleys erratics which do not belong to the drainage-system within
which they occur, but have without doubt been derived from the higher
Alps of Switzerland and Savoy. And the course followed by these foreign
erratics has crossed at all angles that which the local glaciers have
certainly pursued--occasionally, indeed, the one set of erratics has
travelled in a direction exactly opposed to the trend taken by the
others. As examples, I may cite the case of the erratics which occur
in Petit Bugey. In this district we encounter many locally-derived
erratics which have come from Mont du Chat to the west of the Lac
du Bourget--that is to say, they have travelled in a north-westerly
direction. But in the same neighbourhood are found many erratics of
Alpine origin which have been carried from north-east to south-west, or
at right angles to the course followed by the local erratics. Again,
in the valley of the Seran we have evidence in erratics and terminal
moraines of a local glacier which flowed south as far as the Lyons and
Geneva Railway, in the neighbourhood of which, a few miles to the west
of Culoz, its terminal moraines may be observed. This is the extinct
Glacier du Valromey of MM. Falsan and Chantre. Now it is especially
worthy of note that in the same valley we have distinct evidence of an
ice-flow from south to north--_i.e._, _up_ the valley. Erratics and
morainic materials which are unquestionably of Alpine origin have been
followed a long way up the Seran valley--for two-thirds of its length
at least. Before they could have entered that valley and approached the
slopes of Romey, they must have travelled down the valley of the Rhone
from the higher Alps of Savoy in a _south-west_ and _south_ direction
until they rounded the Montagne du Grand Colombier. It was only after
they had rounded this massive mountain-ridge that they could pursue
their course up the valley of the Seran, in a direction precisely
opposite to that which they had previously followed. These and many
similar and even more remarkable examples of the "intercrossings"
of streams of erratics are described by MM. Falsan and Chantre, and
graphically portrayed in their beautiful and instructive work on the
"Ancient Glaciers and Erratic Deposits of the Basin of the Rhone"; and
the explanation of the phenomena given by them is extremely simple
and convincing. The local erratics and moraines pertain partly to the
commencement and partly to the closing stage of the Glacial period.
Long before the south branch of the great glacier of the Rhone had
united with the glacier of the Arve, and this last with the glaciers
of Annecy and Beaufurt, and before these had become confluent with the
glacier of the Isère, etc., the secondary mountain-ranges of Savoy and
Dauphiny and the hills of Bugey were covered with very considerable
snow-fields, from which local glaciers descended all the valleys to the
low-ground. But when the vast ice-flows of Switzerland, Upper Savoy,
etc., at last became confluent, they completely overflowed many of the
hilly districts which had formerly supported independent snow-fields
and glaciers, and deposited their bottom-moraines over the morainic
débris of the local glaciers. In other cases, where the secondary
hill-ranges were too lofty to be completely drowned in the great _mer
de glace_, long tongues of ice dilated into the valleys, and compelled
the local ice out of its course; sometimes, as in the case of the
Valromey, forcing it backward up the valleys down which it formerly
flowed. But when once more the mighty _mer de glace_ was on the wane,
then the local glaciers came again into existence, and reoccupied
their old courses. And thus it is that in the hilly regions at the
base of the higher Alps, and even out upon the low-grounds and plains,
we encounter that remarkable commingling of erratics which has been
described above. Not infrequently, indeed, we find one set of moraines
superposed upon another, just as in the low-grounds of northern
Germany, etc., we may observe one boulder-clay overlying another, the
erratics in which give evidence of transport in different directions.
The observations recorded by MM. Falsan and Chantre, and their
colleagues, thus demonstrate that "intercrossings" of erratics of the
most pronounced character have been brought about solely by the action
of glaciers. In the case of the erratics and morainic accumulations of
the basin of the Rhone, the action of icebergs is entirely precluded.
I may now mention some of the more remarkable examples of
intercrossings of erratics which have been recorded from the glacial
accumulations of north Germany, etc. An examination of the glacial
striæ, _roches moutonnées_, and boulder-clays of Saxony leads to
the conviction, according to Credner, Penck, Torell, Helland, and
others, that the whole of that region has been invaded by the great
Scandinavian _mer de glace_ which flowed into Saxony from NNE. to SSW.
Erratics from southern Sweden and Gothland occur in the boulder-clay,
and the presence of these, taken in connection with the direction
of the glaciation, leaves us no alternative but to agree with the
conclusions arrived at by the Saxon geologists. But, apparently
in direct contradiction of this conclusion, we have evidence to
show that boulders of the same kinds of rock occur in Denmark and
Holland, pointing to a former ice-flow from north-east to south-west
and west. Thus boulders derived from Gothland occur at Gröningen
in Holland, while fragments from the island of Öland are met with
in Faxö; and erratics from the borders of the Gulf of Finland are
encountered at Hamburg. Indeed, when geologists come to examine the
erratics in north Germany and Poland generally, they find evidence of
apparently two ice-flows--one of which went south-south-west, south,
and south-east--spreading out, as it were, in a fan-shape towards the
southern limits reached by the great "Northern Drift,"--while the other
seems to have followed the course of the Baltic depression, overflowing
the low-grounds of northern Prussia, Holland, etc., in a south-west and
west direction. Now, it is quite evident that no one _mer de glace_
could have followed these various directions at one and the same time.
The explanation of the apparent anomaly, however, is not far to seek.
It is reasonable to infer that long before the _mer de glace_ had
attained its maximum dimensions, when as yet it was confined to the
basin of the Baltic and was only able to overflow the northern regions
of Prussia, etc., its course would be determined by the contour of the
pavement upon which it advanced. It would, therefore, be compelled
to follow the Baltic depression, and for a long time it would carry
erratics from Finland, the Baltic islands, and eastern Sweden in a
south-west and west-south-west direction. And this would continue to
be the direction even after a considerable portion of the low-grounds
of Prussia, etc., had been overflowed. But when the ice-sheet was
enabled to advance south into Saxony, Poland, and Lithuania, erratics
from Finland, the Baltic islands, etc., would necessarily cease to
travel towards the west, and hold on a south-south-east, south, and
south-south-west course. Again, when the _mer de glace_ was on the
decline, a time would return when the ice, as before, would be
controlled in its flow by the Baltic depression, and this would give
rise to a further distribution of erratics in a prevalent west-by-south
direction.[T]
[T] For a fuller discussion of the distribution of erratics on
the Continent, I may refer to Appendix, Note B, in _Prehistoric
Europe_, where the reader will find references to the literature of
this interesting subject. [Continental geologists now recognise a
distinct stage of the Ice Age, during which their "Upper Diluvium" was
deposited by a great glacier that occupied the basin of the Baltic.
This "Great Baltic Glacier" appears to have been contemporaneous with
the local ice-sheets and valley-glaciers of the Highlands and other
mountain-tracts of our island. See Article X. 1892.]
No one of late years has been more assiduous in the collection of facts
relating to the intercrossing of erratics in the drift-deposits of
England than Mr. D. Mackintosh.[U] He has written many instructive and
interesting descriptions of the phenomena in question, which he justly
thinks are of prime importance from a theoretical point of view. In a
recent paper[V] he presents us with the results of a systematic survey
of the direction and limits of dispersion of the erratics of the west
of England and east of Wales, which he evidently is of opinion afford
strong support to the iceberg theory, while at the same time they
are directly opposed to the theory of transport by land-ice. I have
attentively considered all the arguments advanced by Mr. Mackintosh in
favour of his views--the one upon which he apparently lays most stress
being that of the intercrossings of erratics observed by him--and I
shall now proceed to point out how the phenomena described by him are
most satisfactorily explained by the land-ice theory. They seem to
me, indeed, to lend additional support to that theory, in the same
manner as the intercrossings of boulders observed in Scotland, northern
Germany, etc., and sub-alpine regions of France. Mr. Mackintosh calls
attention to the fact that erratics of the well-known Criffel granite
are found scattered over a large part of the plain of Cumberland,
from which they extend south along the coast to near the mouth of the
estuary of the Duddon. They reappear on the coast in the neighbourhood
of Blackpool and Liverpool, and again at intervals on the coasts of
north Wales from Flint to Colwyn Bay, and thence to Penmaenmawr and
the neighbourhood of Beaumaris. They are dispersed over the peninsula
of Wirral and the Cheshire plain, etc., and they have been followed
south-east as far as the neighbourhood of Cardington, near Church
Stretton, Burton, Wolverhampton, Stafford, Hare Castle, Macclesfield,
and Manchester. This great stream of boulders, therefore, spreads
out to south-east, south, and south-west: the erratics, to quote
Mr. Mackintosh, "have radiated from an area much smaller than their
terminal breadth." The same is the case, I may remark in passing, with
erratics in the boulder-clays of Scotland, Scandinavia, north Germany,
etc., as also with those in the drift-deposits of the great Rhone
glacier and other ancient glaciers both on the north and south side of
the Alps. Now, the course followed by the Criffel erratics is crossed
at an acute angle by the path pursued by many boulders of Eskdale
granite, and various felspathic rocks derived from the Cumberland
mountains. For example, Cumberland erratics of the kinds mentioned
occur near St. Asaph and Moel-y-Tryfane and in Anglesey, and they
have been followed over a wide district in Cheshire, etc., extending
as far south as Church Stretton and Wolverhampton, and as far east as
Rochdale. More than this, we find that numerous erratics of felstone,
derived from the mountain of Great Arenig, in north Wales, have gone
to north-east as far as Halkin Mountain, in Flintshire, Eryrys, near
Llanarmon, and Chirk, from which last-named place they have been traced
in a south-easterly direction to Birmingham, Bromsgrove, etc. A glance
at the map of England will show that this south-easterly drift of
erratics crosses at an acute angle the paths followed by the Criffel
granite boulders and the erratics derived from Cumberland, so that we
have now several intercrossings to account for. How can this be done by
the land-ice theory?
[U] This enthusiastic geologist died in 1891.
[V] _Quart. Journ. Geol. Soc._, vol. xxxv. p. 425
The explanation seems to me obvious, for the phenomena are, after all,
less striking than similar appearances which have been observed in
Scotland, especially by my colleagues, Messrs. Peach and Horne, in
Caithness and the Orkney and Shetland Islands; and they are certainly
less intricate than the facts recorded by MM. Falsan and Chantre
concerning the intercrossing, interosculation, and direct opposition
of erratic paths in Savoy and Dauphiny. We have only to reflect that
the great _mer de glace_--to which, as I believe, all the English
phenomena are due--did not come into existence and attain its maximum
dimensions in the twinkling of an eye, nor could it afterwards have
disappeared in the same sudden manner. On the contrary, a period
of local glaciation must have preceded the appearance of the great
ice-sheet. At first, and for a long time, permanent snow would be
confined to the higher elevations of the land, and glaciers would
be limited to mountain-valleys; but as the temperature fell the
snow-line would gradually descend, until at last, probably after a
prolonged period, it reached what is now the sea-level. Thus the
formation of _névé_ and glacier-ice would eventually take place over
what are now our low-grounds, and other tracts also, which are now
submerged. It is quite impossible that the vast sheets of ice which
can be demonstrated to have covered Scotland, a large part of England,
Ireland, Scandinavia, and north Germany, and even the limited area of
the Faröe Islands, could possibly have been fed by the snow-fields of
mountain-heights only. The precipitation and accumulation of snow, and
the formation of _névé_ and glacier-ice, must have taken place over
enormous regions in what are now the temperate latitudes of Europe.