One is somewhat astounded, on each re-reading of parts of Muir’s works, at the remarkable knowledge which he had acquired, mainly through his own observations, of glacial processes and their physiographic effects. Muir studied the Sierra canyons, cirques, and peaks at a time when knowledge regarding the principles of glacial erosion was not wide-spread. Geology, in contrast to such sciences as Physics, Chemistry and Astronomy, is a very young branch of learning; more than half of what we know in the geological sciences has been discovered and acquired in the last 75 years. Muir was conversant with the glaciology of his day, as discussed in geologic texts and other geologic literature, but much that he wrote about was not treated in textbooks but related primarily and uniquely to the Sierra and was derived directly from his own extensive and long-continued field studies. His contribution to glacial erosion the more singular when one realizes that little attention had been give to alpine glaciation in western America by other geologists at the time when he worked in the Sierra and he therefore received little stimulus or inspiration from other students of the same natural phenomena. How far he was ahead of his time is perhaps suggested by the fact that it was nearly a half century before another study comparable in scope of the nature of glacial erosion in the Sierra and its effects was undertaken.
Muir recognized very clearly the evidences of ice work in the Sierra and was able to differentiate those areas which had undergone glacial erosion from those which had not been covered. He thereby developed not only a very broad knowledge of the former extent of the ice over the range but also of the paths which it followed in moving from the high areas of accumulation to the lower levels of ablation. He understood fully, for instance, that the ice stream which came down through Tuolumne Meadows bifurcated, and that one branch rose over the 500-foot divide to glide down through what is now the basin of Lake Tenaya and down Tenaya Canyon to join the Merced Glacier, which was fed mainly from an entirely different drainage basin.
By careful observation, inference, and induction Muir reached the important conclusion that the nature of the joint structure in the Sierra granite largely determined the character of the physiographic features modeled by the ice. This seemed a simple idea after it was announced and fully demonstrated, but that is true for many of the most important generalizations of science: ideas that require years and fortunes to discover or demonstrate can be conveyed to later students in a few moments. Muir showed how curved jointing produced the domes found from the Merced to the Kings, and how plane vertical jointing is responsible for the cleaving off of the northern portion of Half Dome. He well said that “the grain of a rock determines its surface forms” under ice attack.
He also demonstrated beautifully that “all the Yosemites in the Sierra occur at the confluence of two or more glacial canyons” and that the greater the number of confluents and their magnitudes and the steeper their gradients the deeper and wider is the Yosemite below their confluence.
Muir also recognized very fully the insignificance of postglacial erosion, and cites in very logical fashion the very sound evidence on which his conclusion was reached. He found the present rock surfaces or hill slopes very little below the surfaces which he could prove had been cut by the ice. He stated that in the high country the postglacial degradation probably did not average more than 3 inches, and in the middle altitudes not more than a foot. While opinions might differ now concerning such exact figures, glacialists would agree that they are of the right order. Muir of course did not know that postglacial time has been only about 15,000 years.
Even though he overestimated the extent of the ice in the Sierra and the total excavation accomplished by it, he was among the first to realize its huge volume and the sculpturing wrought by it. He believed the ice covered the Sierra from summit to base, but we now know that the western half of the range was scarcely reached by it. But his urging its widespread distribution over the mountains was a valuable contribution. His idea that the ice had removed all of the upper part of the granite batholiths, from the slate roof down to the present land surface, a thickness he estimated at a mile, was likewise too large quantitatively, but his writings opened men’s minds with regard to the huge volumes of rock that were removed.
Muir was not aware of multiple glaciation—that the Ice Age consisted of four advances and disappearances of the ice—but he may have recognized some of the evidence for it, for he remarks that the glaciers apparently contracted and expanded constantly and that the moraines differ greatly in their nature and their materials.
Although it was not realized by geologists when Muir wrote about the Sierra glaciers that ice is not a rigid solid but more like a very viscous fluid, and that hence it cannot be pushed uphill for any considerable distance, he contributed very important information and ideas regarding Sierra history and glacier mechanics in demonstrating at numerous localities that the ice had risen up over divides hundreds of feet high. We now know that the motive power for such uphill ice movements resides in a overall downhill slope of the upper surface of the ice upstream from the obstacle or ridge.
Muir’s essays regarding glacial sculpture in the Sierra are remarkable both in their scientific reasoning and in their literary style. In the early stages of a science, or in the early stages of the study of a natural phenomenon, the investigational methods are usually largely descriptive and the inferences or conclusions drawn from the limited data are commonly at least partly speculative and not the result of rigorous analysis. Muir’s habit of making careful and rather complete observations of such phenomena as jointing in the granite and then reasoning rigorously to a conclusion about its effect on the physiographic forms produced under glacier sculpture was one of the early examples in geology of the use of the inductive method which, stated formally in American scientific literature several decades later by Grove Karl Gilbert, T. C. Chamberlin, William Morris Davis, and others, has become the normal and conscious procedure in scientific research.
The style of Muir’s writing was equally unique. It combined scientific precision of expression and rationality of treatment with a grace of statement which afforded much pleasure to the reader. He chose just the right words for each idea, and when the English language did not quite suffice for his purposes he invented such quaint but expressive terms as peaklets, mountainets, and “past flowed rock,” meaning a rock past which the glacier had moved. His writings are marked by that simplicity and clarity which is characteristic of men who know a subject very thoroughly and whose minds comprehend the human meaning of the knowledge they have accumulated.
Muir was one of the small group of men of whom America has had far too few, who published scientific knowledge in fascinating but accurate form, not only for the enjoyment and information of the public, but a an inspiration to young men and women who through innate interest might take up careers in more intensive research on problems of Nature to which he had given them such stimulating introductions.
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