The character profiles which result from sculpture by mountain glaciers.—The lines which are repeated in landscapes carved by mountain glaciers are easy to recognize ([Fig. 405]). The highest horizon lines are the outlines of horns which are separated by cols. Minaret-like palisades, or “files of gendarmes”, often run for long distances as the characteristic comb ridges. Lower down and lacking the lighter background of the sky, we make out with less distinctness the U-valley, either with or without the albs to show that the sculpturing process has been interrupted by uplift.
Fig. 407.—Two views illustrating successive stages in the shaping of tinds or “beehive” mountains.
The sculpture accomplished by ice caps.—In the case of ice caps, the only rock exposed is found in the neighborhood of the margin—the projecting islands known as nunataks. It is essential for the existence of the ice cap that the rock base should have relatively slight irregularities compared to the dimensions of the cap itself. Except in very high latitudes this base must be somewhat elevated, for like mountain glaciers ice caps are nourished by the surface air currents, and their snows are deposited above the snow line.
The Norwegian tind or beehive mountain.—Within temperate or tropical climes the snow line lies so high that only the loftier mountains are able to support glaciers. It follows that those which are formed flow upon relatively high grades with correspondingly high rate of movement and increased cutting power. Within high latitudes the snow is found nearer the sea level, and glaciers are for the most part correspondingly sluggish in their movements as well as less active denuding agents.
To this condition characteristic of high latitude glaciers, there is added in Norway another in the peculiar shape of the basement beneath the recent and the still existing glaciers. The plateau of Norway is intersected by a network of deep and steep walled fjords, and the glaciers have developed as small ice caps perched upon veritable pedestals of rock, over the margins of which their outlet tongues of ice descend on steep slopes into the fjord. The tops of the pedestals thus come to be shaped by the plucking and abrading processes into flat domes ([Fig. 406]), while the knobs of rock, which as nunataks reach above the surface of the ice, divide the outflowing ice tongues at the margin of the pedestal. These tongues being much more active denuding agents, because of their steep gradients, continually lower their beds, thus transforming the earlier knobs of rock into high and steep mountains of more or less circular base. Such “beehive” mountains upon the margins of the fjords are the characteristic Norwegian tinds ([Fig. 407]).
Reading References for Chapter XXVI
I. C. Russell. Quaternary History of Mono Valley, California, 8th Ann. Rept. U. S. Geol. Surv., 1889, pp. 329-371, pls. 27-37.
F. E. Matthes. Glacial Sculpture of the Bighorn Mountains, Wyoming, 21st Ann. Rept. U. S. Geol. Surv., 1900, Pt. ii, pp. 179-185, pl. 23.
W. D. Johnson. Maturity in Alpine Glacial Erosion, Jour. Geol., vol. 12, 1904, pp. 569-578.