The Watchung ridges extend only about eight miles northward of the Paterson gaps, but reach thirty miles southwestward. It is therefore chiefly in the latter direction that we may expect to find examples of incomplete adjustment following superimposition and capture. At Milburn, there is a deep gap in First mountain, and opposite this at Summit (S, [fig. 1]) a partly drift-filled gap in Second mountain: this I am disposed to regard as the former outlet of the Rockaway-Rahway river, which on account of its considerable size was not captured by the Passaic until it had cut its passage across the trap sheets almost to a safe depth. The diverted upper portion—the Rockaway—now joins the Passaic; its crooked course from the Highlands via Boonton (Bn) being a post-glacial irregularity; the beheaded lower portion—the Rahway—heads on the ridge of Second mountain, retains the pair of subsequent streams between the two ridges, and flows in diminished volume to the sea: the divide between the two portions being in its mature stable position on Second mountain.
South of the Milburn gap, there are three streams that maintain water gaps in First mountain, and five head branches of these three streams rise behind the crest of Second mountain. These must be interpreted as remnants of streams that once rose further inland, and whose upper courses have been captured by the victorious upper Passaic; but it is noteworthy that here, at the greatest distance from the gap of the master stream at Paterson, the divides between the diverted and beheaded portions of these southern streams should lie in unstable positions, back of the crest line of Second mountain. This is exactly what the hypothesis of a superimposed origin for these streams would require; and if the complexity of accordance between deduction and fact here presented be duly considered, I believe new confidence may be gained in the hypothesis of superimposition, already rendered likely from other evidence.
The rectangular courses of the streams that cross First and Second mountains southwest of Milburn do not militate against their initial obliquely superimposed courses; for, as Gilbert has shown, oblique courses across tilted beds, alternately hard and soft, will gradually shift until they follow rectangular courses, along the strike of the soft beds and square across the strike of the hard beds. Middle Brook, at the southern bend of First mountain near Bound Brook (B, B), presents the peculiarity of branching east and west while on the trap sheet of the mountain: this may be due to a retention here, where the dip is moderate, of an initially superimposed bifurcation; or to guidance by fractures at this point where the course of the mountain changes rather abruptly; the facts at hand do not serve to make choice between these alternatives.
The lesson of greatest importance in this study lies, to my mind, in the gradual development of accordant subsequent streams in a region where the unchanged superimposed drainage would show no such accordance. Similar adjustment of subsequent streams to structural features may characterize drainage systems that were originally antecedent: and with this principle in mind, I have recently read over with renewed interest Powell's classic study of the Green river where it crosses the Uinta mountains.7 The Green river and the smaller streams of its lateral cañons and valleys are all regarded as antecedent. Let us examine the arguments on which this conclusion rests.
7 Exploration of the Colorado river of the west, Washington, 1875, 152-166. See also the geological map in the Geology of the Uinta mountains, 1876.
The Green river itself rises many miles north of the Uinta range, traverses a relatively low basin before reaching the flank of the mountains, and then instead of turning away, it boldly enters the great uplift and trenches it from side to side in a profound cañon, flowing out to the southwest on its way to the Colorado. There is relatively low ground at the eastern end of the range, several thousand feet lower than the summits of the range on either side of the Green river cañon, and many thousand feet lower than the restored crest of the great uplift; but the river does not follow this open round-about course. Powell says that the river cut through, instead of running around, the great obstruction, because it "had the right of way; ... it was running ere the mountains were formed." Had the mountain fold been formed suddenly, it would have turned the river around it to the east; "but the emergence of the fold above the general surface of the country was little or no faster than the progress of the corrasion of the channel." ... "The river preserved its level, but the mountains were lifted up.... The river was the saw which cut the mountains in two" (152, 153). If this interpretation is correct, the Green river would be the type of a perfect antecedent stream: but it appears to me that the case is probably overstated in that respect. Perhaps it would have been more deliberately stated in a later volume if Powell's intention of describing more fully the three chief kinds of drainage of the region had been carried out.8 Not having seen the region, my comments may have little value; but the context of Powell's report, the description of the immense series of lacustrine beds, over a mile thick, north of the mountains, and the eastward deflection of the river where it traverses the mountains all seem to me to indicate that the Green was by no means continuously successful in maintaining its antecedent course across the uplift. It is by no means a typical antecedent river. The great series of lacustrine beds up-stream from the cañon, with conglomerates where they rest on the northern flank of the mountains, are fully recognized in the report, and must mean that the upper portion of the river was for a time shut back, or ponded. During part of this time, there may have been no overflow across the growing mountains, for the lower lacustrine beds contain fossils indicative of brackish water.9 The intermittent growth of the mountains and the repeated return of lacustrine conditions, with gradually freshening water, is indicated by the strong unconformities that occur at various points in the lacustrine beds, and by the change in the fossil fauna. It must be conceded from this that the upper portion of Green river was repeatedly ponded back by mountain growth across its middle course; we therefore have not now any close indication of its pre-lacustrine course above the mountains; the ancient, or pre-Uinta, upper portion of the river was extinguished by the lacustrine sediments, and to that extent the Green river departs from the perfect antecedent type.
8 Geol. Uinta mountain, page v.
9 Geology of the Uinta mountains, 1876, 84; Chapter III, by C. A. White.
In the second place, if the original Green river existed upon the upper surface of the beds that were at a subsequent date raised to form the Uinta uplift, it does not appear to be clearly proved that its course at that early time was closely coincident with its present course in the mountainous area. The first deformations of the mountain growth may have temporarily interrupted its flow, as is made likely by the lacustrine deposits already referred to; and when the rise in the level of the waters of the lake overtook the uplift, probably at a time of slower mountain growth than that which first formed the lake, the point of overflow may have been many miles to one side of its previous drowned-out course. The moderate elevation of the eastern end of the range, where it connects with the Yampa plateau, may possibly have then been a little higher than a point farther west, where the overflow was consequently located. This is perhaps hardly as probable as the postulates involved in arguing a truly antecedent course for the river; but its impossibility is not as strictly proved as would be necessary before a definite conclusion as to the continuous persistence of an antecedent river could be finally accepted. Such continuity of action must be rare and should be rigorously demonstrated if possible.
It must, moreover, be remembered that Emmons10 is of the opinion that the Colorado river is not antecedent at all, but is superimposed on the eastern portion of the Uinta range from a course that it had chosen upon a sheet of horizontal sediments—the Wyoming conglomerate—which he supposes once stretched unconformably all over the previously deeply eroded surface of the uplifted range, where the cañon is now cut. He quotes facts of two kinds in evidence of this; first, the remnants of the Wyoming conglomerate still lie on ridges as high as those that enclose the river cañons; second, the Green and certain of its branches possess tortuous courses, out of accord with the structure of the range. It might be added that the wide open valley of Brown's park, in the middle of the range is best explained as the product of a pre-Wyoming cycle of erosion by rivers that were extinguished when the Wyoming beds were laid over the mountains. The strongest objection to Emmons' conclusion seems to be the great amount of erosion that it requires; erosion sufficient not only to remove the Wyoming conglomerate from nearly all its former overlap on the Uinta range, where it had buried and extinguished a pre-Wyoming drainage, but also to carry away a vast extension of the formation at the same height north of the range. It may be best to conclude that both antecedent and superimposed processes must be called on: for one must hesitate before admitting that the Wyoming beds stretched all across the country north and east of the Uinta range up to the height at which the remnants are now found on the range; it seems more likely that some part of the height of these remnants is due to a relatively local elevation. As far as this is the case, it gives reason for regarding the Green as an antecedent river; that is, antecedent to the local elevation of the Wyoming beds, but long posterior to the elevation of the Uinta range: but as the river now flows—according to Emmons' theory—on beds lying unconformably below those on which its course was chosen, it is for this reason to be classed as superimposed.