XIII. Illustrations of the viscous theory of glacier motion

Abstract

When Gruner proposed the explanation of glacier motion by the sliding of the ice over its bed, and De Saussure illustrated and confirmed it by considerations drawn from the lubricating action of the earth’s heat melting the ice in contact with the soil, there is no reason to suppose that either of them thought it necessary to take into account the varying form of the channel through which the glacier had to pass, and the consequently invincible harrier presented to the passage of a rigid cake of ice through a strait or narrow aperture when it occurred. This is the more remarkable, because he conceives that the inequalities of the bed or bottom may be overcome by the hydrostatic pressure of the water, which he supposes may be impri­soned between the rock and the ice, so as absolutely to heave the latter over the resisting obstacles. I believe that in no part of De Saussure’s writings will there be found any, the slightest reference to the possibility of the glacier when fairly formed moulding itself to the inequalities of the surfaces over which gravity urges it; nor is there any trace of the correlative fact of an unequal motion of the sides and centre of the ice, which may in some sense be considered as the geometrical statement of the preceding physical fact. The fact of plasticity was suspected by Basil Hall, and more distinctly announced by Rendu, as shown in the first part of this paper; but it could not be proved until the geometrical fact of the swifter motion of the centre of the glacier relatively to the sides was established in 1842. The contrary opinion at that time generally entertained would have been conclusive against the hypothesis of plasticity called forth by the gravity of the mass.