Chapter III: Part 3

32. _Capture of the Anthracite headwaters by the growing Susquehanna_.--Throughout the Perm-Triassic period of denudation, a great work was done in wearing down the original Alleghanies. Anticlines of hard sandstone were breached, and broad lowlands were opened on the softer rocks beneath. Little semblance of the early constructional topography remained when the period of Newark depression was brought to a close; and all the while the headwater streams of the region were gnawing at the divides, seeking to develop the most perfect arrangement of waterways. Several adjustments have taken place, and the larger streams have been reversed in the direction of their flow; but a more serious problem is found in the disappearance of the original master stream, the great Anthracite river, which must have at first led away the water from all the lateral synclinal streams. Being a large river, it could not have been easily diverted from its course, unless it was greatly retarded in cutting down its channel by the presence of many beds of hard rocks on its way. The following considerations may perhaps throw some light on this obscure point.

It may be assumed that the whole group of mountains formed by the Permian deformation had been reduced to a moderate relief when the Newark deposition was stopped by the Jurassic elevation. The harder ribs of rock doubtless remained as ridges projecting above the intervening lowlands, but the strength of relief that had been given by the constructional forces had been lost. The general distribution of residual elevations then remaining unsubdued is indicated in fig. 25, in which the Crystalline, the Medina, and the two Carboniferous sandstone ridges are denoted by appropriate symbols. In restoring this phase of the surface form, when the country stood lower than now, I have reduced the anticlines from their present outlines and increased the synclines, the change of area being made greatest where the dips are least, and hence most apparent at the ends of the plunging anticlines and synclines. Some of the Medina anticlines of Perry and Juniata counties are not indicated because they were not then uncovered. The country between the residual ridges of Jurassic time was chiefly Cambrian limestone and Siluro-Devonian shales and soft sandstones. The moderate ridges developed on the Oriskany and Chemung sandstones are not represented. The drainage of this stage retained the original courses of the streams, except for the adjustments that have been described, but the great Anthracite river is drawn as if it had been controlled by the Newark depression and reversed in the direction of its flow, so that its former upper course on the Cambrian rocks was replaced by a superimposed Newark lower course. Fig. 25 therefore represents the streams for the most part still following near their synclinal axes, although departing from them where they have to enter a synclinal cove-mountain ridge; the headwaters of the Juniata avoid the mass of hard sandstones discovered in the bottom of old Broad Top lake, and flow around them to the north, and then by a cross-country course to the Wiconisco synclinal, as already described in detail. Several streams come from the northeast, entering the Anthracite district after the fashion generalized in fig. 13. Three of the many streams that were developed on the great Kittatinny slope are located, with their direction of flow reversed; these are marked Sq, L and D, and are intended to represent the ancestors of the existing Susquehanna, Lehigh and Delaware. We have now to examine the opportunities offered to these small streams to increase their drainage areas.

The Jurassic elevation, by which the Newark deposition was stopped, restored to activity all the streams that had in the previous cycle sought and found a course close to baselevel. They now all set to work again deepening their channels. But in this restoration of lost activity with reference to a new baselevel, there came the best possible chance for numerous re-arrangements of drainage areas by mutual adjustment into which we must inquire.

I have already illustrated what seems to me to be the type of the conditions involved at this time in figs. 19 and 20. The master stream, A, traversing the synclines, corresponds to the reversed Anthracite river; the lowlands at the top are those that have been opened out on the Siluro-Devonian beds of the present Susquehanna middle course between the Pocono and the Medina ridges. The small stream, B, that is gaining drainage area in these lowlands, corresponds to the embryo of the present Susquehanna, Sq, fig. 25, this having been itself once a branch on the south side of the Swatara synclinal stream, fig. 21, from which it was first turned by the change of slope accompanying the Newark depression; but it is located a little farther west than the actual Susquehanna, so as to avoid the two synclinal cove mountains of Pocono sandstone that the Susquehanna now traverses, for reasons to be stated below (section 35). This stream had to cross only one bed of hard rock, the outer wall of Medina sandstone, between the broad inner lowlands of the relatively weak Siluro-Devonian rocks and the great valley lowlands on the still weaker Cambrian limestones. Step by step it must have pushed its headwater divide northward, and from time to time it would have thus captured a subsequent stream, that crossed the lowlands eastward, and entered a Carboniferous syncline by one of the lateral gaps already described. With every such capture, the power of the growing stream to capture others was increased. Fig. 19 represents a stage after the streams in the Swatara and Wiconisco synclines (the latter then having gained the Juniata) had been turned aside on their way to the Carboniferous basins. On the other hand, the Anthracite river, rising somewhere on the plains north of the Wyoming syncline and pursuing an irregular course from one coal basin to another, found an extremely difficult task in cutting down its channel across the numerous hard beds of the Carboniferous sandstones, so often repeated in the rolling folds of the coal fields. It is also important to remember that an aid to other conditions concerned in the diversion of the upper Anthracite is found in the decrease of slope that its lower course suffered in crossing the coal fields, if that area took any part in the deformation that produced the Newark monocline--whichever theory prove true in regard to the origin of the southeastward flow of the rivers--for loss of slope in the middle course, where the river had to cross many reefs of hard sandstone, would have been very effective in lengthening the time allowed for the diversion of the headwaters.

The question is, therefore, whether the retardation of down-cutting here experienced by the Anthracite was sufficient to allow the capture of its headwaters by the Susquehanna. There can be little doubt as to the correct quality of the process, but whether it was quantitatively sufficient is another matter. In the absence of any means of testing its sufficiency, may the result not be taken as the test? Is not the correspondence between deduction and fact close enough to prove the correctness of the deduction?

33. _Present outward drainage of the Anthracite basins_.--The Lehigh, like the Susquehanna, made an attempt to capture the headwaters of adjacent streams, but failed to acquire much territory from the Anthracite because the Carboniferous sandstones spread out between the two in a broad plateau of hard rocks, across which the divide made little movement. The plateau area that its upper branches drain is, I think, the conquest of a later cycle of growth. The Delaware had little success, except as against certain eastern synclinal branches of the Anthracite, for the same reason. The ancestor of the Swatara of to-day made little progress in extending its headwaters because its point of attack was against the repeated Carboniferous sandstones in the Swatara synclinal. One early stream alone found a favorable opportunity for conquest, and thus grew to be the master river--the Susquehanna of to-day. The head of the Anthracite was carried away by this captor, and its beheaded lower portion remains in our Schuylkill. The Anthracite coal basins, formerly drained by the single master stream, have since been apportioned to the surrounding rivers. As the Siluro-Devonian lowlands were opened around the coal-basins, especially on the north and west, the streams that formerly flowed into the basins were gradually inverted and flowed out of them, as they still do. The extent of the inversion seems to be in a general way proportionate to its opportunity. The most considerable conquests were made in the upper basins, where the Catawissa and Nescopec streams of to-day drain many square miles of wide valleys opened on the Mauch Chunk red shale between the Pocono and Pottsville sandstone ridges; the ancient middle waters of the Anthracite here being inverted to the Susquehanna tributaries, because the northern coal basins were degraded very slowly after the upper Anthracite had been diverted. The Schuylkill as the modern representative of the Anthracite retains only certain streams south of a medial divide between Nescopec and Blue mountains. The only considerable part of the old Anthracite river that still retains a course along the axis of a synclinal trough seems to be that part which follows the Wyoming basin; none of the many other coal basins are now occupied by the large stream that originally followed them. The reason for this is manifestly to be found in the great depth of the Wyoming basin, whereby the axial portion of its hard sandstones are even now below baselevel, and hence have never yet acted to throw the river from its axial course. Indeed, during the early cycles of denudation, this basin must have been changed from a deep lake to a lacustrine plain by the accumulation in it of waste from the surrounding highlands, and for a time the streams that entered it may have flowed in meandering courses across the ancient alluvial surface; the lacustrine and alluvial condition may have been temporarily revived at the time of the Jurassic elevation. It is perhaps as an inheritance from a course thus locally superimposed that we may come to regard the deflection of the river at Nanticoke from the axis of the syncline to a narrow shale valley on its northern side, before turning south again and leaving the basin altogether. But like certain other suggestions, this can only be regarded as an open hypothesis, to be tested by some better method of river analysis than we now possess; like several of the other explanations here offered, it is presented more as a possibility to be discussed than as a conclusion to be accepted.

I believe that it was during the earlier part of the great Jura-Cretaceous cycle of denudation that the Susquehanna thus became the master stream of the central district of the state. For the rest of the cycle, it was occupied in carrying off the waste and reducing the surface to a well finished baselevel lowland that characterized the end of Cretaceous time. From an active youth of conquest, the Susquehanna advanced into an old age of established boundaries; and in later times, its area of drainage does not seem to have been greatly altered from that so long ago defined; except perhaps in the districts drained by the West and North Branch headwaters.

34. _Homologies of the Susquehanna and Juniata_.--Looking at the change from the Anthracite to the Susquehanna in a broad way, one may perceive that it is an effect of the same order as the peripheral diversion of the Broad Top drainage, illustrated in figures 22, 23 and 24; another example of a similar change is seen in the lateral diversion of the Juniata above Lewistown and its rectilinear continuation in Aughwick creek, from their original axial location when they formed the initial Broad Top outlet. They have departed from the axis of their syncline to the softer beds on its southern side; FE of fig. 17 has been diverted to FD of fig. 18.

All of these examples are truly only special cases of the one already described in which the Juniata left its original syncline for others to the south. The general case may be stated in a few words. A stream flowing along a syncline of hard beds (Carboniferous sandstones) develops side streams which breach the adjacent anticlines and open lowlands in the underlying softer beds (Devonian and Silurian). On these lowlands, the headwaters of side streams from other synclines are encountered and a contest ensues as to possession of the drainage territory. The divides are pushed away from those headwaters whose lower course leads them over the fewest hard barriers; this conquest goes on until the upper course of the initial main stream is diverted to a new and easier path than the one it chose in its youth in obedience to the first deformation of the region. Thus the Juniata now avoids the center and once deepest part of the old Broad Top lake, because in the general progress of erosion, lowlands on soft Devonian beds were opened all around the edge of the great mass of sandstones that held the lake; the original drainage across the lake, from its western slopes to its outlet just south of the Jack's mountain anticline, has now taken an easier path along the Devonian beds to the west of the old lake basin, and is seen in the Little Juniata, flowing along the outer side of Terrace mountain and rounding the northern synclinal point where Terrace mountain joins Sideling hill. It then crosses Jack's mountain at a point where the hard Medina sandstones of the mountain were still buried at the time of the choice of this channel. In the same way, the drainage of the subordinate basin, through which the main lake discharged eastward, is now not along the axis of the Juniata-Catawissa syncline, but on the softer beds along one side of it; and along the southern side because the easier escape that was provided for it lay on that side, namely, via the Tuscarora and Wiconisco synclines, as already described. The much broader change from the Anthracite to the Susquehanna was only another form of the same process. Taking a transverse view of the whole system of central folds, it is perceived that their axes descend into the Anthracite district from the east and rise westward therefrom; it is as if the whole region had received a slight transverse folding, and the transverse axis of depression thus formed defined the initial course of the first master stream. But this master stream deserted its original course on the transverse axis of depression because a lateral course across lowlands on softer beds was opened by its side streams; and in the contest on these lowlands with an external stream, the Susquehanna, the upper portion of the Anthracite was diverted from the hard rocks that had appeared on the transverse axis. The distance of diversion from the axial to the lateral course in this case was great because of the gentle quality of the transverse folding; or, better said, because of the gentle dips of the axes of the longitudinal folds. This appearance of systematic re-arrangement in the several river courses where none was expected is to my mind a strong argument in favor of the originally consequent location of the rivers and their later mutual adjustment. It may perhaps be conceived that antecedent streams might imitate one another roughly in the attitude that they prophetically chose with regard to folds subsequently formed, but no reason has been suggested for the imitation being carried to so remarkable and definite a degree as that here outlined.

35. _Superimposition of the Susquehanna on two synclinal ridges_.--There is however one apparently venturesome postulate that may have been already noted as such by the reader; unless it can be reasonably accounted for and shown to be a natural result of the long sequence of changes here considered, it will seriously militate against the validity of the whole argument. The present course of the middle Susquehanna leads it through the apical curves of two Pocono synclinal ridges, which were disregarded in the statement given above. It was then assumed that the embryonic Susquehanna gained possession of the Siluro-Devonian lowland drainage by gnawing out a course to the west of these synclinal points; for it is not to be thought of that any conquest of the headwaters of the Anthracite river could have been made by the Susquehanna if it had had to gnaw out the existing four traverses of the Pocono sandstones before securing the drainage of the lowlands above them. The backward progress of the Susquehanna could not in that case have been nearly fast enough to reach the Anthracite before the latter had sunk its channel to a safe depth. It is therefore important to justify the assumption as to the more westerly location of the embryonic Susquehanna; and afterwards, to explain how it should have since then been transferred to its present course. A short cut through all this round-about method is open to those who adopt in the beginning the theory that the Susquehanna was an antecedent river; but as I have said at the outset of this inquiry, it seems to me that such a method is not freer from assumption, even though shorter than the one here adopted; and it has the demerit of not considering all the curious details that follow the examination of consequent and adjusted courses.

The sufficient reason for the assumption that the embryonic Susquehanna lay farther west than the present one in the neighborhood of the Pocono synclinals is simply that--in the absence of any antecedent stream--it must have lain there. The whole explanation of the development of the Siluro-Devonian lowlands between the Pocono and Medina ridges depends simply on their being weathered out where the rocks are weak enough to waste faster than the enclosing harder ridges through which the streams escape. In this process, the streams exercise no control whatever over the direction in which their headwaters shall grow; they leave this entirely to the structure of the district that they drain. It thus appears that, under the postulate as to the initial location of the Susquehanna as one of the many streams descending the great slope of the Kittatinny (Cumberland) highland into the Swatara syncline, its course being reversed from northward to southward by the Newark depression, we are required to suppose that its headwater (northward) growth at the time of the Jurassic elevation must have been on the Siluro-Devonian beds, so as to avoid the harder rocks on either side. Many streams competed for the distinction of becoming the master, and that one gained its ambition whose initial location gave it the best subsequent opportunity. It remains then to consider the means by which the course of the conquering Susquehanna may have been subsequently changed from the lowlands on to the two Pocono synclines that it now traverses. Some departure from its early location may have been due to eastward planation in its advanced age, when it had large volume and gentle slope and was therefore swinging and cutting laterally in its lower course. This may have had a share in the result, but there is another process that seems to me more effective.

In the latter part of the Jura-Cretaceous cycle, the whole country hereabout suffered a moderate depression, by which the Atlantic transgressed many miles inland from its former shoreline, across the lowlands of erosion that had been developed on the litoral belt. Such a depression must have had a distinct effect on the lower courses of the larger rivers, which having already cut their channels down close to baselevel and opened their valleys wide on the softer rocks, were then "estuaried," or at least so far checked as to build wide flood-plains over their lower stretches. Indeed, the flood-plains may have been begun at an earlier date, and have been confirmed and extended in the later time of depression. Is it possible that in the latest stage of this process, the almost baselevelled remnants of Blue mountain and the Pocono ridges could have been buried under the flood-plain in the neighborhood of the river?

If this be admitted, it is then natural for the river to depart from the line of its buried channel and cross the buried ridges on which it might settle down as a superimposed river in the next cycle of elevation. It is difficult to decide such general questions as these; and it may be difficult for the reader to gain much confidence in the efficacy of the processes suggested; but there are certain features in the side streams of the Susquehanna that lend some color of probability to the explanation as offered.

Admit, for the moment, that the aged Susquehanna, in the later part of the Jura-Cretaceous cycle, did change its channel somewhat by cutting to one side, or by planation, as it is called. Admit, also, that in the natural progress of its growth it had built a broad flood plain over the Siluro-Devonian lowlands, and that the depth of this deposit was increased by the formation of an estuarine delta upon it when the country sank at the time of the mid-Cretaceous transgression of the sea. It is manifest that one of the consequences of all this might be the peculiar course of the river that is to be explained, namely, its superimposition on the two Pocono synclinal ridges in the next cycle of its history, after the Tertiary elevation had given it opportunity to re-discover them. It remains to inquire what other consequences should follow from the same conditions, and from these to devise tests of the hypothesis.

36. _Evidence of superimposition in the Susquehanna tributaries_.--One of the peculiarities of flood-plained rivers is that the lateral streams shift their points of union with the main stream farther and farther down the valley, as Lombardini has shown in the case of the Po. If the Susquehanna were heavily flood-plained at the close of the Jura-Cretaceous cycle, some of its tributaries should manifest signs of this kind of deflection from their structural courses along the strike of the rocks. Side streams that once joined the main stream on the line of some of the softer northeast-southwest beds, leaving the stronger beds as faint hills on either side, must have forgotten such control after it was baselevelled and buried; as the flood plain grew, they properly took more and more distinctly downward deflected courses, and these deflections should be maintained in subsequent cycles as superimposed courses independent of structural guidance. Such I believe to be the fact. The downstream deflection is so distinctly a peculiarity of a number of tributaries that join the Susquehanna on the west side (see figure 1) that it cannot be ascribed to accident, but must be referred to some systematic cause. Examples of deflection are found in Penn's creek, Middle creek and North Mahantango creek in Snyder county; West Mahantango between the latter and Juniata county; and in the Juniata and Little Juniata rivers of Perry county. On the other side of the Susquehanna, the examples are not so distinct, but the following may be mentioned: Delaware and Warrior runs, Chillisquaque creek and Little Shamokin creek, all in Northumberland county. It may be remarked that it does not seem impossible that the reason for the more distinct deflection of the western streams may be that the Susquehanna is at present east of its old course, and hence towards the eastern margin of its flood plain, as, indeed its position on the Pocono synclinals implies. A reason for the final location of the superimposed river on the eastern side of the old flood plain may perhaps be found in the eastward tilting that is known to have accompanied the elevation of the Cretaceous lowland.

It follows from the foregoing that the present lower course of the Susquehanna must also be of superimposed origin; for the flood plain of the middle course must have extended down stream to its delta, and there have become confluent with the sheet of Cretaceous sediments that covered all the southeastern lowland, over which the sea had transgressed. McGee has already pointed out indications of superimposed stream courses in the southeastern part of the State;[22] but I am not sure that he would regard them as of the date here referred to.

[Footnote 22: Amer. Journ. Science, xxxv, 1888, 121, 134.]

The theory of the location of the Susquehanna on the Pocono synclinal ridges therefore stands as follows. The general position of the river indicates that it has been located by some process of slow self-adjusting development and that it is not a persistent antecedent river; and yet there is no reason to think that it could have been brought into its present special position by any process of shifting divides. The processes that have been suggested to account for its special location, as departing slightly from a location due to slow adjustments following an ancient consequent origin, call for the occurrence of certain additional peculiarities in the courses of its tributary streams, entirely unforeseen and unnoticed until this point in the inquiry is reached; and on looking at the map to see if they occur, they are found with perfect distinctness. The hypothesis of superimposition may therefore be regarded as having advanced beyond the stage of mere suggestion and as having gained some degree of confirmation from the correlations that it detects and explains. It only remains to ask if these correlations might have originated in any other way, and if the answer to this is in the negative, the case may be looked upon as having a fair measure of evidence in its favor. The remaining consideration may be taken up at once as the first point to be examined in the Tertiary cycle of development.

37. _Events of the Tertiary cycle_.--The elevation given to the region by which Cretaceous baselevelling was terminated, and which I have called the early Tertiary elevation, offered opportunity for the streams to deepen their channels once more. In doing so, certain adjustments of moderate amount occurred, which will be soon examined. As time went on, much denudation was effected, but no wide-spread baselevelling was reached, for the Cretaceous crest lines of the hard sandstone ridges still exist. The Tertiary cycle was an incomplete one. At its close, lowlands had been opened only on the weaker rocks between the hard beds. Is it not possible that the flood-plaining of the Susquehanna and the down-stream deflection of its branches took place in the closing stages of this cycle, instead of at the end of the previous cycle? If so, the deflection might appear on the branches, but the main river would not be transferred to the Pocono ridges. This question may be safely answered in the negative; for the Tertiary lowland is by no means well enough baselevelled to permit such an event. The beds of intermediate resistance, the Oriskany and certain Chemung sandstones, had not been worn down to baselevel at the close of the Tertiary cycle; they had indeed lost much of the height that they possessed at the close of the previous cycle, but they had not been reduced as low as the softer beds on either side. They were only reduced to ridges of moderate and unequal height over the general plain of the Siluro-Devonian low country, without great strength of relief but quite strong enough to call for obedience from the streams along side of them. And yet near Selin's Grove, for example, in Snyder county, Penn's and Middle creeks depart most distinctly from the strike of the local rocks as they near the Susquehanna, and traverse certain well-marked ridges on their way to the main river. Such aberrant streams cannot be regarded as superimposed at the close of the incomplete Tertiary cycle; they cannot be explained by any process of spontaneous adjustment yet described, nor can they be regarded as vastly ancient streams of antecedent courses; I am therefore much tempted to consider them as of superimposed origin, inheriting their present courses from the flood-plain cover of the Susquehanna in the latest stage of the Jura-Cretaceous cycle. With this tentative conclusion in mind as to the final events of Jura-Cretaceous time, we may take up the more deliberate consideration of the work of the Tertiary cycle.

The chief work of the Tertiary cycle was merely the opening of the valley lowlands; little opportunity for river adjustment occurred except on a small scale. The most evident cases of adjustment have resulted in the change of water-gaps into wind-gaps, of which several examples can be given, the one best known being the Delaware wind-gap between the Lehigh and Delaware water-gaps in Blue mountain. The wind-gap marks the unfinished notch of some stream that once crossed the ridge here and whose headwaters have since then been diverted, probably to the Lehigh. The difficulty in the case is not at all how the stream that once flowed here was diverted, but how a stream that could be diverted in the Tertiary cycle could have escaped diversion at some earlier date. The relative rarity of wind-gaps indicates that nearly all of the initial lateral streams, which may have crossed the ridges at an early epoch in the history of the rivers, have been beheaded in some cycle earlier than the Tertiary and their gaps thereafter obliterated. Why the Delaware wind-gap stream should have endured into a later cycle does not at present appear. Other wind-gaps of apparently similar origin may be found in Blue mountain west of the Schuylkill and east of the Susquehanna. It is noteworthy that if any small streams still persevere in their gaps across a hard ridge, they are not very close to any large river-gap; hence it is only at the very headwaters of Conedogwinet creek, in the northern part of Franklin county, that any water is still drawn from the back of Blue mountain. Again, these small stream gaps do not lie between large river-gaps and wind-gaps, but wind-gaps lie between the gaps of large rivers and those of small streams that are not yet diverted. Excellent illustration of this is found on the "Piedmont sheet" of the contoured maps issued by the United States Geological Survey. The sheet covers part of Maryland and West Virginia, near where the North Branch of the Potomac comes out of the plateau and crosses New Creek mountain. Eleven miles south of the Potomac gap there is a deep wind-gap; but further on, at twenty, twenty-five and twenty-nine miles from the river-gap are three fine water-gaps occupied by small streams. This example merely shows how many important points in the history of our rivers will be made clear when the country is properly portrayed on contoured maps.

A few lines may be given to the general absence of gaps in Blue Mountain in Pennsylvania. When the initial consequent drainage was established, many streams must have been located on the northward slope of the great Cumberland highland, C, C, fig. 21; they must have gullied the slope to great depths and carried away great volumes of the weak Cambrian beds that lay deep within the hard outer casings of the mass. Minor adjustments served to diminish the number of these streams, but the more effective cause of their present rarity lay in the natural selection of certain of them to become large streams; the smaller ones were generally beheaded by these. The only examples of streams that still cross this ridge with their initial Permian direction of flow to the northwest are found in two southern branches of Tuscarora creek at the southern point of Juniata county; and these survive because of their obscure location among the many Medina ridges of that district, where they were not easily accessible to capture by other streams.

38. _Tertiary adjustment of the Juniata on the Medina anticlines_.--The lower course of the Juniata presents several examples of adjustment referable to the last part of the Jura-Cretaceous cycle and to the Tertiary cycle. The explanation offered for the escape of this river from its initial syncline did not show any reason for its peculiar position with respect to the several Medina anticlines that it now borders, because at the time when it was led across country to the Wiconisco syncline, the hard Medina beds of these anticlines were not discovered. It is therefore hardly to be thought that the location of the Juniata in the Narrows below Lewistown between Blue Ridge and Shade mountain and its avoidance of Tuscarora mountain could have been defined at that early date. But all these Medina anticlines rise more or less above the Cretaceous baselevel, and must have had some effect on the position taken by the river about the middle of that cycle when its channel sank upon them. Blue Ridge and Black Log anticlines rise highest. The first location of the cross-country stream that led the early Juniata away from its initial syncline probably traversed the Blue Ridge and Black Log anticlines while they were yet buried; but its channel-cutting was much retarded on encountering them, and some branch stream working around from the lower side of the obstructions may have diverted the river to an easier path. The only path of the kind is the narrow one between the overlapping anticlines of Blue Ridge and Shade mountains, and there the Juniata now flows. If another elevation should occur in the future, it might happen that the slow deepening of the channel in the hard Medina beds which now floor the Narrows would allow Middle creek of Snyder county to tap the Juniata at Lewistown and lead it by direct course past Middleburgh to the Susquehanna; thus it would return to the path of its youth.

The location of the Juniata at the end of Tuscarora mountain is again so definite that it can hardly be referred to a time when the mountain had not been revealed. The most likely position of the original cross-country stream which brought the Juniata into the Wiconisco syncline was somewhere on the line of the existing mountain, and assuming it to have been there, we must question how it has been displaced. The process seems to have been of the same kind as that just given; the retardation of channel-cutting in the late Cretaceous cycle, when the Medina beds of Tuscarora anticline were discovered, allowed a branch from the lower part of the river to work around the end of the mountain and lead the river out that way. The occurrence of a shallow depression across the summit of the otherwise remarkably even crest of Tuscarora mountain suggests that this diversion was not finally accomplished until shortly after the Tertiary elevation of the country; but at whatever date the adjustment occurred, it is natural that it should pass around the eastern end of the mountain and not around the western end, where the course would have been much longer, and therefore not successfully to be taken by a diverting stream.

While the quality of these processes appears satisfactory, I am not satisfied as to the sufficiency of their quantity. If diversion was successfully practiced at the crossing of the Tuscarora anticline, why not also at the crossing of Jack's mountain anticline, on which the river still perseveres. It is difficult here to decide how much confidence may be placed in the explanation, because of its giving reason for the location of certain streams, and how much doubt must be cast upon it, because it seems impossible and is not of universal application.

39. _Migration of the Atlantic-Ohio divide_.--There are certain shifted courses which cannot be definitely referred to any particular cycle, and which may therefore be mentioned now. Among the greatest are those by which the divide between the Atlantic and the Ohio streams has been changed from its initial position on the great constructional Nittany highland and Bedford range. There was probably no significant change until after Newark depression, for the branches of the Anthracite river could not have begun to push the divide westward till after the eastward flow of the river was determined; until then, there does not seem to have been any marked advantage possessed by the eastward streams over the westward. But with the eastward escape of the Anthracite, it probably found a shorter course to the sea and one that led it over alternately soft and hard rocks, instead of the longer course followed by the Ohio streams over continuous sandstones. The advantage given by the greater extent of soft beds is indicated by the great breadth of the existing valleys in the central district compared with the less breadth of those in the plateau to the west. Consider the effect of this advantage at the time of the Jurassic elevation. As the streams on the eastern slope of the Nittany divide had the shortest and steepest courses to the sea, they deepened their valleys faster than those on the west and acquired drainage area from them; hence we find reason for the drainage of the entire Nittany and Bedford district by the Atlantic streams at present. Various branches of what are now the Alleghany and Monongahela originally rose on the western slope of the dividing range. These probably reached much farther east in pre-Permian time, but had their headwaters turned another way by the growth of the great anticlinal divide; but the smaller anticlines of Laurel ridge and Negro mountain farther west do not seem to have been strong enough to form a divide, for the rivers still traverse them. Now as the headwaters of the Juniata breached the eastern slope of the Nittany-Bedford range and pushed the divide westward, they at last gained possession of the Siluro-Devonian monocline on its western slope; but beyond this it has not been possible for them yet to go. As the streams cut down deeper and encountered the Medina anticline near the core of the ridge, they sawed a passage through it; the Cambrian beds were discovered below and a valley was opened on them as the Medina cover wore away. The most important point about this is that we find in it an adequate explanation of the opposite location of water-gaps in pairs, such as characterize the branches of the Juniata below Tyrone and again below Bedford. This opposite location has been held to indicate an antecedent origin of the river that passes through the gaps, while gaps formed by self-developed streams are not thought to present such correspondence (Hilber). Yet this special case of paired gaps in the opposite walls of a breached anticline is manifestly a direct sequence of the development of the Juniata headwaters. The settling down of the main Juniata on Jack's mountain anticline below Huntingdon is another case of the same kind, in which the relatively low anticlinal crest is as yet not widely breached; the gaps below Bedford stand apart, as the crest is there higher, and hence wider opened; and the gaps below Tyrone are separated by some ten or twelve miles.

When the headwater streams captured the drainage of the Siluro-Devonian monocline on the western side of the ancient dividing anticline, they developed subsequent rectangular branches growing like a well-trained grape vine. Most of this valley has been acquired by the west branch of the Susquehanna, probably because it traversed the Medina beds less often than the Juniata. For the same reason, it may be, the West Branch has captured a considerable area of plateau drainage that must have once belonged to the Ohio, while the Juniata has none of it; but if so, the capture must have been before the Tertiary cycle, for since that time the ability of the West Branch and of the Juniata as regards such capture appears about alike. On the other hand, Castleman's river, a branch of the Monongahela, still retains the drainage of a small bit of the Siluro-Devonian monocline, at the southern border of the State, where the Juniata headwaters had the least opportunity to capture it; but the change here is probably only retarded, not prevented entirely; the Juniata will some day push the divide even here back to the Alleghany Front, the frontal bluff of the plateau.

40. _Other examples of adjustments_.--Other examples of small adjustments are found around the Wyoming basin, fig. 26. Originally all these streams ran centripetally down the enclosing slopes, and in such locations they must have cut gullies and breaches in the hard Carboniferous beds and opened low back country on the weaker Devonians. Some of the existing streams still do so, and these are precisely the ones that are not easily reached by divertors. The Susquehanna in its course outside of the basin has sent out branches that have beheaded all the centripetal streams within reach; where the same river enters the basin, the centripetal streams have been shortened if not completely beheaded. A branch of the Delaware has captured the heads of some of the streams near the eastern end of the basin. Elsewhere, the centripetal streams still exist of good length. The contrast between the persistence of some of the centripetal streams here and their peripheral diversion around Broad Top is a consequence of the difference of altitude of the old lake bottoms in the two cases. It is not to be doubted that we shall become acquainted with many examples of this kind as our intimacy with rivers increases.

41. _Events of the Quaternary cycle_.--The brief quaternary cycle does not offer many examples of the kind that we have considered, and all that are found are of small dimensions. The only capturing stream that need be mentioned has lately been described as a "river pirate;"[23] but its conquest is only a Schleswig-Holstein affair compared to the Goth- and Hun-like depredations of the greater streams in earlier cycles.

[Footnote 23: Science, xiii, 1889, 108.]

The character of the streams and their valleys as they now exist is strikingly dependent in many ways on the relation of the incipient quaternary cycle to the longer cycles of the past. No lakes occur, exception being made only of the relatively small ponds due to drift obstruction within the glaciated area. Waterfalls are found only at the headwaters of small streams in the plateau district, exception again being made only for certain cases of larger streams that have been thrown from their pre-glacial courses by drift barriers, and which are now in a very immature state on their new lines of flow. The small valleys of this cycle are shallow and narrow, always of a size strictly proportional to the volume of the stream and the hardness of the enclosing rocks, exception being made only in the case of post-glacial gorges whose streams have been displaced from their pre-glacial channels. The terraces that are seen, especially on the streams that flow in or from the glaciated district, are merely a temporary and subordinate complication of the general development of the valleys. In the region that has been here considered, the streams have been seldom much displaced from their pre-glacial channels; but in the northwestern part of the State, where the drift in the valleys seems to be heavier, more serious disturbance of pre-glacial courses is reported. The facts here referred to in regard to lakes, falls, gorges, terraces and displaced streams are to be found in the various volumes of the Second Geological Survey of the State;[24] in regard to the terraces and the estuarine deflections of the Delaware and Susquehanna, reference should be made also to McGee's studies.[25]

[Footnote 24: Especially Carll, Reports I_{3}, I_{4}; White, Reports G_{5}, G_{6}; Lewis, Report Z.]

[Footnote 25: Amer. Journ. Science, xxxv, 1888, 367, 448; Seventh Annual Rep. U. S. G. S., 1888, 545.]

42. _Doubtful cases_.--It is hardly necessary to state that there are many facts for which no satisfactory explanation is found under the theory of adjustments that we have been considering. Some will certainly include the location of the Susquehanna on the points of the Pocono synclines under this category; all must feel that such a location savors of an antecedent origin. The same is true of the examples of the alignment of water-gaps found on certain streams; for example, the four gaps cut in the two pairs of Pocono and Pottsville outcrops at the west end of the Wyoming syncline, and the three gaps where the Little Schuylkill crosses the coal basin at Tamaqua; the opposite gaps in pairs at Tyrone and Bedford have already been sufficiently explained. The location of the upper North Branch of the Susquehanna is also unrelated to processes of adjustment as far as I can see them, and the great area of plateau drainage that is now possessed by the West Branch is certainly difficult to understand as the result of conquest. The two independent gaps in Tussey's mountain, maintained by the Juniata and its Frankstown branch below Tyrone are curious, especially in view of the apparent diversion of the branch to the main stream on the upper side of Warrior's ridge (Oriskany), just east of Tussey's mountain.

43. _Complicated history of our actual rivers_.--If this theory of the history of our rivers is correct, it follows that any one river as it now exists is of so complicated an origin that its development cannot become a matter of general study and must unhappily remain only a subject for special investigation for some time to come. It was my hope on beginning this essay to find some teachable sequence of facts that would serve to relieve the usual routine of statistical and descriptive geography, but this is not the result that has been attained. The history of the Susquehanna, the Juniata, or the Schuylkill, is too involved with complex changes, if not enshrouded in mystery, to become intelligible to any but advanced students; only the simplest cases of river development can be introduced into the narrow limits of ordinary instruction. The single course of an ancient stream is now broken into several independent parts; witness the disjointing and diversion of the original Juniata, which, as I have supposed, once extended from Broad Top lake to the Catawissa basin. Now the upper part of the stream, representing the early Broad Top outlet, is reduced to small volume in Aughwick creek; the continuation of the stream to Lewistown is first set to one side of its original axial location and is then diverted to another syncline; the beheaded portion now represented by Middle creek is diverted from its course to the Catawissa basin by the Susquehanna; perhaps the Catawissa of the present day represents the reversed course of the lower Juniata where it joined the Anthracite. This unserviceably complicated statement is not much simplified if instead of beginning with an original stream and searching out its present disjointed parts, we trace the composition of a single existing stream from its once independent parts. The Juniata of to-day consists of headwaters acquired from Ohio streams; the lake in which the river once gathered its upper branches is now drained and the lake bottom has become a mountain top; the streams flow around the margin of the lake, not across its basin; a short course towards Lewistown nearly coincides with the original location of the stream, but to confound this with a precise agreement is to lose the true significance of river history; the lower course is the product of diversion at least at two epochs and certainly in several places; and where the river now joins the Susquehanna, it is suspected of having a superimposed course unlike any of the rest of the stream. This is too complicated, even if it should ever be demonstrated to be wholly true, to serve as material for ordinary study; but as long as it has a savor of truth, and as long as we are ignorant of the whole history of our rivers, through which alone their present features can be rightfully understood, we must continue to search after the natural processes of their development as carefully and thoroughly as the biologist searches for the links missing from his scheme of classification.

44. _Provisional Conclusions_.--It is in view of these doubts and complications that I feel that the history of our rivers is not yet settled; but yet the numerous accordances of actual and deductive locations appear so definite and in some cases so remarkable that they cannot be neglected, as they must be if we should adhere to the antecedent origin of the river courses.

The method adopted on an early page therefore seems to be justified. The provisional system of ancient consequent drainage, illustrated on fig. 21, does appear to be sufficiently related to the streams of to-day to warrant the belief that most of our rivers took their first courses between the primitive folds of our mountains, and that from that distant time to the present the changes they have suffered are due to their own interaction--to their own mutual adjustment more than to any other cause. The Susquehanna, Schuylkill, Lehigh and Delaware are compound, composite and highly complex rivers, of repeated mature adjustment. The middle Susquehanna and its branches and the upper portions of the Schuylkill and Lehigh are descendants of original Permian rivers consequent on the constructional topography of that time; Newark depression reversed the flow of some of the transverse streams, and the spontaneous changes or adjustments from immature to mature courses in the several cycles of development are so numerous and extensive that, as Löwl truly says, the initial drainage has almost disappeared. The larger westward-flowing streams of the plateau are of earlier, Carboniferous birth, and have suffered little subsequent change beyond a loss of headwaters. The lower courses of the Atlantic rivers are younger, having been much shifted from their Permian or pre-Permian courses by Newark and Cretaceous superimposition, as well as by recent downward deformation of the surface in their existing estuaries. No recognizable remnant of rivers antecedent to the Permian deformation are found in the central part of the State; and with the exception of parts of the upper Schuylkill and of the Susquehanna near Wilkes-Barre, there are no large survivors of Permian consequent streams in the ordinary meaning of the term "consequent." The shifting of courses in the progress of mature adjustment has had more to do with determining the actual location of our rivers and streams than any other process.

Harvard College, June, 1889.

TOPOGRAPHIC MODELS.

BY COSMOS MINDELEFF.

Of the many methods by which it has been sought to represent the relief of a country or district, only two have been at all widely used. These methods are, in the order of their development, by hachured and by contoured maps. Both have advantages and both have serious disadvantages. Without entering into the controversy that is even yet raging over the relative merits of the two systems, some slight notice of what each claims to accomplish is necessary.

The representation of relief by hachures is a graphic system, and in the best examples we have is an attempt to show, upon a plane surface, the actual appearance of a given area under given conditions of lighting,--as in the Dufour map of the Alps. Of course certain details that would really disappear if the assumed conditions were actual ones, must be shown upon the map,--so that it is, after all, but a conventional representation. The very best examples are, for this and other reasons, unsatisfactory, and far more so is this the case in the vastly larger class of medium grade and poor work.

The contour system represents relief by a series of lines, each of which is, at every point throughout its length, at a certain stated elevation above sea-level, or some other datum-plane; in other words, each contour line represents what would be the water's edge, if the sea were to rise to that elevation. It possesses the advantage of great clearness, but fails to a large degree in the representation of surface detail; moreover, one must have considerable knowledge of topography, in order to read the map correctly.[1]

[Footnote 1: For specimens of representation of the same subject on different scales, in both the hachure and contour systems, see plate from "Enthoffer's Topographical Atlas."]

To those who must give first place to the quantity of relief rather than the quality, as, for example, the geologist or the engineer, a contoured map is now considered essential. On the other hand, where quality of relief is the prime consideration and the quantity a secondary one, as, for example, for the use of the army, a hachured map is considered the best. The method of hachures may be roughly characterized as a graphic system with a conventional element, and the contour method as a conventional system with a graphic element,--for if the contour interval is small enough a sort of shading is produced which helps considerably the idea of relief.