The recent movements of glaciers may be noted by the following signs:—
When the ice is advancing, the glaciers generally have a more convex outline, the icefalls are more broken into towers and spires, and piles of fresh rubbish are found shot over the grass of the lower moraines. Moraines which have been comparatively recently deposited by advancing ice are disturbed, show cracks, and are obviously being pushed forward or aside by the glacier. There is a tendency for the glacier to terminate in a vertical front, or “Chinese wall,” as distinguished from the sloping snout of a stationary or retreating glacier, owing to the more rapid advance of the upper layers. If the advance is rapid the overhanging layers will fall, and an ice talus will collect in front. An inspection of the ice will often disclose horizontal lines of sheering which in a side view are seen to rise towards the terminal wall. Search should be made along these sheer planes for included moraine material, and any proofs of elevation of this material noted.
When the ice is in retreat it terminates in a gently sloping snout. The marks of its further recent extension are seen fringing the glacier both at the end and sides in their lower portions, the glacier fails to fill its former bed, and bare stony tracts, often interspersed with pools or lakelets, lie between the end of the glacier and the mounds of recent terminal moraines.
Where a glacier has retreated to any considerable extent, careful observations of the form of its bed are of value. What is the nature of the rock surfaces exposed—convex or concave; are they rubbed smooth on their leesides; how far have the contours of the cliffs or slopes, or the sides of any gorge, been modified where they have been subjected to ice-friction? Is there any evidence that the ice has flowed over large boulders, or loose soils, such as gravel, without disturbing them? How has it affected rocks of different hardness, for instance, veins of quartz in a less hard rock? Generally, do the appearances indicate that the glacier has excavated, or only abraded and polished its bed; that it has scooped out new rock-basins, or only cleaned out, scratched, and preserved from filling-up by alluvial deposits or earthslips, existing basins? What is the general character of the valley bottom and the slopes above and below the most conspicuous ancient moraines?
With a view to ascertaining the oscillations in length of a glacier, the traveller or surveyor should, if possible, mark on the ground the position of the end of the glacier at the time of his visit, so that the next visitor will be able to measure the movement that has taken place since his predecessor’s visit. Leaving out of question elaborate trigonometrical methods, such, for instance, as have been carried out on the Rhone Glacier in Switzerland, the following means may be adopted for recording the position of the glacier front with considerable accuracy. Paint some signs on large boulders, not too far from the end of the glacier, and measure their distance from it by a tape (Richter’s system), or build a low wall of stones of a few yards in length, and, say 15 to 20 inches in height, some distance from the ice-end, and measure this distance (Gosset’s system). It is to be recommended that the stones of these walls should also be painted. In either case the date and distance should be painted on the stones. If the traveller himself returns after some interval—even after only two or three weeks—he will be able to judge of the movement of the glacier, and he will have laid down a basis for further observations by future travellers. A plane-table sketch on a fairly large scale with contours or form lines of the ice front and its immediate vicinity is of the greatest value, as it records not only the position but also the form of the glacier. The sketch should, of course, show the position of the painted boulders or other fixed points near the ice. To ascertain the recent retreat of a glacier, measure the distance from the ice front to the most advanced terminal moraine, where vegetation first shows itself. The bare ground recently left by glaciers is easily recognisable. The diminution of volume is best measured by ascertaining the height of bare soil left on the sides of the lateral moraines in the portion of the glacier within the zone of vegetation. All photographic representations of the glacier end, and of the ground which has been freed from the glacier ice, are of great value. Those will be of most service that show the position of the glacier-snout with relation to some conspicuous rock or other feature in the local scenery. Each photograph should be dated, and the bearings and distance of the camera with reference to any such feature accurately noted.
Neighbouring glaciers often furnish very different results as regards advance or retreat, owing to the fact that steep glaciers anticipate in their oscillations those the beds of which are less inclined.
One of the results most to be desired is an exact knowledge of the dates:
- I. Of the maximum extension of the ice.
- II. Of the commencement of retreat.
- III. Of the minimum.
- IV. Of the commencement of fresh increase.
and even information giving approximate dates—say, to within a decade, may prove to be of value.
In dealing with a mountain group, therefore, the traveller should note where he can get the information as to the past, the date of the commencement of the actual movement of each glacier, and in all cases whether the ice is in advance, or retreat, or stationary. Of course the rate of forward movement, or velocity of the ice, and the oscillations in the extension of the ice must be kept carefully distinct. Observations should also be made on the presence of blue bands, and their relation to the lines of stratification in the névé noted.