Glacial Sculpture
Within the present generation sculpture by glaciers has received much attention and has involved a reconsideration of the ability of ice to erode which in turn involves a crystallization of views of the mechanics of moving ice. The evidence for glacier erosion has remained largely physiographic and rests on a study of land forms. In fact, the inadequacy of structural features or of river corrasion to account for flat-floored, steep-walled gorges, hanging valleys, and many lake basins, rather than a knowledge of the mechanics of ice has led to the present fairly general belief that glaciers are powerful agents of rock sculpture. The details of the process are not yet understood.
Erosion by glaciers enters the arena of active discussion in 1862–63. The possibility had been suggested by Esmark (1827) and by Dana (1849) in the description of fiords and by Hind (1855) with reference to the origin of the Great Lakes. It appears full-fledged in Ramsay’s classic, which was published simultaneously in England and in America.72 The argument runs as follows: There is a close association of ancient glaciers and lakes especially in mountains; glaciers are amply able to erode; evidences of faulting, special subsidence, river erosion, and marine erosion are absent from the lake basins of Switzerland and Great Britain. To quote Ramsay:
“It required a solid body grinding steadily and powerfully in direct and heavy contact with and across the rocks to scoop out deep hollows, the situations of which might either be determined by unequal hardness of the rocks, by extra weight of ice in special places, or by accidental circumstances, the clue to which is lost from our inability perfectly to reconstruct the original forms of the glaciers.”
“I believe with the Italian geologists, that all that the glaciers as a whole effected was only slightly to deepen these valleys and materially to modify their general outlines, and, further (a theory I am alone responsible for), to deepen them in parts more considerably when, from various causes, the grinding power of the ice was unusually powerful, especially where, as in the lowlands of Switzerland, the Miocene strata are comparatively soft.”
Whittlesey (1864)73 considered that the rock-bound lakes and narrow bays near Lake Superior were partly excavated by ice. LeConte (1875)74 records some significant observations in a pioneer paper on glacier erosion which has not received adequate recognition. He says:
“... I am convinced that a glacier, by its enormous pressure and resistless onward movement, is constantly breaking off large blocks from its bed and bounding walls. Its erosion is not only a grinding and scoring, but also a crushing and breaking. It makes by its erosion not only rock-meal, but also large rock-chips.... Its erosion is a constant process of alternate rough hewing and planing.
If Yosemite were unique, we might suppose that it was formed by violent cataclysms; but Yosemite is not unique in form and therefore probably not in origin. There are many Yosemites. It is more philosophical to account for them by the regular operation of known causes. I must believe that all these deep perpendicular slots have been sawn out by the action of glaciers; the peculiar verticality of the walls having been determined by the perpendicular cleavage structure.”... A lake in Bloody Canyon “is a pure rock basin scooped out by the glacier at this place.... These ridges [separating Hope, Faith, and Charity valleys] are in fact the lips of consecutive lake basins scooped out by ice.
... Water tends to form deep V-shaped canons, while ice produces broad valleys with lakes and meadows.... I know not how general these distinctions may be, but certainly the Coast range of this State is characterized by rounded summits and ridges, and deep V-shaped canons, while the high Sierras are characterized on the contrary by sharp, spire-like, comb-like summits, and broad valleys; and this difference I am convinced is due in part at least to the action of water on the one hand, and of ice on the other.”
King (1878)75 assigned to glacial erosion a commanding position in mountain sculpture. In regard to the Uintas, he says:
“Glacial erosion has cut almost vertically down through the beds carving immense amphitheatres with basin bottoms containing numerous Alpine lakes.... Post-glacial erosion has done an absolutely trivial work. There is not a particle of direct evidence, so far as I can see, to warrant the belief that these U-shaped canons were given their peculiar form by other means than the actual ploughing erosion of glaciers....”
These contributions from the Cordilleras corroborating the conclusions of Ramsay (1862), Tyndall (1862), Jukes (1862), Hector (1863), Logan (1863), Close (1870), and James Geikie (1875), made little impression. The views of Lyell (1833), Ball (1863), J. W. Dawson (1864), Falconer (1864), Studer (1864), Murchison (1864, 1870), Ruskin (1865), Rutimeyer (1869), Whymper (1871), Bonney (1873), Pfaff (1874), Gurlt (1874), Judd (1876), prevailed, and the conclusions of Davis in 188276 fairly expressed the prevailing belief in Europe and in America:
“The amount of glacial erosion in the central districts has been very considerable, but not greatly in excess of pre-glacial soils and old talus and alluvial deposits. Most of the solid rock that was carried away came from ledges rather than from valleys; and glaciers had in general a smoothing rather than a roughening effect. In the outer areas on which the ice advanced it only rubbed down the projecting points; here it acted more frequently as a depositing than as an eroding agent.”
During the past quarter-century the cleavage in the ranks of geologists, brought about by Ramsay’s classic paper, has remained. Fairchild and others in America, Heim, Bonney, and Garwood in Europe argue for insignificant erosion by glaciers; and Gannet, Davis, Gilbert, Tarr in America followed by Austrian workers present evidence for erosion on a gigantic scale. A perusal of the voluminous literature in the Journal and elsewhere shows that the difference of opinion is in part one of terms, the amount of erosion rather than the fact of erosion; it also arises from failure to differentiate the work of mountain glaciers and continental ice sheets, of Pleistocene glaciers and their present diminished representatives. The irrelevant contribution of physicists has also made for confusion.
It is interesting to note that the criteria for erosion of valleys by glaciers has long been established and by workers in different countries. Ramsay (1862) in England outlined the problem and presented generalized evidence. Hector (1863) in New Zealand pointed out the significance of discordant drainage, the “hanging valleys” of Gilbert. The U-form, the broad lake-dotted floor, and the presence of cirques and the process of plucking were probably first described by LeConte (1873) in America. The truncation of valley spurs by glaciers pointed out by Studer in the Kerguelen Islands (1878) was used by Chamberlin (1883) as evidence of glacial scouring.