A Source Book in Geology, 1900-1950

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Author: Willard Drake Johnson  | Date: 1904

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The Profile of Maturity in Alpine Glacial Erosion

*

My own acquaintance with the phenomena of glaciation of the alpine type had its beginning in the Sierra Nevada, in 1883, in the latitude of the Yosemite Valley—the so-called High Sierra. Prevailing opinion as to that region, it appeared, ranged between the two extreme views indicated; namely that, as regards quantitative effects in degradation more especially, glaciation had been widely destructive of the preglacial topography, on the one hand; on the other, that it had been relatively protective. But there was no recognition of distinctive forms—beyond "U-canyons" and moraines. I had little notion, therefore, as to what I should discover; only an open mind and a lively curiosity.

I was a maker of topographic maps, of some experience, and had a topographer’s familiarity with the erosion aspects of mountains; but only of unglaciated mountains. I had as well, however, something of the inquisitiveness of the physiographer as to the origin and development of topographic forms.

The first station occupied in this work of survey was Mount Lyell, one of the most widely commanding summits of the vast mountainous tract of the High Sierra.

From Lyell there was disclosed a scheme of degradation for which I had not been in the least prepared. No accepted theory of erosion, glacial or other, explained either its ground-plan outlines or its canyon-valley profiles; and, so far as I can see, none makes intelligible its distinctive features now. The canyons, at their heads, were abnormally deep; they were broadly flat-bottomed rather then

the ratio of bottom width to depth often being several to one; and their head walls, as a rule, stood as nearly upright, apparently, as scaling of the rock would permit. I characterized them, figuratively, as "down at the heel." In many instances the basin floor, of naked, sound rock in large part, and showing a glistening polish on wet surfaces, was virtually without grade, its drainage an assemblage of shallow pools in disorderly connection; and not infrequently the grade was backward, a half-moon lake lying visibly deep against the curving talus of the head wall, and visibly shallowing forward upon the bare rock-floor.

The amphitheater bottom terminated forward in either a cross-cliff or a cascade stairway, descending, between high walls, to yet another flat. In this manner, in steps from flat to flat, commonly enough to be characteristic, the canyon made descent. In height, however, the initial cross-cliff at the head dominated all. The tread of the steps in the long stairway, as far as the eye could follow, greatly lengthened in down-canyon order. In that order, also the phenomena of the faintly reversed grade and of the rock-basin lakes rapidly failed. Apparently, at the canyon head, the last touch of vanishing glaciation had been so recent that filling had not been initiated, while down-stream, incision of the step cliffs and aggradation of the flats had made at least a beginning in the immense task of grade adjustment; the tread of the step was graded forward, but so insensibly, as a rule, that its draining stream lingered in meanders on a strip of meadow, as though approaching base-level. These deep-sunk ribbon meadows, still thousands of feet above the sea and miles in length, reflecting in placid waters their bordering walls or abnormally steep slopes, presented an anomaly of the longitudinal profile in erosion no less impressive than that of the upright canyon heads.

In ground plan, the canyon heads crowded upon the summit upland, frequently intersecting. They scalloped its borders, producing remnantal-table effects. In plan as in profile, the inset arcs of the amphitheaters were vigorously suggestive of basal sapping and recession. The summit upland—the preglacial upland beyond a doubt—was recognizable only in patches, long and narrow and irregular in plan, detached and variously disposed as to orientation, but always in sharp tabular relief and always scalloped. I likened it then, and by way of illustration I can best do so now, to the irregular remnants of a sheet of dough, on the biscuit board, after the biscuit tin has done its work.

In large part, apparently, a preglacial summit topography had been channeled away. By sapping at low levels, by retrogressive undercutting on the part of individual ice-streams at their amphitheater heads in opposing disorderly ranks, the old surface had been consumed, leaving sinking ridges, meandering dulled divides, low cols or passes, and passageways of transection pointing to piracy and to wide shiftings of the glacial drainage. There was not wanting a scattering of the more evanescent sharp forms of transition which the hypothesis would require, as thin arêtes, small isolated table caps, needle-pointed Mätterhorn pyramids with incurving slopes, and subdued spires (in the massive granite tracts) with radiating spurs inclosing basin lakes. . . .

. . . . .

. . . The adjusted grade in river erosion is a smooth curve, lessening in declivity in the direction of flow. The glacier, however, by ablation, is diminished in volume as it lengthens; it is normally deepest close to its head; and possibly it is most effective in scour-erosion in proportion as it is deep. It must, in that event, tend to produce a valley "down at the heel."

The reverse grade, on amphitheater floors especially, occurs with sufficient frequency to be regarded as a type form. Rock-basin lakes, beginning at the amphitheater head, sometimes have notable length, several times the canyon width. The upper surface of the glacier here, on the other hand, invariably declines forward. Thus, in specific instances, it is not merely inference, but fact, that the glacier is deepest at the rear, and excavates there to a forward-rising grade.

It is, furthermore, implied that forward inclination of bed is not essential to glacier movement. It is not necessary, merely to determine that question, to inquire intimately into the nature of glacial motion. Fundamental in that motion, apparently, is the weight of the ice; and if the glacier at bottom, under its own weight, is not strictly viscous, it is apparently at least viscoid, responding in effect to the law of liquid pressures.

A viscous substance, heaped upon a level surface, spreads in mounded disk form, deepest at the center. Its flow-curve, in any radial vertical plane, advances from the bottom. The tendency to flow movement is proportioned to depth—to load; it diminishes toward the outer margin. The outer portions, therefore, move too slowly, and are affected by horizontal, forward thrust. They are retarded at the same time by basal friction, and in consequence present a bulged and swelling front, implying, over a broad marginal tract, rising lines of flow. But the glacier is terminated forward, and is thinned toward its termination, by combined melting and evaporation—i.e., by ablation; and, by ablation, it may be inferred, the constantly bulging front is planed away. The glacier may be regarded as made up of two layers—a superficial, relatively rigid layer, and a basal layer, mobile under the weight of the other; or of a zone of fracture and a zone of flow. In the thinning frontal region, the upper layer, or cover, is brought into contact with the bed. Rearward, it is lifted; though at the same time there it is planed away. Hence, rising lines of flow in effect extend to the surface; for the cover is to be regarded as a zone of rigidity merely, constant only as to position, and thickening, from the mobile ice below, as it is thinned by ablation above. Rates of glacial motion, measured along the surface, therefore will be deceptive. On these assumptions, the line of most rapid advance in the glacier mass is from near the bed, at the rear, to the surface, near the front. Along the bed, motion slows forward; and as pressure upon the bed diminishes in that direction, presumably abrasive erosion is most vigorous toward the rear. The accepted view as to the flow-curve of the river is that, normally, it advances most rapidly at the surface. Deep rivers, however, are found to advance from a point measurably below the surface. If rivers had the great depth of glacial streams, possibly it would appear that the curve of flow which they actually have is but the reverse curve due to bed friction, extended to the surface because the surface is near. It would seem to be a safe assertion that descending grade of bed is not essential to river motion, only decline of the river surface toward the level of discharge; and that, in a long canyon with level floor, terminating at the sea, a river, one or two thousand feet in depth and maintained at that depth at its head, would advance with essentially the same flow-curve as that here attributed to the glacier. The value of such speculation consists in the indication it affords that appeal to the observed flow-curve of the river, in rebuttal, may not be valid.

The long ribbon meadow of the lower canyon course, no less than the ponded amphitheater floor, I think, invites interpretation as the manifestation of a tendency on the part of the glacier to channel excessively up-stream. And in this overdeepening toward the canyon head, I suspect, the two agencies of horizontal sapping and of vertical corrosion powerfully co-operate.

The ultimate effect, upon a range of high-altitude glaciation, would be rude truncation. The crest would be channeled away, down to what might be termed the base-level of glacial generation. Where, among the determining causes of glaciation, high latitude rather than high altitude is operative, the base-level of degradation may lie below the sea, deepest centrally and shallowing outward. Given a land area initially, the glacier itself, as degradation approached its maximum, would replace the land, affording the necessary above-sea surface for snow accumulation. The degradation limit would be determined by the lifting power of the sea.

The hypothesis, at this stage, is of much less importance than recognition of the anomalies of fact. of which it offers a tentative, even venturesome, explanation. In the fiorded regions of the globe, notably in the Patagonian Andes, of which a well-controlled reconnaissance survey has recently been completed, we have examples not only of fiords deepening backward for many miles into rising grades, but of fiord lakes, in parallel series, penetrating from foothills on the one side to foothills on the other, transecting a range. In explanation of such deep channels, whether occupied by arms of the sea, by lakes, or by feebly moving streams on meander bottoms, the appeal to grades, it seems to me, will be most cogent.

* From Journal of Geology 12 (1904), 569–578.

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Chicago: Willard Drake Johnson, "The Profile of Maturity in Alpine Glacial Erosion," A Source Book in Geology, 1900-1950 in A Source Book in Geology, 1900-1950, ed. Kirtley F. Mather (Cambridge: Harvard University Press, 1967), 55–59. Original Sources, accessed September 20, 2018, http://www.originalsources.com/Document.aspx?DocID=CYA6JEU9F3929LL.

MLA: Johnson, Willard Drake. "The Profile of Maturity in Alpine Glacial Erosion." A Source Book in Geology, 1900-1950, Vol. 12, in A Source Book in Geology, 1900-1950, edited by Kirtley F. Mather, Cambridge, Harvard University Press, 1967, pp. 55–59. Original Sources. 20 Sep. 2018. www.originalsources.com/Document.aspx?DocID=CYA6JEU9F3929LL.

Harvard: Johnson, WD, 'The Profile of Maturity in Alpine Glacial Erosion' in A Source Book in Geology, 1900-1950. cited in 1967, A Source Book in Geology, 1900-1950, ed. , Harvard University Press, Cambridge, pp.55–59. Original Sources, retrieved 20 September 2018, from http://www.originalsources.com/Document.aspx?DocID=CYA6JEU9F3929LL.