A CYCLE OF EROSION. ITS STAGES.
From what has preceded it is clear that the topography of a region undergoing erosion will change greatly from time to time. The first effect of erosion is to roughen the surface by cutting out valleys, leaving ridges and hills. The final effect is to make it smooth again by cutting the ridges and hills down to the level of the valleys.
Fig. 63.—Diagram showing three parallel valleys in a land surface.
Fig. 64.—Diagram to illustrate the lowering of the surface by valley erosion. The successive cross profiles of the valleys are represented by the lines 1–1, 1–1′, 2–2, 2–2′, etc.
The base-level of erosion has already been defined; but the mode of its development may now be illustrated in the light of the preceding discussion. Suppose a land surface affected by a series of parallel young valleys without tributaries ([Fig. 63]). Between them there is a series of upland plateaus. The profile of the surface between two adjacent valleys is represented in section by the uppermost line in [Fig. 64]. As the valleys are widened from 1–1 and 1,′–1′, to 2–2 and 2′–2′, the intervening plateau is correspondingly narrowed. When the valleys have attained the form represented by 3–3 and 3′–3′, the intervening upland has been narrowed to a ridge, a, and the valley flats have become wide. With continued erosion the ridge will be lowered (to b and below), and in time the surface will approach a plain. In this condition it is known as a peneplain (an “almost-plain”). Finally, when running water has done its utmost, the ridges will be essentially obliterated and a base-leveled plain (e, e′, e″) results. The figure expresses the fact that the base-level develops laterally from the axis of the valley. It also develops headward from the seaward end of the valley. Similarly, taking into account all the valleys which affect it, the seaward margin of a base-leveled plain is developed first, and thence it extends itself inland.
Fig. 65.—Diagram showing the dissection of the upland shown in [Fig. 64] by tributary valleys.
Tributaries are tolerably sure to develop along each main valley. The heads of the tributaries work back across the uplands between the main valleys, dissecting them into secondary ridges ([Fig. 65]). Tributaries will develop on the tributaries, and these tertiary valleys dissect the secondary ridges into those of a lower order. This process of tributary development goes on until drainage lines of the fourth, fifth, sixth, and higher orders are formed ([Fig. 66]). Since the process of valley development under such circumstances is also the process of ridge dissection, a stage is presently reached where the ridges are cut into such short sections that they cease to be ridges, and become hills instead. Even then the processes of erosion do not stop, for the rain-water falling on the hills washes the loose material from their surfaces, and starts it on its seaward journey. Thus the “everlasting hills” themselves are lowered, and, given time enough, will be carried to the sea. Under these conditions, as under those already discussed, the final result of stream erosion is the reduction of the land to base-level. The base-leveled surface, as before, would not be absolutely flat. The area reduced by each stream will have a slight gradient down-stream, and from each lateral divide toward the axis of the valley. The crests of the scarcely perceptible elevations which remain will be in the position of the former divides, and these will be highest where most distant from the sea by the course which this part of the drainage took. Even the insensible divides between streams flowing in a common direction may disappear, for when valleys have reached their limits in depth, their streams do not cease to cut laterally. Meandering in their flat-bottomed valleys, they often reach and undercut the divides ([Pl. VII]), whether they be high or low. By lateral planation, therefore, the divides between streams may be entirely eaten away.
Fig. 66.—Diagram showing tributaries of several orders developed from the conditions sketched in the text.
It has now been seen that by whatever method erosion by running water proceeds, whether there be many valleys, or few or none, the final result of subaërial erosion must be the production of a base-level. It has also been seen that the base-level is first developed at the lower ends of the main streams, and that it extends itself systematically up the main valleys and up all tributaries. The time involved in the reduction of a land area to base-level is a cycle of erosion.
It will have been evident from the preceding pages that the terms “grade,” “graded plain,” and “base-level” and “base-leveled plain,” are somewhat variously, and therefore somewhat confusingly, used. “Grade is a condition of essential balance between corrasion and deposition.”[29] A graded valley is one in which deposition and corrasion are, in the vertical sense, balanced. Its angle of slope is most variable, and is dependent on the capacity of the stream for work, and on the work it has to do. A weak river must have a higher gradient than a strong one; a stream with much sediment must have a higher gradient than one with little, and a stream with a load of coarse material must have a higher gradient than one with a load of fine. Thus the graded valley of the lower Mississippi has an inappreciable angle of slope, but the graded valleys of many of its tributaries have slopes of hundreds of feet per mile. Since both the size of the stream and the amount and coarseness of its load at a given place vary from time to time, it is clear that the inclination of a graded valley must vary also, and further, that it must be in process of continual readjustment. With the changing conditions of advancing years the slope of a graded valley normally decreases. The same principles apply to graded surfaces outside of valleys.
In the continual readjustment of grades incident to a river’s normal history the land is brought nearer and nearer to sea-level without ceasing to be at grade. When the inclination of a graded surface becomes so low that it is sensibly flat, the surface may be said to be at base-level, although this does not mean that the surface can never be degraded further. If the term be used in this way, it is clear that there is no sharp line of distinction between a graded surface and a base-leveled surface, and as the terms are now commonly applied no such distinction exists.
If the term base-level were made synonymous with sea-level, as has been proposed,[30] the term might as well be discarded, for sea-level could always be used in its stead. Furthermore, streams often erode below sea-level. The bottom of the channel of the Mississippi is below sea-level for some 400 miles above its debouchure, and locally (Fort Jackson) it is nearly 250 feet below. This deep channel is the result of the erosive activity of the stream, not of subsidence. Again, the sea-level is itself inconstant. The extent of its changes cannot now be measured, but they have probably been more considerable in the course of geological history than has been commonly recognized. It is true that they take place slowly, as far as known, but it is also true that the duration of an erosion cycle is sufficiently long for even very slow changes to reach great magnitude. The sea-level, therefore, can hardly be accepted as the absolute base-level, unless (1) the absolute base-level is a variable, and unless (2) the absolute base-level be a surface below which rivers may cut to the extent of at least 250 feet.
The ocean may be looked upon as a barrier which in a general way limits the down-cutting of running water; for only very large streams cut much below its level. Other barriers, such as lakes, and the outcrops of hard rock in a stream’s bed, have a comparable, though more temporary, effect on the development of valley plains above. Plains thus developed have been called temporary base-levels. They differ from other graded plains in being controlled primarily by a barrier below, rather than by conditions which exist above.
Since river valleys have a beginning and pass through various stages of development before the country they drain is base-leveled, it is important to recognize their various stages of advancement. Nor is this difficult. An old valley and a young one have different characteristics, and the one would no more be mistaken for the other by those who have learned to interpret them, than the face of an aged man would be mistaken for that of a child.
Fig. 67.—A gully developed by a single shower. (Blackwelder.)
The cycle begins with the beginning of valley development, and at that stage drainage is in its infancy. The type of the infant valley is the gully or ravine (Figs. [67] and [68]). It has steep slopes and a narrow bottom. [Fig. 1 of Plate IV] represents similar, or rather older, ravines in contour (shore of Lake Michigan, just north of Chicago). With age, the valley widens, lengthens, and deepens, and passes from infancy to youth. In this stage also the valleys are relatively narrow, and the divides between them broad. They may be deep or shallow, according to the height of the land in which they are cut, and the fall of the water flowing through them; but in any case the streams flowing through them have done but a small part of the work they are to do before the country they drain is base-leveled. Figs. [69] and [70], respectively, represent youthful valleys in regions of moderate and great relief. [Fig. 2, Plate IV], shows a youthful valley in a region of slight relief (near Casselton, N. D., Lat. 46° 40′, Long. 97° 25′). The uppermost line in [Fig. 64] likewise represents topographic youth, as shown in cross-profile.
Fig. 68.—A gully somewhat older than that shown in [Fig. 67]. (Alden.)
Fig. 69.—A young valley in a region of slight relief.
Not only are narrow valleys said to be young, but the territory affected by them is said to be in its topographic youth, since but a small part of the time necessary to reduce it to base-level has elapsed. An area is in its topographic youth when considerable portions of it are still unaffected by valleys. Thus the areas (as a whole), as well as the valleys, represented on [Plate IV], are in their topographic youth. It is often convenient to recognize various sub-stages, such as early, middle, and late, within the youthful stage of valleys or topographies. The different parts of the areas shown on [Plate IV], for example, represent different stages of youth.
Youthful streams, as well as youthful topographies, have their distinctive characteristics. They are usually swift; their cutting is mainly at the bottom rather than at the sides, and their courses are often marked by rapids and falls.
As valleys approach base-level they develop flats. As the valleys and their flats widen, and as their tributaries increase in numbers and size, a stage of erosion is presently reached where but little of the original upland surface remains. The country is largely reduced to slopes. In this condition the drainage and the topography which it has determined are said to be mature. Mature topography is shown in contours in the figures of [Plate V], and in the northern part of [Plate VI], where slopes, rather than upland or valley flats, predominate. [Fig. 1 of Plate V] represents an area in southeastern Kentucky (Lat. 37° 12′, Long. 83° 10′); [Fig. 2], an area in western Virginia. [Plate VI] represents an area in southern California, somewhat west of San Bernardino. The three areas are alike in representing mature drainage, though not of equal stages of advancement. The striking differences of topography of the three areas are the result of differences in rock structure and altitude, and will be considered later. Mature topography is also shown in [Fig. 71], where the relief is moderate, and in Figs. [72] and [73], where it is great. Figs. [72] and [73] illustrate clearly the universal tendency of rivers in regions of notable relief to develop new flats well below the old surface of the region. At the same time that these low-lying flats are developing, tributary drainage is dissecting and roughening the upper surfaces. This process is well shown in [Fig. 73]. In both Figs. [72] and [73] the summits of the mountains on either side of the valleys appear to have had about the same elevation. The new flat is therefore developed at the expense of the old flat. As will be seen in the sequel, the first flat which a stream develops along its course is usually somewhat above base-level. It is a graded flat.
PLATE V.
U. S. Geol. Surv.
Scale, 2+ mile per inch.
Fig. 1. KENTUCKY.
U. S. Geol. Surv.
Scale, 2+ mile per inch.
Fig. 2. VIRGINIA.
PLATE VI.
U. S. Geol. Surv.
Scale, 1+ mile per inch.
PARTS OF LOS ANGELES AND SAN BERNARDINO COUNTIES, CALIFORNIA.
Fig. 70.—The valley (canyon) of the Yellowstone. A young valley in an elevated region.
Fig. 71.—Mature erosion topography in a region of slight relief, Iowa. (Calvin.)
Fig. 72.—Mature erosion in a mountain region. From mouth of Gray Copper Gulch, Silverton, Colo., quadrangle. (Cross, U. S. Geol. Surv.)
The same processes which have made young valleys mature will in time work further changes. When the gradients of the valleys have become low and their bottoms wide, and when the intervening ridges and hills have become narrow and small, the drainage and the drainage topography have reached old age, and the streams are in a condition of senility. This is illustrated by [Fig. 1, Plate VII] (central Kansas), and in section by the third and lower lines in [Fig. 64]. Topographic old age sometimes has a different expression; this is shown in [Fig. 74], where most of the surface has been brought low. The elevations which rise above the general plain are small in area, but have abrupt slopes. This phase of old-age topography is usually the result of the unequal resistance of the rock degraded. The effects of unequal rock-resistance will be considered later.
Fig. 73.—Mature erosion in a mountain region. Silverton, Colo. (Cross, U. S. Geol. Surv.)
The marks of old streams are as characteristic as those of young ones. They have low gradients and are sluggish. Instead of lowering their channels steadily they cut them down in flood, and fill them up when their currents are not swollen. They meander widely in their flat-bottomed valleys ([Fig. 1, Pl. VII], Central Kansas) and their erosion, except in time of flood, is largely lateral.
If the processes of degradation were to continue until the land surface was brought to sea-level, and this might be done by solution though not by mechanical erosion of running water, the rivers would no longer flow, and the drainage system would have reached the end of its history—death.
Not only do valleys normally pass from birth to youth, from youth to maturity, and from maturity to old age, but a single river system may show these various stages of development in its various parts. Thus in the area shown in [Fig. 2, Plate VII] (north central Kansas), there is a tract (extreme southwest) where the erosion history is scarcely begun. The zone of land a little farther northeast, and just reached by the heads of the valleys (same figure), is in its youth. The well-drained and uneven tract southwest of the flat of the Solomon River is in maturity, while the flat of the main valley has the general characteristics of old age.
Fig. 74.—A peneplained surface where the elevations are small but steep-sided. Near Camp Douglas, Wis. (Atwood.)
The age of valleys in terms of erosion is also expressed more or less perfectly by their cross-sections. The line 1–1 (and 1′–1′) of [Fig. 64] represents in cross-section a narrow V-shaped valley. Such a section is always indicative of youth. The stream which developed it cut chiefly at its bottom, not at its sides. It was therefore rapid, and rapid streams are young. The line 2–2, (2′–2′) ([Fig. 64]) shows the same valley at a later and maturer stage when downward cutting has nearly ceased. The widening of the valley by slope wash has become relatively more important than before, and the stream has so far lost velocity as the result of diminished gradient as to be unable to carry away all the detritus washed down from the sides. As a result of deposition at the bases of the side slopes, a concave curve has been developed. Up the valley from the point where such a section as is represented by 2–2 occurs, the valley may still have a section similar to that represented by 1–1.
PLATE VII.
U. S. Geol. Surv.
Scale, 2+ mile per inch.
Fig. 1. KANSAS.
U. S. Geol. Surv.
Scale, 2+ mile per inch.
Fig. 2. KANSAS.
PLATE VIII.
U. S. Geol. Surv.
Scale, 1+ mile per inch.
ABOUT 15 MILES SOUTHWEST OF ST. LOUIS, MISSOURI.
Still later stages of development are represented by the cross-sections 3–3 and 4–4. Not only has the valley become larger, but the stream has deposited detritus (not shown in the figure) in the bottom of its valley, developing an alluvial flat. On this flat the stream meanders, and the valley may be widened by the undercutting of the bluffs wherever the stream in its wanderings reaches them ([Pl. VIII], near St. Louis). A valley might possess the characteristics shown by the cross-sections 3–3, 2–2, and 1–1, [Fig. 64], in its lower, middle, and upper courses, respectively.
The preceding discussion, and the illustrations which accompany it, give some idea of the topography which characterizes an area in various stages of its erosion history. Whether the valleys are deep or shallow, and the intervening ridges high or low, depends on the original height of the land and its distance from the sea. The higher the land, and the nearer it is to the sea, the greater the relief developed by erosion. A plateau near the sea may become mountainous in the mature stage of its erosion history, while a plain in the same situation would only become hilly. A plateau in the heart of a continent would have less relief in its maturity than one of equal elevation near the sea, since the grade-plain in the former position is higher than in the latter. Plates [IV] and [IX] show youthful topography where the relief is relatively slight, and Plate [X] shows youthful topography where the relief is great. Similarly, Plates [V] and [VI] show mature topography where the relief is great, and [Fig. 1, Plate III], shows mature topography where the relief is relatively slight.
Topographic youth, topographic maturity, and topographic old age are also indicated in other ways, and especially by the presence of features which rivers tend to destroy. If, for example, the surface of the land, well above the valley bottoms, is marked by numerous ponds and marshes, it is clear that drainage has not yet progressed beyond its early stages, for, unless the lakes be very deep, valleys working back into the land will find and drain them before topographic maturity has been reached. Their presence is evidence that the region where they occur has not yet been thoroughly dissected by erosion lines, and therefore has not reached maturity. Still other marks of topographic youth, such as rapids, falls, etc., as well as marks of topographic maturity and old age, will be mentioned in the following pages.