Fragments of Earth Lore: Sketches & Addresses Geological and Geographical

by James Geikie

The Intercrossing of Erratics in Glacial Deposits.[Q]

[Q] The Scottish Naturalist, 1881.

Among the many phenomena connected with the glacial deposits of this 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.