BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 33, PP. 587-598 SEPTEMBER 30, 1922

PENEPLAINS AND THE GEOGRAPHICAL CYCLE1

BY W. M. DAVIS

, (Presented, before the, Society December 29, 1921)

CONTENTS Page General features of a peneplain...... 587 Subsequent streams and valleys...... 589 The geographical cycle and its complications...... 590 Climatic changes during a cycle of erosion...... 591 Misconception of the geographical cycle as a rigid scheme...... 593 Essential features of the geographical cycle...... 594 Induction and deduction in the scheme of the cycle...... 595 Deduction in the scheme of the arid cycle...... 596 Unlike use of the cycle in geology and geography...... 597

G e n e r a l F e a t u r e s o p a P e n e p l a i n

If a landmass of whatever structure and large area be upheaved un­ equally to considerable altitudes in its interior area and if it then stand still for an indefinitely long period, it will be eventually degraded to a plain. In order to avoid the necessity of assuming so indefinitely long a period of still-stand, and in order at the same time to detain attention upon the gently undulating surface that such a region will have before it is worn down to a plain, the term peneplain was invented some thirty years ago. As no one, I believe, proposes to call the surface of ultimate degradation a “plane,” I see no sufficient reason for calling the penultimate surface a “peneplane,” as some have proposed; the original spelling, peneplain, is preferred. When the term was first proposed it was vaguely defined; and it has recently come to my knowledge that the vagueness of its definition has led some students of the physiographic evolution of the Appalachians to make for themselves a fuller definition, which I hope they will publish. In the meantime the following statement is offered of my own view.< upon the subject, as they have been gradually developed during the

1 Manuscript received by the Secretary o£ the Society January 24, 1922.

XXXIX—B u l l . G e o l . S o c . A m ., V o l . 3 3 , 1 9 2 1 ( 5 8 7 )

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growth, of the many ideas that are compactly embodied in the scheme of the cycle of erosion or the geographical cycle. A peneplain developed on a large landmass of homogeneous structure should be margined along its retrograding ocean shore by sealevel delta- free valley plains, alternating with low, wave-cut bluffs on faint inter­ valley swells; and should very gradually ascend to greater altitude and greater relief at an interior divide, beyond which similar features would be repeated in reverse order to a farther ocean. The survival of large mountain-like hills along the interior divide where the initial upheaval was greatest is not inconsistent with the occurrence of a well developed peneplain—broad swells of gentle convexity between wide valley floors— over the greater part of the area between the divide and the ocean. Penck has proposed the term, mosore, for the residual hills that survive along the divide, not by reason of greater resistance, but by reason of representing a greater original mass to be consumed. Prom the mosores along the main divide, gradually dwindling trains of hills would follow the secondary divides. There is no break in the long sequence of slow changes by which the smaller hills of the secondary divides and the larger hills of the main divides are gradually reduced to so small a relief that they, too, may be regarded as part of a peneplain. The term pene­ plain should therefore be taken as especially applicable to certain advanced phases of land sculpture not sharply separated from the phases that precede and follow. If a peneplain have a breadth of 1,000 miles or more, or if a peneplain be developed in the interior of a large continent, its interior part—not merely the residual hills of fairly strong relief, but also the gently undu­ lating swells that rise but little over the broad valley floors—may have altitudes of from 1,000 to 3,000 feet or more above sealevel. Hence an evenly uplifted peneplain, now undergoing dissection, should not be ex­ pected to stand everywhere at the same altitude, and the present altitude of even its best developed parts should not be taken as necessarily giving a measnre of its uplift, as if it had previously stood at sealevel. The uplift may have been several thousand feet less than the altitude. Cvijic has emphasized this point in his discussion of the physiography of the Balkan region. Similarly, if the coastal two-thirds of a broad peneplain be flexed or faulted down near or beneath sealevel, while the inner third remain at its former altitude, the rivers there will at once proceed to in­ cise new valleys beneath their former valley floors; hence the mere occur­ rence of valleys incised in a peneplain should not be taken as evidence of uplift. Philippson has urged the importance of this interesting principle in an account of the Slate Mountains of the middle Rhine.

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A peneplain formed in a region of varied structure will be first devel­ oped in areas of weak rocks, wherever they are situated; it will be later developed in areas of moderately resistant rocks; and by that time the weak-rock areas may be degraded to true plains. The most resistant rocks will long survive as knobs or ridges, now commonly known as monadnocks.

S u b s e q u e n t S t r e a m s a n d V a l l e y s The development of subsequent streams along belts of weak structure and the corresponding diminution of consequent streams is a character­ istic feature of a well advanced cycle of erosion; and while subsequent streams are developing along belts of weak structure consequent divides will be largely replaced by subsequent divides on belts of resistant struc­ ture. But be it noted that the physiographic value of these two stream terms does not lie so much in the indications that they give of stream and valley origins as in the suggestions that they offer regarding the relation of the streams and valleys to their surroundings. A consequent stream or valley following the original slope of a body of inclined strati­ fied rocks will usually have similar rocks and similar forms on both sides; but subsequent streams and valleys are usually characterized by different rocks and by different forms on the two sides; and, further­ more, in a region of slanting structure, where a master consequent is joined by one subsequent stream, it will usually be joined also by another subsequent stream coming from the opposite direction, both subsequents being developed on the same tranverse weak belt. It is curious to note that the term subsequent, which has been proposed to name a class of streams and valleys in southern Ireland clearly described by Jukes in 1862, has not gained general acceptance among American geologists and physiographers, not even among those who adopt Powell’s trio of 1875—consequent, antecedent, and superimposed (shortened by McGee to superposed). Yet it has been pointed out that, by reason of Powell’s failure to extend the trio to a quartet by adding subsequent as a fourth member, he was led into serious errors in the inter­ pretation of the streams of the Uinta Mountains; and that Dutton made similar errors in the region of the Colorado Canyon of Arizona for the same reason. Even Gilbert referred to Powell's trio as if it contained a complete genetic classification of streams. Whether the name “subse­ quent” should be used for the fourth member of the group is unim­ portant; but that no fourth member shoujd be recognized is a serious mistake. The spontaneous development of subsequent streams in regions of tilted, strong and weak strata is an essential process in the advance

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•of the cycle of erosion. The addition of three other classes of streams— insequent, obsequent, and resequent—thus enlarging the original trio to a septet, is a refinement of much less importance than the recognition of the class of subséquents.

T h e geographical C y c l e a n d i t s C omplications Two of my European correspondents have lately written me that they must give up the cycle of erosion because it does not include certain facts to 'which they call attention. One group of facts is the well-opened valleys in areas of relatively weak rocks; and it is urged that these valleys were never “young” in the sense of being narrow; they were widened as well as deepened while the region was raised. I believe it is true that my own writings do not contain any explicit statement of this rather evident idea; the nearest I have come to it is to say that if a region of resistant rocks be slowly uplifted its valleys will be widened as they are deepened. “In such a case there would have been no early stage of dissection in which the streams were enclosed in narrow valleys with steep and rocky walls; the stage of youth would have been elided and that of maturity would have prevailed from the beginning, but with constantly increasing relief as long as uplift lasted.”2 Immediately mature valleys of this kind would evidently be much more common in areas of weak than of resistant rocks;* it may indeed be a rare exception to find “young” valleys, in the sense of narrow, steep-sided valleys, in weak-rock areas. In such cases the valleys are probably, Minerva-like, born mature. It is certainly a puzzling and regrettable omission not to have explicitly stated a matter as manifest as that; but what is still more puzzling is why the omission of a subordinate matter of this kind should be regarded as a reason for discarding the whole scheme of the cycle of erosion. The omission clearly gives reason for amending and improving the scheme; but, in view of the abundance of regions in which typical “young” valleys are found in resistant rocks and in which a relatively rapid uplift is thereby proved, the scheme of the cycle seems to me still worth preserving, as well as worth improving as far as possible. Even if the general presentation of the scheme sets out from a region of relatively resistant rocks uplifted with comparative rapidity, in which the first-cut valleys of the larger streams are typically “young,” the

2 Complications of the geographical cycle. Proc. 8th Internat. Geogr. Congr., 1904. Washington, 1905, pp. 150-163 ; see p. 153. * Since writing this article I find that explicit, though brief, mention of the more probable occurrence of immediately mature valleys in weak than in hard rocks is made in my "Erklärende Beschreibung der Landformen,’’ 1912, p. 147.

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presentation may soon be advisedly expanded to include regions of resistant rocks which are uplifted slowly and in which the first-cut- valleys of the larger streams are maturely open or “mature-born” from the beginning; also regions of weak rocks in which, even if the uplift be relatively rapid, the first-cut valleys will be “mature-born” ; and in which, if the uplift be slow, the first-cut valleys may be deepened so little in excess of the down-wearing of the inter-valley uplands that the general expression of the surface will at once be “old.” Indeed, one may conceive of a region that, after a first very slow uplift, is uplifted more rapidly, but eventually stands still for an indefinite period; and in such a case the expression of the first-developed valleys would be “old” ; then as the rate of uplift increased, the “old” valleys would be first incised by “mature,” and then by “young” valleys; and finally the “young” valleys would, during the ensuing still-stand period, gradually gain the appearance of “mature” and “old” valleys. Nevertheless, it does not seem advisable to base the general terminology of the whole scheme on so special a case, but rather to modify a generally applicable termi­ nology so as to adapt it to such a case and to various other cases, as need arises. The other dissenting correspondent proposes to give up the scheme of the cycle because he has found that the small headwater streams of an uplifted and partly dissected peneplain occupy wide-open but shallow valleys, even though the trunk rivers flow in narrow, deep, and steep­ sided valleys. I had myself come upon such headwater valleys during an excursion in central Prance in the winter of 1898-99, and have pub­ lished an explicit account of them, as well as what seems to me a simple and sufficient explanation of them, in my Berlin lectures of 1908-09.3 The general scheme must therefore be modified so as to recognize that, even if trunk rivers incise typical “young” valleys in an uplifted pene­ plain, their little headwater branches will probably have “mature-born” valleys from the beginning of the new cycle of erosion. Hence my feeling is here again: Modify the scheme to include this principle as a matter of course; but why give up all the rest of the scheme because a modification is needed here!

Clim a tic Ch a n g es during a Cycle of E rosion But besides these two correspondents there are other geographers, particularly Hettner of Heidelberg and Passarge of Hamburg, who reject the scheme of the oycle as essentially incorrect in omitting many

* Die erklärende Beschreibung der Landformen. Leipzig, 1912, See pp. 62, 259.

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details and complications which they hold to he indispensable. Passarge, for example, ascribes so great a value to past changes of climate that he is disinclined, to say the least, to regard any existing mature or old land form as the product of a one-climate erosion cycle; not merely that the climate would change during the progress of a cycle from cold and rainy on the initial highlands of an uplifted region to warmer and drier on the old worn-down lowlands; but that, by reason of the shifting of climatic zones, the tilting of the earth’s axis, and other possible causes, the processes of “normal” subaerial erosion in middle latitudes, for example, are not likely to endure through so long a period of time as an erosion cycle. He is therefore unwilling to look upon most existing land forms as the product of the prolonged action of their present-day erosional processes. He questions the sufficiency of soil creep on the forested slopes of subdued mountains, like those of North. Carolina, to continue the degradation that such mountains have already suffered, and ascribes their present form to the action of some other-than-normal processes in the past; similarly, he discredits the explanation of the Inselberglandschaften of sub-arid east Africa—isolated residual moun­ tains rising sharply over degraded rock-plains, like those of southeastern California—by the processes now in operation upon them, and calls in the aid of one or more pluvial periods, even though the visible forms give no evidence of pluvial action. That there have been climatic changes during the progress of certain cycles of erosion is plain enough in glaciated regions, and also in certain subarid regions where the records of extinct lakes are still preserved; but if the Great Basin of Utah and Nevada be taken as a sample of such a subarid region, the remarkable thing about it is the relatively trifling amount of work that was done there during the Quaternary humid epochs; they were mere transitory episodes in the immensely longer period or cycle of erosion that would be required for the general degradation of the region to a low peneplain. Where the work of the humid episodes is not recorded in lake-shore terraces, or cliffs or in basin- floor sediments, it is not recognizable. The forms of the Great Basin are essentially those of a long-continued subarid cycle of erosion, on which the details produced during the brief episodes of humidity are of very subordinate value. Hence, while the effects of changes of climate should, of course, be considered wherever necessary, it is practically help­ ful to discuss steady-going cycles of erosion during which a normal (humid), or subarid, or other climate has prevailed, and to apply such discussions to the description of regions like the North Carolina mount­

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ains or the Great Basin ranges. As far as my inquiries have gone, American geographers and geologists do not share Passarge’s misgivings on this matter.

M isconception o f t h e geogeaphical C y c l e a s a k i g i d S c h e m e

The various objections to the general scheme of the cycle of erosion above alluded to are, in my own opinion, founded chiefly on a misap­ prehension of it as a rigid and complete concept. I t has never been considered either rigid or complete in my mind, but rather a growing idea, always plastic, so that it might be corrected where wrong, or modi­ fied and extended where accommodation is needed for new facts and processes. The scheme was first conceived simply in relation to ordinary or. “normal” erosion by rain and rivers, by weather and water acting on a landmass of horizontal structure; it was slowly extended to include the action of the same normal processes on various kinds of structures; and later to include various kinds of processes; yet even in the first statement a certain breadth of view was reached, for it was there said: “The channels Ivalleys] will be narrow and steep walled in regions of relatively rapid elevation, but broadly open in regions that have risen slowly; and I believe that rate of elevation is thus of greater'importance than climatic conditions in giving the canyon form to a valley.” 4 I t is quite possible that a misconception regarding the scheme has arisen from the fact that certain brief presentations which I have made of it have emphasized the elementary case of a region which first suffers a rather rapid uplift and which thereafter stands still for an indefinitely long period,5 and have given less attention to cases of greater complica­ tion, such as those of slow and of variable uplift alluded to above. But even the brief mentions made of possible complications supplementary to the elementary case appear to me quite sufficient to show that they also inhere in the generalized concept of upheaval at various rates and in various manners, associated with degradation by various processes in various combinations. Such complications and variations would not characterize a rigid scheme into which the facts of nature must be forced to fit, as if it were Procrustes’ bed; but they certainly do char­ acterize a plastic scheme, the very essence of which is its capacity to adapt itself to the facts of nature, and the main object of which has

4 Geographic classification, illustrated by a study of plains, plateaus, and their deriva­ tives. Proc. Amer. Assoc., 1884. 5 The geographical cycle. Geogr. Journ., 1899. The geographical cycle. Verhandl. VII Internat. Geogr. Kongr. Berlin, 1889. 1900, 221-231.

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always been to treat the forms of the land in a rational and revolutionary manner.

E s s e n t i a l F e a t u r e s o f t h e geographical C y c l e The last point deserves fuller statement. The essence and object of the scheme do not lie in its name, “cycle of erosion,” to which indeed objection has been made because its processes are not circular in the sense of returning at the end to their beginning—an objection that has less weight today than when it was first offered, because it is now generally recognized that not only must plains and plateaus be in the end worn down to surfaces about as smooth as those with which they began their cycle of change, but also that most of the existing mountain ranges of the world began their present cycle of erosional change in peneplains very much like those down to which they will be again eventually worn if no disturbance happens. Nor do the essence and object of the scheme lie in the adoption of such words as young, mature, and old—two of them, young and old, having been introduced by Chamberlin—as names for the successive stages of an ordinary cycle of erosion ; any other names that the geographical world prefers will serve; but it is a satisfaction, quand même, to know that a physiographer so philosophical as Gilbert held that the analogy on which this terminology is based is good because it indicates a “close resemblance in some striking particulars, coupled with differences in other respects ;” and it is truly gratifying to read his opinion : “In my judgment, there are few groups of terms which serve better than does this group the purpose of concisely expressing an idea. Its strength inheres, first, in the aptness and completeness of‘the analogy, and, second, in the perfect familiarity of the group of facts to which the unfamiliar facts are likened. . . . The aptness and the familiarity make the terms perma­ nently mnemonic, so that the use of any one of them brings to mind not only the sequence, but relative position in the sequence.” 6 The essence and object of the scheme of the cycle does not lie in its terminology, but in its capacity to set forth the reasonableness of land forms and to replace the arbitrary, empirical methods of description formerly in universal use, by a rational, explanatory method in accord with the evolutionary philosophy of the modern era. All the older descriptions of land forms treated each form by and for itself. The idea that certain groups of forms may be arranged in a genetic sequence based upon structure, process and stage, and the further idea that the

8 Style in scientific composition. Science, vol. xxi, 1905, pp. 28-29.

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different form-elements of a given structural mass are at each stage of its physiographic evolution systematically related to one another, ■were not then recognized. The terminology by which these ideas are set forth is a subordinate matter, although it is of course desirable that some one reasonably consistent terminology should be generally adopted as a matter of convenience. But it is today inevitable that the fundamental ideas of the reasonableness of land forms and the systematic relations of their elements should be accepted and made use of in their description. The reasonableness is not exhibited only in the sequence of forms and the systematic relations of their elements during the uninterrupted progress of a single cycle; it is also exhibited, although in a more complex manner, when one cycle is interrupted at any stage of its progress by a movement which introduces another cycle; or by a volcanic or climatic accident which for a time disturbs its ordinary progress. Great practical advantage fallows from the recognition and utilization of these ideas, for they serve to impress, first, the helpful physiographic principle that every structural mass has, at any stage of its erosional development, a reasonable surface form; and, second, the equally helpful principle that if a structural mass is moved at any time, the form that it had gained when the movement took place must always be specified, as well as the new attitude into which it was placed by the movement and the changes it has suffered since the movement—always provided that the changes since the movement have not wholly obliterated the pre-move- ment forms; in that case, whatever importance the movement may have in a geological study, geographical study is not responsible for it.

I n d u c t i o n a n d D e d u c t i o n i n t h e S c h e m e o k t h e C y c l e

A frequent misconception of the scheme- of the cycle is one that regards it as based chiefly on deduction. It is true that various expositions of the scheme have been cast in a deductive form, because that is the most convenient form in which to present a general idea to mature readers; and it is also true that, as in all geological and physiographic reasoning, deduction frequently either anticipates observation or fills in gaps where observation is lacking; but to imagine that the scheme as a whole is deductive, in the sense of being contrived independent of observation and induction, is absurd. Its different phases are all composite results of seeing and thinking, and of thinking and seeing; and it would be impossible today to state in just what proportion these two comple­ mentary processes have contributed to the total results. As a matter of fact, even the penultimate stage of a normal cycle of erosion first

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came to my attention and understanding through observation in the summer of 1883, when I was studying the series of rocks below . the Cretaceous coal horizon in Montana for the Northern Transcontinental Survey, under the direction of Professor Pumpelly. Areas of the Great Plains of nearly horizontal strata that I then traversed gradually came to be understood as plains of degradation, because they were here and there surmounted by lava-capped tables and dike-ridges, or even by mountainous residuals held up by the extra resistance of a network of dikes. It was several years after that beginning was made before the idea of peneplanation was extended to regions of disordered rocks in Pennsylvania and New England; and then again the facts observed, compared, and generalized in inductions preceded the attainment of the explanation by deduction. The same is true regarding the scheme of the cycles of marine abrasion and of glacial erosion. As to the latter, it was the sight of the hanging side valleys high above the broad floor of the valley of the Ticino, the Yal Leventina, in the southern Alps, which first led me to search for their explanation by a strong deepening of the main valley by glacial erosion in 1899, a year after Gannett had, then unknown to me, seen similar facts about Lake Chelan and reached a similar explanation for them. But the disappointing thing about this advance is that it had not been made deductively years before; and the fact that it was not so made must be explained chiefly by the inattention of geologists and geographers to the immense aid that deduction gives to observation when the origin and the meaning of things are searched for.

Deduction in t h e S c h e m e of t h e arid Cycle An exception to the statement that induction and deduction have usually gone hand in hand, in so far as my own work on cycles of erosion is concerned, must be made in the case of the arid cycle; for there, following a clue siiggested by Passarge’s account of his observations in South Africa, I deduced a whole sequence of changes far in advance of their observational verification; and in spite of repeated search in the records of explorers of desert regions, many of the deduced stages are still without their observed counterparts. If the records of these explorers gave specific examples of observed forms that contradicted the features of the deduced forms, the scheme of the arid cycle might be condemned as wrong; but as a matter of fact their descriptions are, as a rule, so vague that it is impossible to know just what sort of forms they are describing. Their records neither prove nor disprove the arid cycle.

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A few examples of high-level intermont basins are known in Tibet, the wet-weather drainage of which appears, from the fragmentary descrip­ tions now available, to have been rather recently diverted to adjoining basins of lower level; but the accounts of the connecting gorges are so brief, so inadequate, that it is impossible to say whether they are conse­ quences of spontaneous captures or not. In my own experience, the nearest approach to the capture of a higher arid basin by a lower one was seen in 1914 from a train on the Western Pacific Eailroad about 100 miles west of Salt Lake City, where ascent was made through a mountain notch from a lower to a higher basin floor; but much extension of the gorge by headward erosion will be necessary before the whole drainage of the higher basin will become tributary to the lower one.

U n l i k e U s e o f t h e C y c l e i n G e o l o g y a n d G e o g r a p h y

A closing paragraph may be given to the unlike use of the cycle of erosion by geologists and geographers. For geologists, cycles of erosion, either very brief or nearly complete, are events in the past history of the earth, and as such are to be treated like any other past events. Their recognition has contributed greatly to the completion of those chapters of geological investigation which are concerned with the evolution of land areas, by showing not only that erosion has taken place at one epoch or another, but also by making it clear how far erosion proceeded in each epoch. The problem of the Great Basin ranges might be here reviewed to advantage, for it was not correctly solved until the cycles of erosion that it involved were explicitly recognized and defined. On the other hand, for geographers—or rather for those geographers who are particularly interested' in the careful and intelligible description of the stage-setting in which the human drama is played—the scheme of the cycle of erosion has its chief value not as a means of reproducing the past, but as a means of describing the present. In illustration of this point: If the district of Snowdon, in north Wales, or of Janies Peak, in the Eocky Mountains front range, is described by a geologist, each of the successive incomplete cycles of erosion, normal and glacial, that the mountain and its district have suffered deserves equal attention and all of them should be set forth in historical sequence. If Snowdon or James Peak is described by a geographer, its existing form should be directly stated as a consequence of the successive cycles of erosion it has suffered. The sole reason for such explanatory description by a geographer is, not that it gives a knowl­ edge of the past, but that it gives a more effective knowledge of the

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present than any empirical description does. Both , the mountains here named exhibit the subdued forms of late-mature normal erosion;, that is, dome-like summits from which rounded spurs descend in various direc­ tions; and in both these normal features are greatly modified by the recent excavation of huge glacial cirques in the valley heads between the diverging spurs ; hence the present remnant of the dome-like summit is undercut on several sides by great cirque-head cliffs, and not far below the summit remnant the deepening and widening of the glacial troughs reduces the sloping spurs to sharply serrate, coxcomb-like crests. The single word “recent,” used to qualify the excavation of the cirques and troughs, suffices to show that postglacial changes are small. The dis­ tinction here made between the use of the cycle scheme for geological and geographical purposes deserves more attention than it usually receives.

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