Quarterly Journal of the Geological Society of London/Volume 33/On the Action of Coast-Ice on an Oscillating Area

49. On the Action of Coast-Ice on an Oscillating Area. By Prof. John Milne, F.G.S., of the Imperial College of Engineering, Tokio, Japan. (Read June 20, 1877.)

[Abridged.]

[In this paper the author commenced by discussing those theories which have been proposed to account for glacial markings by the assumption of a polar ice-cap. He argued that even if such vast sheets of ice as are accepted by many geologists could have produced the grooves and other markings, and the drift deposits ascribed to their agency, the action of the sea during subsequent submergence and upheaval of the regions where these phenomena are displayed would certainly have effaced all such traces of ice-action. Other objections might also be raised to the hypothesis of great ice-sheets; and he thought it desirable to consider fully "whether other agents may not have shared the work with which they have been credited." He proceeds as follows:—]

Excepting glacier-action, which will generally have taken place in elevated regions, I think it can be shown that the modelling and scratching has in many instances been produced by an agent the existence of which is certain, and which acted under conditions favourable to the preservation of its work. Such an agent is coast or floe-ice acting on a rising area.

Examples of the work done, and being done, by such an agent under the stated conditions I shall take from Newfoundland, Labrador, and Finland. Every year the shores of these countries are surrounded by a fringe of ice. During the process of freezing and at other times, by various causes, such as tides, currents, wind, the driving in of pack-ice from the sea, this is forced up and down upon the shore on which it rests. By actions such as these, which extend sometimes 100 yards back above high water, the shore line is scratched, scoured, and rounded. Boulders and angular stones travel along the coast, and are often deposited in banks and lines far removed from the cliffs or mountain masses from which they were originally detached. These actions are perhaps best seen upon the small islands which form archipelagos along the shores of all those countries to which I have referred.

Lying well out from the east coast of Newfoundland there is an island called Funk Island, which, through the action of the floe-ice by which it is annually invaded, has, I believe, received not only the ordinary marks due to the moulding of ice, but also its contour as a whole. It is about half a mile in length, very low and flat, and is situated right in the stream of arctic ice coming south from Baffin's Bay and Labrador. The northern end of this island, which has every year to face the pressure of the vast fields of ice which are borne down upon it, is visibly worn down and covered with erratic boulders, whilst the opposite extremity is a low but abrupt cliff.

Passing by this extreme case, and considering the generality of islands as exhibited in these three districts (but more especially perhaps those of Finland) with regard to the way in which they are acted on by ice, I divide them into the following three groups:—

1. Rocks which at high water are hidden from view, but at low water appear as small islands.

2. Those small islands which are always above sea-level, but yet are annually covered with ice, which is driven up their sides and over their summits by outside pressure.

3. Those islands whose summits are beyond the influence of ice. The tops of these are generally covered with trees and vegetation, whilst others, which are usually not so high, have only a black colour, probably due to a growth of lichen.

Islands emerging from an ocean, as these appear to be doing, must successively pass through the stages I have enumerated. During the first two of these stages they are wholly within the influence of coast-ice, as may be clearly seen from those islands on the Finnish coast, which have not only been moulded, but are kept of a whitish colour by the scouring they continually undergo. This latter character is especially noticeable in the islands of the third class, the upper parts of which have been raised so high that they are now beyond the influence of the scouring agency; these parts are black with a growth of lichen, whilst those at a lower level, which are annually invaded by the ice, are kept of a whitish colour. New, between the dark upper parts of such islands as I have classed in this last group and their lower parts, so far as I could see, there is a continuity in contour between the undulations and curves along the margin of the water (which I know to have been produced by coast-ice) and those which lie at higher levels. From this it seems to me an inevitable conclusion that the upper mouldings were produced in the same way as those below, and have since been raised to their present position.

Should two or three, or, still better, a whole archipelago of these islands slowly rise to unite and form a continuous land surface, I do not think it would be unlike many parts of Finland.

Turning from the islands to the adjoining mainland, it seems natural to conclude that the actions which produced the features in the one were identical with those producing the features in the other.

In these arguments it must be remembered that not only does the action of coast-ice upon an oscillating area explain many phenomena of ordinary occurrence, but it also readily explains some phenomena, such as the appearance of erratics at levels higher than the parent rock from which they were derived, which would be difficult to account for by either the action of sheets of ice or of glaciers.

Icebergs I have not considered, because, both from my own observations and more especially from the observations of those whose labours lie amongst them, I believe them, as compared with all other forms of ice, incomparable as transporters of material, and, from mathematical considerations of their flotation, incapable of producing any great effect in grinding the shoals on which they may occasionally ground.

Many of the phenomena presented by our northern countries and attributed to ice-action are readily explained by the supposition of coast-ice acting on a rising area, but only with difficulty when either glaciers or ice-sheets are supposed to have been the grinding agents. In such cases, all that I ask is that coast-ice may have its due.

Cosmical changes, or even changes in the geography of land and sea, I think have been sufficient to bring coast-ice as far south as any of the low-lying regions which exhibit traces of ice-action. These changes may also on the higher ground have given rise to glaciers which ground out lake-basins and moulded valleys; but that these changes produced large ice-caps, filling oceans and covering countries, I do not see that we are justified in supposing until there is a greater convergence of such evidence as can be brought to bear upon the subject.

If we take a map of Northern Europe on which are indicated the general direction of ice-markings, any inference which can be drawn from such directions, which all point more or less at right angles to the sea-coast, is as favourable to the view that they were produced by coast-ice acting on a rising area as it is to the fact that they point out the direction in which the great ice-sheets travelled.

In conclusion, I will say that one thing appears to me certain—namely, that, even if we accept the most favourable views of large ice-caps, the appearances presented by many countries, which have hitherto been ascribed to their action, ought rather, for reasons already stated, to have been accredited to the action of coast-ice on a rising area.

(For the Discussion on this paper, see p. 861.)