When the strata are vertical, three distinct cases may arise (Fig. 105). The stream may flow (1) with the strike (aa); (2) at right angles to the strike (bb); or (3) oblique to it (cc) at any angle whatsoever. It is perhaps not possible to say which of these positions is most favorable for erosion, for the character of the rock, the thickness of its layers, its ability to stand with steep slopes, and the strength of the currents concerned, would influence the result. A stream which flows at right angles to the strike (bb, Fig. 105) would cross more cleavage-planes in a given distance than a stream flowing in any other direction, and would strike the outcropping edges of layers at the angle of greatest advantage. A stream flowing along the strike (aa), on the other hand, has better opportunity to sink its channel on cleavage-planes, and the current oblique to the strike (cc), has some of the advantages of each of the others.
When the strata are inclined five cases may arise. (1) The stream may be parallel to the strike (aa, Fig. 106), when it makes no difference which way the current flows; it may be at right angles to the strike (bb′), and (2) flowing with the dip (toward b′), or (3) against it (toward b); it may be oblique to the strike, and flowing (4) in the general direction of dip (toward c′); or (5) in the opposite direction (toward c). As before, the stream flowing at right angles to the strike would cross the largest number of layers in a given distance, and so have an opportunity to take advantage of more cleavage-planes than a stream in any other position. But in the case of inclined strata a new element enters into the problem. When the stream flows parallel to the strike, the valley which is in process of deepening is not sunk vertically, but is shifted more or less in the direction of the dip (Fig. 107). This is called monoclinal shifting. The result is that there is a constant tendency to undermine (sap) the valley bluff on the down-dip side, and this process of sapping will, according to its rate, accelerate the growth of the valley, especially in width. Monoclinal shifting is favored by the presence of a hard layer (H), as shown in Fig. 107, if this stratum is the bed of the stream.
In the second and third cases mentioned above, the only difference is in the angle at which the current strikes the outcropping edges of layers and laminæ. The mechanical advantage is with the stream which flows with the dip. In the fourth and fifth cases something will depend on the angle which the stream’s course makes with the strike. In all these cases, as in those where the strata are vertical, much will depend on the thickness and resistance of the layers and on the strength of the currents concerned.
Climate has both a direct and an indirect effect on erosion. Its direct influence is through precipitation, evaporation, changes of temperature, and wind; its indirect, through vegetation. Like declivity and rock structure, climate does not affect all elements of erosion equally.
The chief elements of climate are temperature, moisture, and atmospheric movements; the principal factors which influence it are latitude, altitude, distance from the sea, direction of prevailing winds, and topographic relations.
The effects of variations in temperature on rock weathering have already been discussed (p. 43). They are chiefly mechanical, and are seen at their best where the daily range is great.
High temperature favors chemical action, and the weathering of rock by decomposition is at its best in the presence of abundant moisture in regions where the temperature is uniformly high. Furthermore, a warm moist climate favors the growth of vegetation, the decay of which supplies the water with organic acids which greatly increase its solvent power. The climatic conditions favoring mechanical weathering are therefore different from those favoring chemical weathering. High temperature and abundant moisture and vegetation are found in many tropical regions, and here the rock is often decomposed to greater depths, on the whole, than in high latitudes. How far this is the result of rapid weathering, and how far of slow removal, due in part to the protective influence of the plants, cannot be affirmed. If the weathered material is not removed, it will presently become a mantle thick enough to retard the processes which brought it into existence.
So long as the water of the surface and that in the soil remains unfrozen, temperature affects neither corrasion nor transportation. But in middle and high latitudes the surface is frozen for some part of each year. During this time corrasion is at a minimum, for although the streams continue to flow there is relatively little water running over the surface outside the drainage channels, and that little is relatively ineffective. Under some conditions, therefore, temperature affects both corrasion and transportation.
The humidity of the atmosphere has an influence even more important than that of temperature on the rate of erosion, and its influence is exerted on each of the elements of that complex process. A moist atmosphere favors oxidation, carbonation, hydration, and the growth of vegetation, all of which promote certain phases of rock weathering. On the other hand, humidity tends to prevent sudden and considerable variations in temperature, thus checking the weathering effected by this means. Precipitation, the most important single factor in determining the rate of erosion, is dependent on atmospheric humidity. Its amount, its kind (rain or snow), and its distribution in time, are the elements which determine its effectiveness in any given place.
Other things being equal the greater the amount of precipitation the more rapid the corrasion and transportation. Much, however, depends on its distribution in time. A given amount of rainfall may be distributed equally through the year, or it may fall during a wet season only. The maximum inequality of distribution would occur if all the rainfall of a given period were concentrated in a single shower. With such concentration the volume of water flowing off over the surface immediately after the down-pour would be greater than under any other conditions of precipitation, and since velocity is increased with volume, and erosive power with velocity, it follows that the erosive power of a given amount of water would be greater under these circumstances than under any other. Furthermore, a larger proportion of the precipitation would run off over the surface under these circumstances than under any other, for less of it would sink beneath the surface and less would be evaporated. If erosive power and rate of erosion were equal terms, this would therefore be the condition for greatest erosion; but erosive power and rate of erosion do not always correspond. If the water falling in this way could get hold of all the material it could carry, extreme concentration of precipitation would be the condition favorable for most rapid erosion. But if the amount of available material for transportation is slight, a large part of the force of the water could not be utilized in erosion. It follows that if there were a large amount of disintegrated material on the surface, erosion would be greater the greater the concentration of precipitation. If, on the other hand, there were but little disintegrated material on the surface, frequent showers, with intervening periods when conditions were favorable for weathering, that is, for preparing material for transportation, might be more favorable for rapid erosion. While the total energy of running water available for erosion under these conditions would be less than before, there might in the long run be more material for transport; for weathering in the presence of moisture, and all that goes with it, might be more effective in preparing material for transportation, than weathering during the long periods of drought which would occur if the precipitation were concentrated to its maximum. Temperature favoring, the uniform distribution of moisture through the year would allow the growth of vegetation, which, although favoring some processes of weathering, retards erosion in general. While therefore it is not possible to say what distribution of rainfall favors most rapid erosion without knowing the nature of the surface on which it is to fall, enough has been said to show that the problem is by no means a simple one. Some of the most striking phases of topography developed by erosion, such as those of the Bad Lands (Figs. 75 to 78, and 108), are developed where the rainfall is unequally distributed in time, and too slight or too infrequent to support abundant vegetation.
During its fall, and immediately after, rain is more effective than an equal amount of snow; but the snow may be accumulated through a considerable period of the year, and then melted rapidly, when it has an effect comparable to that which would be produced by the concentration of the rainfall into a limited period of the year. If the ground beneath be frozen when the snow melts (and this is often the case) the erosion accomplished by the resulting water will be diminished.
Except in dry regions, where wind-work sometimes exceeds water-work, the movements of the atmosphere are of less importance directly than precipitation in determining the rate of erosion. But even in regions which are not arid the winds have much to do with the rate of evaporation and the distribution of rainfall, so that their indirect effect is great. Even their direct effects in moist climates are not to be lost sight of, for even here the surface is sometimes dry enough to yield dust and sand, and the uprooting of trees so disturbs the surface as to make earthy débris more accessible to wind and water. Where trees gain precarious footholds on steep slopes, as they often do, they are likely to be overturned as soon as they are large enough to offer considerable resistance to the wind, and in the overturning, large quantities of rock are sometimes loosened and carried down the slope by gravity. This phase of destructive work is seen at its best on the walls of gorges, where trees often flourish until their tops project above the rim of the valley.
Through vegetation, climate influences erosion in ways which are easily defined qualitatively, but not quantitatively. Both by its growth (wedge-work of roots) and by its decay (supplying CO2, etc., to descending waters) it favors certain phases of weathering; but, on the other hand, it retards corrasion and transportation both by wind and water. This is well shown along the banks of streams and on the faces of cliffs, in clay, sand, etc. Its aggregate effect is probably unfavorable to erosion by mechanical means, and favorable to that by chemical processes.
Erosion in high arid regions differs from that in regions of abundant rainfall in several ways. It is obvious that the valleys will develop more slowly in the former, that they will remain young longer, that the period necessary for the dissection of the surface is greater, that the watercourses will be less numerous, and that fewer of them will have permanent streams. There are certain other differences which are less obvious. If the arid region be high and composed of heterogeneous strata, the topography which erosion develops is more angular (Fig. 83) than that of the humid region. This is because there is less rock decay, and less vegetation to hold the products of decay. The more resistant beds of rock therefore come into greater prominence, especially on slopes, where they develop cliffs (Figs. 109 and 110). These general principles find abundant illustration in the plateaus of the western part of the United States,[48] where the cliffs are by no means confined to the immediate valleys of the streams (Fig. 1, Pl. XII).
In the preceding pages incidental reference has been made to the results of inequalities of rock resistance. This topic will now be considered more fully.
Rapids and falls.—Returning for a moment to the hypothetical island with which our study of erosion began, let a horizontal layer of hard rock be assumed to run through it (H, Fig. 111). As the rain falls on the land and runs off over it, wear will be less rapid where the hard layer comes to the surface than at the higher or lower levels. As a result, the slope will become steeper at and below the outcrop of the hard layer, and less steep immediately above it, as shown by ab in Fig. 111. Under these conditions the water passing over the hard ledge constitutes rapids. The increased erosion which accompanies the increased velocity makes the rapids more rapid. The process may continue until the water falls, rather than flows over the hard layer (cd, Fig. 111). With continued rainfall the edges of the hard layer, together with the slopes above and below, would continue to recede toward the center of the island. Under conditions of absolute homogeneity of material, save for the hard layer specified, no valley would be developed, and therefore no stream.
If the surface was so changed as to allow of the development of a valley (p. 63) the same principles would be applicable. As an active stream passes from a hard layer to one less resistant, the greater wear on the latter gives origin to rapids. At first the rapids would be slight (a, Fig. 112), but would become more considerable (b) as time and erosion go on. When the bed of the rapids becomes sufficiently steep, the rapids become falls[49] (cd). When the water falls rather than flows over the rock surface below the hard layer, erosion assumes a new phase. The hard layer is then undermined, and the undermining causes the falls to recede. This phase of erosion is sometimes called sapping.
If the hard layer which occasions a fall dips up-stream (Fig. 112), its outcrop in the stream’s bed becomes lower as the fall recedes (e). When it has become so low that the water passing over it no longer reacts effectively against the less resistant material beneath (f), sapping ceases, and the point of greatest erosion may be shifted from the soft material beneath the fall to the hard layer itself. The actual rate of erosion at this point may be no greater than before, though the relative rate is. Under these circumstances the vertical edge of the hard layer will presently be converted into an incline (f), and as this takes place the fall becomes rapids. The conversion of the falls into the rapids begins about the time the lower edge of the hard stratum in the channel reaches grade. By continuation of the process which transformed the falls into rapids, the rapids become less rapid, and when the upper edge of the hard layer has been brought to grade, the rapids disappear (h, Fig. 112). The history of rapids which succeed falls is the reverse of that which preceded. The later rapids are steepest at the beginning of their history, the earlier at their end. Stated in other terms, rapids are steepest when nearest falls in time. Slight differences in hardness in successive layers often occasion successive falls or rapids (Fig. 114).
If the hard layer which occasions the falls be horizontal, instead of dipping up-stream, the general result would be the same; but, other things being equal, the duration of the falls developed under these conditions would be greater, since they must recede farther before becoming rapids.
If the layers of unequal hardness in a stream’s bed be vertical and the course of the stream at right angles to the strike, rapids, and perhaps falls, will develop (Fig. 115). The chances for falls are greater, the greater the difference in hardness. Falls developed under these conditions, as well as the rapids preceding and following, would remain constant in position until the resistant layer was brought to grade, but they would ultimately disappear as in the preceding cases. Falls are not likely to develop where the strata of the stream’s bed dip down-stream, though they may develop even under these conditions if the gradient of the stream is greater than the dip of the strata (Fig. 116).
The inequality of resistance in the rock which occasions a fall may be original or secondary. In the case of Niagara Falls[50] (Fig. 113) relatively resistant limestone overlies relatively weak shale. At the Falls of St. Anthony (Minneapolis) limestone overlies friable sandstone. The falls of the Yellowstone and the Shoshone Falls of the Snake River (Idaho), are in igneous rock. In the former case the unequal resistance is occasioned by unequal decay of the rock, due perhaps to the rise of hot vapors which have decomposed the rock along the lines of their ascent; in the latter, a more resistant sort of igneous rock overlies a less resistant.
Structural features, such as jointing, sometimes give rise to falls, or determine their distinctive features (Fig. 117), even where the formations involved are of uniform hardness. A joint plane has the effect of a weak vertical or highly inclined bed. If an open joint is discovered in a stream’s bed, the water enters it. If it finds an outlet below, a channel is worn along the new line of flow, with rapids or falls where the water descends. Rock originally homogeneous may be much fractured in some parts, while it remains unbroken in others. Where a stream passes from the solid to the broken portion rapids, or even falls, may develop.
Falls may originate in still other ways. If for any reason a stream is forced out of its valley, it may in its flow find entrance to another valley, or to another part of its own valley, over a steep slope. If the structure of the slope favors, a fall may speedily develop. The Falls of St. Anthony are an example, the Mississippi having been turned out of its earlier course by deposits of glacial drift. Again, if an obstruction of any sort, such as a flow of lava, dams a stream, rapids or falls are developed where the water overflows the dam. When a main valley is notably deepened by glaciation the drainage from tributary valleys may fall into it, if the tributaries were not equally deepened. Falls which originated in this way are common in the western mountains of the United States, as well as in most mountain regions recently affected by local glaciers (Fig. 118).
One waterfall often breeds others. Thus where a fall recedes beyond the mouth of a tributary stream, the tributary falls. The Falls of Minnehaha, on a small tributary to the Mississippi, near Minneapolis, may serve as an illustration. In such cases the falls may not develop from rapids. Once in existence, the fall of a tributary follows the same history as that of a main stream.
Streams which have falls are relatively clear.[51] If a stream favorably situated for the development of a fall carried a heavy load, deposition would take place below the rapids, and the tendency would be to aggrade the channel at that point and so to prevent the development of the fall. Falls occur only on streams which have relatively high gradients. This means that the streams which have falls are well above base-level, and streams well above base-level are young. Falls therefore are a mark of topographic youth.
The fall of the Niagara[52] (Pl. IX) is one of the most remarkable known, both because of its large volume of water and its great descent, between 160 and 170 feet. The rate at which the fall is receding is a matter of interest not only in itself, but because, once determined, it may be made to serve as a unit of measurement for certain important events in geological history. It was formerly conjectured that this fall was receding at the rate of one to three feet per century, but it was not until recent years that its actual rate of recession was approximately fixed. By surveys executed in 1842 and 1890 it has been determined that its average rate of recession between those dates was something like 4½ feet per year, or about 150 times as great as the highest estimate stated above. It is to be noted that this is the average rate of recession, for all parts of the ledge over which the water falls are not receding at the same rate. The point of the “Horseshoe” has, during the same time, gone back at more than twice this rate.[53]
Rapids and falls sometimes occasion the development of pot-holes (Fig. 119), a peculiar rather than important erosion feature. The holes are excavated in part by the falling and eddying of silt-charged water, but chiefly by stones which the eddies move. Pot-holes which are not now in immediate association with rapids or falls often point to the former existence of rapids or falls.
Rock terraces.—The tendency to sapping shown in many waterfalls is also shown in the weathering and erosion of the sides of a valley where a hard layer outcrops above the bottom, and the profile of the side slopes of the valley simulates that of the stream; that is, the slope becomes gentle just above the hard layer, and steep, or even vertical, at and below its outcrop. This is illustrated by Fig. 120, where the hard layer through which the stream has sunk its valley stands out as a rock terrace on either side of the valley. Such terraces are not rare and are popularly believed to be old “water-lines”; that is, to represent the height at which the water once stood. In one sense this interpretation is correct, since a river has stood at all levels between that of the surface in which its valley started, and its present channel, but the shelf of hard rock does not mean that the river, after attaining its present channel, was ever so large as to fill the valley to the level of the terrace. Rock terraces may also result from changes of level.
Narrows.—Inequalities in hardness occasion another peculiarity common to valleys. If a stream crosses vertical or highly inclined strata of unequal hardness, its valley is usually constricted at the crossing of the harder layers. If such a constriction be notable it is called a narrows, or sometimes a water-gap (Figs. 121, 159, and Fig. 2, Pl. XII). The Appalachian Mountains afford numerous examples. The constriction arises because the processes which widen the valley are less effective on the hard layer than on the less resistant ones on either hand. Though most narrows are due to the superior resistance of the rock where they occur, they are sometimes the result of other causes.
Narrows are much more conspicuous in certain stages of erosion than in others. While a valley is still so young as to be narrow at all points, no narrows will be conspicuous; but at a later stage in its history, when the valley is otherwise wide, narrows are more pronounced. At a still later stage, when the hard strata themselves approach base-level, the narrows again become inconspicuous.
From what has preceded it is clear that rapids or falls are likely to occur at narrows, especially in the early part of their history.
Other effects on topography.—Inequalities in the hardness of rock develop certain peculiarities of topography other than those of valleys. The less resistant portions of a land area more or less distant from streams are worn down more readily than those which are more resistant. If great areas of high land be capped with hard rock they are likely to remain as plateaus after surrounding areas of less resistance are brought low. If the hard capping affects a small area instead of a large one, the elevation is a butte, a hill, or a mountain, instead of a plateau (Fig. 110). Many buttes and small mesas are but remnants of former plateaus (Mesa Lauriano, N. M., Fig. 1, Pl. XII). A feature of buttes and mesas capped by hard rock is the steep slope or cliff corresponding to the edge of the hard bed (Figs. 78 and 109).
If the rock of a region be stratified and the layers tilted, the removal of the softer beds leaves the harder ones projecting above the general level in the form of ridges or “hog-backs” (Figs. 122 and 123). Dikes of igneous rock, harder than the beds which they intersect, likewise become ridges after the degradation of their surroundings. The plugs of old volcanic vents and other igneous intrusions of limited area often constitute conspicuous hills or mountains after erosion has removed their less resistant surroundings (Fig. 124). Inequalities of hardness are therefore responsible for many hills and ridges. In the isolation of the hills and ridges picturesque coves are developed, where the attitude and distribution of the weak and strong rocks are propitious. The bottoms of the coves are located on the weak rocks, and above them rise the precipitous slopes of the resistant ones. Round valley (Fig. 1, Pl. XVII, High Bridge, N. J., quadrangle, U. S. Geol. Surv.) and the coves about the head of Hiawassee River (Dahlonega, Ga., quadrangle) are examples.
Ridges and hills resulting from the unequal degradation of unequally resistant terranes are not equally prominent at all stages in an erosion cycle. In early youth the material surrounding the hard bodies of rock has not been removed; in early maturity considerable portions of their surroundings still remain about them; but in late maturity or early old age the outcropping masses of hard rock have been more perfectly isolated and are most conspicuous. Most of the even-crested ridges of the Appalachian system, as well as many others which might be mentioned, became ridges in this way. In the final stages of an erosion cycle the ridges of hard rock are themselves brought low. Isolated remnants of hard rock which remain distinctly above their surroundings in the late stages of an erosion cycle (Fig. 124) are known as Monadnocks, the name being derived from Mount Monadnock, N. H., an elevation of this sort developed in a cycle antedating the present.
Adjustment of streams to rock structures.—Valleys (gullies) locate themselves at the outset without immediate regard to the hardness and softness of their beds. It is primarily the slope about the head of a gully which determines its line of growth, though relative hardness often determines the details of slope, even in the early stages of an erosion cycle. Once established, streams tend to hold their courses, even if this involves the crossing of resistant layers.
While a region where more and less resistant layers of rock come to the surface is in a youthful stage of erosion, some of the valleys (and therefore the streams) are likely to be located on the less resistant rock, some on the more resistant, and some partly on the one and partly on the other. The streams on the weaker rock will deepen their valleys more rapidly than the others, and those which flow across stronger and weaker rocks alternately will deepen their valleys more rapidly than those which run on hard rock all the time. The former conclusion is self-evident. The latter appears from the fact that rapids will be likely to develop at the crossing of each hard layer, thus accelerating erosion at those points. Such a stream therefore not only has less hard rock to erode than one which flows on resistant rock all the time, but it erodes that which it does cross much faster.
Streams which do not cross hard layers therefore have an advantage over those which do, and the tributaries to such streams, since they join deeper mains, have an advantage over the tributaries to the others. The valleys of the former may lengthen until their heads reach the latter, and capture their streams. This sequence of events is illustrated in the accompanying diagrams (Figs. 125–27). Even where several streams cross the same resistant bed, piracy is likely to take place among them, for some are sure to deepen their valleys faster than others, because of inequalities of volume, load, or hardness. This is illustrated by Figs. 128–30. An actual case is shown in Figs. 131, 132. Though piracy may take place when streams do not flow over rock of unequal hardness (p. 103), it is much more common where unequal resistance of the rock puts one stream at a disadvantage as compared with another.
The changes in the courses of streams, by means of which they come to sustain definite and stable relations to the rock structure beneath, are known as processes of adjustment.[54] Since streams and valleys adjust themselves to other conditions as well, this phase of adjustment may be called structural adjustment. Structural adjustment is not uncommon among rivers flowing over strata which are vertical or highly inclined, since in these positions the hard and soft strata are most likely to come to the surface in frequent alternation. The smaller streams suffer capture and adjustment first, since, as a rule, they have shallower valleys. It often happens that main streams, because of their deeper valleys, hold courses not in adjustment with structure (the Delaware, the Susquehanna, etc.), while tributary streams are captured, diverted, and adjusted. The capture of a tributary, however, leads both to the diminution of its main and to the increase of its captor, and the weakened stream may ultimately fall a prey to the one which is strengthened.
The processes of adjustment go on until the streams flow as much as possible on the weaker beds, and as little as possible on the stronger, when adjustment is complete. This amounts to the same thing as saying that the outcrops of the hard layers tend to become divides. In many cases an area is so situated that there is no escape for its drainage except across resistant rock. In this case its drainage is completely adjusted when as few streams as possible cross the resistant rock, and these by the shortest routes.
Adjustment has been carried to a high degree of perfection in most parts of the Appalachian system. Here, as in all other mountains of similar structure, strata of unequal hardness were folded into ridges. In this case, the folds have been truncated by erosion, exposing the more and the less resistant beds (H and S respectively) in alternate belts along the flanks of the truncated folds (ab and cd, Fig. 133). The streams, especially the lesser ones, now flow along the strike of the softer beds much more commonly than elsewhere, and where they cross the hard layers it is usually at right angles to the strike. This is shown in Fig. 134, where the arrows indicate the direction of strike. In the history of these rivers, however, a factor is involved which has not yet been considered, and these streams will be referred to later.