The position or situation of the entrance is important. In almost all cases it has a northerly exposure, and is sheltered against entering winds. If these two conditions do not exist the ice supply surely suffers. Sometimes the entrance is more or less tortuous. In some cases it is protected by a fringe of trees. Still, there is no absolute rule about entrances. The Friedrichsteinerhöhle faces about due south, and at midday in summer, the sun shines all the way down to the ice floor, causing mists to form. In the Kolowratshöhle, the entrance is badly sheltered against the wind and this undoubtedly affects the supply in summer and causes more rapid melting there than in some other cases.[58]
[58] See Part III.: Decorah, [page 178].
Freezing boulder taluses invariably have the ice near the surface, and probably it is never a dozen meters distant from the open air. These taluses are one of the strongest links in the chain of evidence proving the winter’s cold theory. The snow and ice on the surface of the taluses and on the surface of the boulders in gullies melts away, while it still lingers underneath the boulders. It seems self-evident that the melting snow water has run to the lowest level and there congealed, and then remained because it was better sheltered than the ice outside.
The subsoil ice of the tundras of Siberia and Alaska is almost identical with the ice of boulder formations, except that it extends under larger areas. It is the product of a climate where there is a long, rigorous winter and it is not surprising that the ice is found at greater depths than in more southerly latitudes.[59] The depth to which the ice extends is, of course, determined by the depth to which the winter’s cold can penetrate the soil. There is no doubt that the causes of this ice are local, that is, that it is due to the long prevailing low temperatures.
[59] See Part III.: Alaska, [page 166]; Klondike, [page 167]; Kowak River, [page 166]; Kotzebue Sound, [page 166].
The freezing wells of which the most conspicuous examples are at Brandon, Owego, Decorah, and in Montana, seem also due to local causes and the ice is never far from the surface, that is, not over twenty meters; and apparently also it forms above the water horizons which supply the wells.
The ice sheet on Mount Etna[60] does not seem to be at any great depth. It apparently had a different origin from most subterranean ice masses, in that the snow probably fell first and was then covered by a flow of lava. It is, therefore, almost sui generis in its mode of formation, unless there are similar sheets on other volcanoes, which is probable in a country like Iceland, and which is said to be the case in Tierra del Fuego,[61] But the original cause of the ice sheet on Etna was the same as all other subterranean ice masses, namely the cold of winter.
[60] See Part III., [page 210].
[61] See Part III., [page 190].
Evaporation and Movements of Air.—The formation of subterranean ice is sometimes assigned partly to evaporation or to expansion of the air. The theory is an old one, and both scientific and non-scientific men have advanced it.[62]