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 ) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/33/3/587/3414417/BUL33-0587.pdf by guest on 27 September 2021 588 W. M. DAVIS---- PENEPLAINS AND THE GEOGRAPHICAL CYCLE 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. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/33/3/587/3414417/BUL33-0587.pdf by guest on 27 September 2021 GENERAL FEATURES OF A PENEPLAIN 5 8 9 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/33/3/587/3414417/BUL33-0587.pdf by guest on 27 September 2021 590 W. M. DAVIS---- PENEPLAINS AND THE GEOGRAPHICAL CYCLE •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.