Many explanations of the origin of fjords are to be found in works on geography and geology as well as in guide books, but none of them seems fully to meet the case.
Fjords are the natural result of the laws which govern the movement of glaciers; they are formed by erosion of the beds of glaciers as they flow into the sea and in the course of centuries become displaced inland. For a further account the reader is referred to my two papers on the theory of glacier mechanism.Footnote * Footnote †
The Laws of Glacier Movement. Just as liquid precipitation in temperate countries follows its course to the ocean under the impulse of gravity, so does the solid precipitation transformed by compression into ice in Polar regions make its way seawards, eroding its bed as it flows and excavating a rock channel. The erosion is slight in the accumulation areas, but increases greatly at lower levels where I have calculated that it is of the order of 1 cm. a year, as opposed to no more than 0.2 to 1.0 mm. a year for the whole glacier System; the latter figures have been established by the measurement of the amount of solid material found in the streams issuing from glaciers.
Erosion takes place uniformly throughout the·bed of the ice stream, so that the glacier gradually sinks into the ground parallel to the surface. At the same time it cuts its way headward (see Fig. 1).
Fig. 1
The erosive power is supplied by the detritus which the glacier pushes along its bed; this detritus, too, is almost uniform throughout the lower part of the ice stream, so that the bed also shifts headward parallel to the surface, leaving a nearly vertical rock wall on either side.
In its displacement inland the glacier leaves at its seaward end a channel which becomes the fjord. Its breadth corresponds with that of the glacier and decreases slightly uphill, being a function of the mass of the glacier and the detritus it transports.
The glacier entering the water erodes the rock until a depth is reached at which its erosive power is reduced to nil by the melting of the ice. It will be realized that so long as the erosive power of a glacier remains constant the depth of a fjord in a rock of homogeneous texture will also remain constant. Nevertheless the erosive power and the depth must diminish as the glacier enters the sea, for under the influence of the salt water the ice will melt more rapidly than in the fresher water of the fjord fed by the glacial streams. In this way is formed the rocky threshold so commonly present at the seaward ends of fjords.
The depth of the fjord depends on the erosive power of the glacier that formed it. Thus a glacier of outstandmg size will give rise to a fjord of equal importance. Sediments and rocky debris transported by the glacier are deposited on the bottoms of fjords and break up the regularity of their beds. Nevertheless glacial lakes are often present in the higher reaches of a glacier System and contain great quantities of water; from time to time these burst their barriers and produce immensely powerful floods. It is conceivable that, under favourable conditions, these might wash the bed clean and transport the Sediments out to sea.
As the glacier excavates its way into the ground it leaves its two walls, which may retain their vertical cross-section or may by degrees, according to the nature of the rock, become less steep by weathering.
I have shown in my theory of glacier formation that there is a relationship on the one hand between the breadth of a glacier and its depth and on the other between its normal depth and its mean slope—a relationship varying, of course, with the nature of the rock. In this way I have set forth the relationship between the slope, depth and width of glaciers varying with varying degrees of roughness (coefficients de rugosité) of the rock.
The diagram in my book (1944) gives the observations made in a large number of glaciers and shows that they are in good accord with the formula R.K. given there (p. III).
If we apply these data to the largest of the Norwegian fjords—the Sogne Fjord—we find that the time it has taken to form, based on an annual wastage of 1 cm., has been 250,000 to 450,000 years, a figure which admittedly aims only at giving the order of magnitude of time.
The calculations are accurate especially for glaciers which only carry small quantities of detritus. To meet these conditions their courses must be relatively short and their rock beds resistant, conditions which are to be found on the coast of Norway. On the Swedish coasts, on the other hand, as in Switzerland, the glaciers have excavated a series of lakes, which, however, develop in accordance with the same laws that govern fjord formation.
The more important prehistoric glaciers flowing into the sea with gentle slopes and much morainic material, such as those which fashioned the great river-beds flowing into the Arctic Ocean in northern Asia, have almost entirely filled up the fjords with alluvium, but the forms of these rivers still betray their origin as glaciers and fjords.
In the Swiss glacial valleys, cut in homogeneous rock, the glaciers usually cut back their beds in a straight line in a manner analogous with that which gives rise to fjords. Thus one often finds in the Alps valleys with vertical sides and even sometimes, in their upper sectors, the remnants of a glacier whose width corresponds with the gorge it has cut in the course of centuries. Examples of this are to found in many places, notably in the Lauterbrunnen valley in the Bernese Oberland and the Blindenthal in the Valais.
