Chapter III: Part 3

With the changes of the physical geography clearly in mind, the rivers of Connecticut may now be examined in respect to their conditions of origin, the number of cycles through which they have lived, and the approach they have made to mature old age. But at the very outset a serious difficulty is encountered, for the geological structure of the state is nowhere well described, nor have topographic maps of all the districts yet been issued. Since the structural details are to some extent unknown it is unwise in many cases to attempt more than tentative conclusions. Several of the problems to be presented cannot be considered as settled. Considerable progress toward a final settlement will have been made, however, if the conditions of the problems are made clear, various hypotheses suggested, and the attention of workers in this field called to these questions.

_Early drainage._ Of the drainage of Connecticut during Jurassic and Cretaceous times very little can be said. It is not even known whether it was consequent upon the Jurassic tilting and faulting, or whether these deformations were so slow in their movement that the rivers persisted in spite of them. It may have been that the larger rivers were victorious, while the smaller were conquered and compelled to assume new consequent courses. Whatever was their origin there must have been abundant opportunities during the long erosion which resulted in the Cretaceous baselevel, and again in the period of revived and quickened degradation succeeding the post-Cretaceous uplift, for the streams to adjust themselves in a large degree to the geological structure. The contrast of hard and soft beds and the great elevation must have been potent factors in bringing to pass such a result. We expect to find the streams so far re-adjusted as to render improbable the discovery of their manner of origin.

_The Housatonic, a re-adjusted stream._ The best example of re-adjustment is found in the northwestern part of the state where the Housatonic and some of its branches follow well adjusted courses. From its headwaters, near Pittsfield, Mass., to New Milford, Conn., it has nearly all the way chosen its course along the Cambrian crystalline limestones in preference to the harder granites and gneisses on either side. The stratigraphical relationships of the limestone are not fully understood, but they seem to be deeply eroded anticlines and synclines, whose axes plunge north or south at various angles. The course of the river, if the drainage was consequent, was at first along the synclinal valleys, passing from one to another across the lowest points in the anticlinal ridge between them. But by a series of changes[43], resulting from the differential rates of erosion as hard or soft beds became exposed, the river previously to the Cretaceous baseleveling, seems to have re-adjusted its course to the softer limestones. However, there are several places where this conformity to structure does not seem to be the law; where the river departs from a limestone valley to flow for a time in the crystallines, only to return to the limestone again. The most marked instance of this is in the towns of Sharon and Cornwall, where the river leaves the limestone valley, which continues to the southwest, and flows for ten miles in a narrow gorge in the gneiss, only to again enter at its northern end a long narrow bed of limestone. The following seems to be the probable explanation. When the land stood at the elevation represented by the Cretaceous peneplain, these hard beds were below or but very slightly above baselevel, and were therefore undiscovered by the stream or had just begun to make themselves known late in the cycle. Had they been reached early in the cycle, when the stream was far above baselevel and presumably before many of its tributaries had been developed, and when it was therefore a smaller river, it is quite probable that further re-adjustments would have occurred, and the stream been led away from the hard rocks onto the softer beds to the west; but when they were reached the stream had cut so deeply and so nearly to baselevel that it was safe from capture. After the elevation of the peneplain the stream was revived and disclosed more and more of these hard beds, but was then, owing to the development and head-water growth of its tributaries, too important a river to be diverted by any rival. A river of this kind may be said to be “conformably superimposed” in distinction to one which is superimposed from an unconformable cover.

_Revived streams._ It is important to recognize the effect of the post-Cretaceous uplift upon the rivers at that time established. As the land was baseleveled and the velocity of the streams decreased, they lost in large degree their cutting power and sluggishly meandered more or less in broad flood-plains. During and for a period after the uplift, their cutting power was restored to them by virtue of their increased velocity and they excavated the deep narrow valleys which we find in the crystalline highlands. The upper course of the Housatonic is a good example of a river re-adjusted to the structure during one cycle, revived by uplift to a second cycle of erosion, and in places “conformably superimposed” upon structures from which it would have been led away in the ordinary course of re-adjustment. Its tributaries, the East Aspetuck, Still, Shepaug, and Pomeraug follow courses re-adjusted in one cycle and revived in a later uplift.

We can assert with the more confidence that such was the history of the upper Housatonic, because we find in other states, in regions whose history has been the same, similar examples of “conformably superimposed” and “revived” streams. The Musconetcong and Pequest, highland rivers of New Jersey, are streams “revived” from mature old age to vigorous youth and “conformably superimposed” upon saddles of gneiss between two limestone valleys.[44]

_Unconformable rivers._ In considering the course of the lower Housatonic we meet with some difficulty at the outset. In the southern part of the town of New Milford the river leaves the limestone belt which continues with some slight interruptions to the Hudson, and swings sharply into the crystalline plateau in a southeasterly course until it is joined by the Naugatuck, when their united waters flow south for a few miles to the sound. The course of the lower Connecticut is even more surprising. At Middletown it leaves the broad open Triassic sandstone lowland, and through a gorge enters the plateau, which has an average elevation of 600 to 700 feet. In this plateau of crystallines the river has sunk its valley nearly to sea-level. The slopes are steep compared to the lines in the sandstone lowland, and the contrast between the two parts of the river is one of the striking features of Connecticut scenery. Several theories may be framed to account for the curious behavior of these two rivers, but none of them are free from all difficulty.

_As a consequent river._ The lower Connecticut has been thought[45] to be a revived river, whose course was consequent upon the post-Triassic tilting and faulting. The faulted monocline seems to have had the shape of a half-boat, ends to the north and south, and one gunwale rising toward the west, the combined effect of the tilting and faulting being to swing the river to the southeast, where the keel of the boat was lowest. The probable existence of faults, with upthrow on the east, along the eastern margin of the Triassic rocks, is a difficulty in the way of the complete acceptance of this theory. Unfortunately too little is known about the structure of the western plateau to say whether the course of the lower Housatonic could be accounted for on such an hypothesis. On this theory the Connecticut would be consequent upon the Jurassic deformation, and revived by the post-Cretaceous uplift.

It may be suggested that the southeast courses are due to the tilting of the peneplain at the time of elevation, the plateau now being, as we have seen, much higher in the northwestern part of the state than elsewhere. But the acceptance of this theory necessitates a degree of smoothness and absence of even mild relief in the peneplain, which is hardly possible. The present average slope of the plateau is but a few feet per mile, and it seems incredible that so gentle a tilting could force rivers as large as these to take new courses. Besides, if the Housatonic and Connecticut were deflected, why were not the smaller streams--the Naugatuck and Quinnebaug--also given southeastern deflections? Clearly, this explanation is not the correct one.

_Superimposition._ It has been suggested that these courses may be inherited from a Cretaceous cover, which formerly stretched over Connecticut for a considerable distance, but of which no traces now remain in the state. On parts of Long Island the Cretaceous deposits are found, and it is not inherently impossible nor improbable that they once stretched far over the main land. In New Jersey[46] several lines of evidence seem to show that the Cretaceous beds formerly extended across the Triassic, probably to the margin of the highland plateau. The curious drainage of the Watchung Crescent is one evidence of this, but the other proofs are along entirely different lines, so that there is apparently good evidence that the Cretaceous beds extended twenty-five miles or more farther inland. If, in the time which has elapsed since the deposition of these beds, there has been erosion sufficient to strip them off from such a broad area in New Jersey, may they not, in Connecticut, under presumably similar conditions, have been equally eroded?

There is much which makes this hypothesis attractive, and, as the facts were first studied, it seemed the most likely one. It affords a good explanation, not only for the courses of the Housatonic and Connecticut, but also for other rivers along the sound. It seems, also, at first thought, to be well supported by analogy from New Jersey. But a closer study of the situation in that state reveals marked differences in the attendant circumstances. There the soft Triassic sandstone must have been worn down to a lowland early in the Cretaceous cycle, perhaps by the close of Jurassic time or thereabouts, while the harder crystallines retained a strong relief. The slight subsidence, which marked the beginning of marine Cretaceous in New Jersey, allowed the Cretaceous sea to transgress rapidly the baseleveled sandstones to the foot of the crystalline hills, but not to cover them to any extent. It is not probable that the crystallines in Connecticut had been brought nearer to baselevel than those in New Jersey at the time of the Cretaceous deposits. There is no evidence to show that the subsidence was greater in Connecticut than in New Jersey, and, therefore, from _a priori_ considerations, the conclusion would seem to follow that the subsidence, which permitted the Cretaceous sea to cover the Triassic sandstone area of New Jersey, was not sufficient to permit the sea to cover the then unsubdued crystalline hills of Connecticut. Although this hypothesis is not to be hastily thrown aside, for theoretical reasons, yet it would seem necessary to hold it very lightly, at least until some positive proof is found of the former existence of the Cretaceous or some later formation in that region. The first suggestion, that the lower Connecticut was a consequent river in the Cretaceous cycle and was revived by the post-Cretaceous uplift, is, at the present state of knowledge, the most probable.

_The Farmington._ The roundabout course of this river presents another interesting problem, which is not free from difficulties. From its source in Massachusetts it flows southeast across the crystallines to the village from which it takes its name, where it turns abruptly north along the Triassic sandstones for ten or twelve miles, when with another wide sweep it crosses the trap ridges at Tariffville by a deep gorge, and resumes its southeasterly course to the Connecticut. Of this latter part I will speak later, but now arise the questions, “what has been the history of this river,” and “why does it turn north at Farmington?”

_The Farmington in the Tertiary cycle._ A course more accordant with the structure would seem to be south along the Quinnipiac and Mill river valleys to the sound at New Haven. As has been said before (page 376), Prof. Dana has expressed the opinion that the gorge at Tariffville was occupied by the Farmington in Tertiary times, and that the Westfield river gap further north and the gorge of the Quinnipiac southwest of Meriden are also of earlier date than the glacial epoch. One reason has also been given why I differ from him in regard to the Quinnipiac and Tariffville gorges--they are narrower and steeper than those made in similar rocks during the Tertiary cycle. But more than this, the constructional topography, resulting from the tilting and faulting of the region, could not, it would seem, have caused the Farmington to take its present course. Even if it had taken this roundabout course during the baseleveling of the country, it must, since it would have had to cross three trap sheets, have been captured and led to the sea by the shorter and easier way along the sandstone area. The fact that the Connecticut probably persisted in its consequent course is no argument for similar conditions for the Farmington, because the latter is much the smaller stream, and so more easily captured. Nor could the river have been forced into this course during or after the post-Cretaceous uplift, for the land was then raised more at the north than at the south, and any changes from this cause would have been to confirm the river in its southward course. It is very probable, therefore, that in at least the latter part of the Tertiary cycle, the Farmington did not have its present course, but followed the open sandstone valley, along the course of the Quinnipiac and Mill rivers of to-day. The earlier history of this river is purely conjectural; one fact may shed a little light upon it, a fact which may indicate that this course was an adjusted one taken during Tertiary times.

_In pre-Tertiary times. Origin of Cook’s Gap._ A few miles southeast of where the river emerges from the crystallines, the trap ridge is cut by a deep wind notch--Cook’s Gap--through which the New York and New England Railroad passes west from New Britain. As was pointed out some time ago by Prof. Davis,[47] this is not a fault gap, because the alignment of the ridge is not broken, but it is probably an abandoned water gap, the head-waters of the stream which formerly occupied it having been abstracted by a rival, which did not have to cross a hard trap ridge. Perhaps this river was the ancestor of the present Farmington, and in that case its history would seem to have been as follows. A stream consequent upon the constructional topography after the faulting and tilting at the close of the Triassic, it had its upper course on the crystallines, its lower on the sandstones and buried trap sheets. In its old age it crossed by a shallow gap the trap sheet, which had been uncovered by erosion. In the second or Tertiary cycle it was simply a revived stream quickened to a new life by the post-Cretaceous uplift of the peneplain. This uplift gave opportunity to a rival stream, which did not have to cross the hard trap beds to intercept the waters of the old Farmington, and lead them out by a shorter, easier path, probably down the sandstone valley west of the trap ridge. The path across the trap was abandoned, and the notch became a wind gap; the river following its new course, until the incursion of the ice-sheet interrupted its normal development. This is of course almost entirely speculative. Cook’s Gap is best explained as an abandoned river gap; the Farmington is the nearest river of a size proportional to the size of the gap, and the hypothesis is a rational one. There is, however, no direct evidence that the Farmington once occupied Cook’s Gap.

_The Tariffville cut._ Before attempting to answer the second question, “why the river flows north at Farmington?” let us consider for a moment the history of the Tariffville cut. The river occupies a gorge whose sides are steep and talus covered, but which is not at all clogged with drift. There is naturally no room at or near the water level, even for the wagon road, place for which has been blasted near the top of the gorge. The profile of the gap shows a gentle ascent from the top of the gorge, up to the nearly level crest line of the ridge. That is to say, the recent gorge has been cut in the bottom of a sag in the ridge. We have already given our reasons for believing that the gorge here is much younger than the Westfield river gap; that it is a part of the work of the next cycle; that it is post-Tertiary. The sag, however, in the bottom of which the gorge is cut, is clearly of the earlier cycle. The bottom of the sag is much above the level to which the rivers had cut their valleys in the late Tertiary, and, therefore, it is certain that a river could not have occupied it at the close of that cycle. It was probably an abandoned water-gap whose stream had been captured in the same way and in the same cycle as the river, which formerly occupied Cook’s Gap.

The fact that the sag and gorge, although located very near a fault line, do not correspond to it, but are transverse and independent of it, is instructive and needs a moment’s attention. It seems probable that the stream consequent upon the faulted blocks would have flowed down the slope of the tilted block and then along the fault line at the foot of the fault cliff and would have held this course during the baseleveling of the country. When the area was baseleveled the stream must have swung from side to side in its broad flood plain, and thus departed from the fault line. When it was revived by the post-Cretaceous uplift, it was confined to the course it had unwittingly taken on the sandstones just above the hard ridge, and it was forced to cut down through the trap. Subsequently a rival, which did not have to work against this obstacle, abstracted its head waters and the gap was abandoned. The accompanying diagrams may make this easier to understand. Figure 3 is a cross-section of the faulted monocline, R showing the position of the river along the foot of the fault cliff. The line B L represents the surface of the country after baseleveling, the trap outcrops forming _low_ hills (much exaggerated in the diagram). Figure 4 shows the dislocated trap sheets, the fault line and the winding course of the river, which has abandoned the fault line except where it passes between the low trap hills. Here the country is at baselevel. Figure 5 represents the region after the elevation and resulting erosion. The trap ridges have become more pronounced, and have migrated eastward in the direction of the dip. The river has been slowly let down upon the northern one from the sandstone at point G and has there cut into the solid trap.

The transverse notch of Cook’s Gap, already described, was probably located in a somewhat similar manner, but the case is not so clear as at Tariffville.

_Gravel terraces of the Farmington._ A consideration of some facts concerning the height and slope of the terraces along this part of the river may give a clue to the answer to our question. One-half a mile east of Tariffville and east of the trap ridge, the highest terrace is 210 to 215 feet. Half a mile south of the same place but west of the ridge the height is 275 feet.[48] The top of the gorge at Tariffville is about 190 feet above the sea-level. It does not seem probable that these highest terraces were ever continuous over all the Farmington valley. But if they represent the level reached by the maximum flood accompanying the melting of the glacier, the great difference in their height on the two sides of the trap ridge, in connection with the other evidence already noted, gives strong reason for believing that the gorge as it exists to-day had not then been cut. A mile and a half east of Tariffville there is a lower terrace which is wide-spread. Its general height is about 190 feet, in places a little more. In this terrace the lower part of the Farmington has cut a trench 90 to 100 feet deep. The shape of the valley makes clear the fact that before this trench was cut the river flowed at about the 190 foot level, which is the height of the bottom of the sag at Tariffville. On the west side of the trap ridge there is also a more or less wide-spread terrace at about the same height. It seems very probable therefore that the river was raised to the level of the old sag in the trap ridge by the building of these terraces.

The present average southward slope of the highest terraces west of the trap ridge from Northampton, Mass., to Farmington, Conn., forty-four miles, is seven inches per mile,[49] and Professor Dana is inclined to believe that this is approximately the slope at the time the terraces were built. The character of the deposits shows that the current which formed these deposits flowed south. The present river, flowing north, falls twenty feet between Farmington and Tariffville, or 1⅔ feet per mile. The reversal of the river was probably determined by two factors. Near the village of Farmington, the waters of 200 square miles of territory are poured into the valley by the upper Farmington and its tributary, the Pequabuck. During the terrace building stage the great mass of débris contributed by these streams was deposited where the steep gradient of the highlands was exchanged for the gentle slope of the lowland. The main north and south valley was thus choked by the débris of its tributaries and a long stretch of comparatively still water extended north from Farmington, in which nearly horizontal deposits were made. South of Farmington the terrace deposits are much coarser than to the north, and the face of the terraces is much greater. It is not impossible that, as the deposits between Farmington and the Massachusetts state line approached nearer and nearer to horizontality, the waters of the upper Farmington began to divide, part flowing north and part south, the northward flowing portion finding an outlet at the sag at Tariffville. If this was the case, the terraces between Farmington and Tariffville must have had a slight slope to the north. Their present southward slope could readily be accounted for by the re-elevation of the land after the disappearance of the ice. This explanation rests upon the ability of the upper Farmington and the Pequabuck to have completely dammed the southward flowing current and turned it northward by the great mass of their deposits. If this was not the case, and there may be some doubt on the matter, the subsidence which accompanied the later stages of the ice-retreat is the other factor in the problem. It is estimated that an average depression of 1.25 feet[50] through the Connecticut valley would restore it to an altitude approximating that at the close of glaciation. It seems highly probable that these terrace-deposits were built before the maximum depression was reached. If this was the case, the depression would be efficient in reversing the Farmington, and this factor would supplement the first. It is impossible at present to say to what extent these two factors enter into the problem. That they are not mutually exclusive is evident, and that they are together quantitatively competent seems certain. Among the several hypotheses which have been considered, this seems the most probable, and in the light of the present evidence the most rational.

At first thought it might seem that if the Farmington was reversed by the differential subsidence of the land, the Connecticut ought to have suffered a similar fate, and since it did not, the explanation cannot apply to the Farmington. But the terraces of the Connecticut have a much greater southward slope than those on the smaller river, and the depression was not sufficient to reverse the stream. The conditions on the two sides of the trap ridge were not the same.

To sum up, then, the history of the Farmington seems to have been as follows: Its original consequent course was southeast on the crystallines and perhaps across the trap ridge at Cook’s Gap, from which course it was turned in the Tertiary cycle by a stream whose course was approximately that of the Mill river of to-day. The damming of the valley by the deposits of the Upper Farmington, and the depression in the north accompanying the ice retreat, reversed the river at Farmington, and it took a new course on the terrace deposits, escaping by the sag in the trap at Tariffville into the Connecticut valley.

_The Quinnipiac._ The gorge of the Quinnipiac, already mentioned several times, seems closely comparable to the gorge of the Farmington. It is not of the Tertiary cycle, and is best referred to the inter-glacial or post-glacial epochs. We should expect the Quinnipiac, instead of turning eastward, to cut through this sandstone ridge, to continue southward along the Mill river valley. Dana[51] finds from the heights of the terraces that the drainage of the terrace-building period was not along the Quinnipiac, but along the Mill river, and concludes that the Quinnipiac gorge was obstructed by an ice-dam. I have not as yet studied it enough in detail to do more than express the opinion here reiterated, that this gorge is later than the cycle in which the open sandstone lowland on either side of it was excavated. Its topographic form would put it in the cycle which has been called post-Tertiary.

_The Scantic._ In the Scantic we have a typical example of a river whose lower course is manifestly of a later date than the upper. In this it is similar to several of our Atlantic rivers, notably those of North Carolina, whose upper courses are on the Piedmont crystallines, being probably established previous to the Cretaceous baseleveling, and whose lower courses stretch seaward over the unconsolidated Tertiary deposits of the coastal plain. As the plain of these recent deposits emerged from the sea, the rivers were forced to extend their courses eastward over the freshly raised surface to the retreating shore line. The Scantic river has a similar history. Its upper course in southern Massachusetts on the crystalline plateau is a remnant of the drainage established before Cretaceous baseleveling and revived by the subsequent uplift. How much that revived drainage has been modified by drift can only be determined by long field study, but the topography, as read from the topographical atlas would seem to indicate, that it has not been much. The valleys were undoubtedly clogged with drift, and the drainage area may be somewhat modified, but the drainage seems to be substantially along the same lines.

Just below the village of Hampden, the Scantic leaves the plateau and enters the Triassic lowland. From this point to its mouth at the Connecticut, opposite Windsor, a distance of twenty miles, it flows nearly all the way through the gravel, sand and clay deposits of the period of ice-retreat. The topography of the lower course of the river is entirely characteristic of a stream which has recently attacked a level, easily eroded district. The inter-stream surfaces are broad and flat; the descent to the stream bed which is sunk seventy or eighty feet below the general surface is exceedingly steep. These two lines, that of the inter-stream surface and that of the valley side, meet at a sharp angle. The side streams are as yet very short, and have cut narrow gorges down to the main river. Tributary to them are deep side ravines, whose bottoms ascend rapidly to the inter-stream surface, the whole making a dendritic system of drainage in its earlier stages. The Scantic, having reached base level in its lower course, has developed a narrow flood-plain.

Manifestly this part of the river valley is of much later date than the upper part. If, during the period of ice-retreat, the lower Connecticut valley was an estuary, the Scantic was a much shorter river than at present. Its mouth could not have been far from the point where now it leave the crystallines, but as the land was elevated and estuarine conditions gave place to fluviatile, the Scantic lengthened “mouthward,” consequent upon the minor inequalities of the newly made beds. The effect would be substantially the same if the terraces were built by great valley floods, as Dana supposes. In pre-glacial times this river, in common with several other rivers rising on the crystallines and flowing into the Connecticut, had courses of various lengths over the Triassic sandstones, but these old valleys are lost entirely, the later trenches in the terrace deposits being altogether independent of them.

_Other examples._ The lower Hockanum, Farmington, Park, and the entire length of many short streams are similar to the lower Scantic, and originated under similar conditions. Stony Brook, a little stream north of Windsor Locks, presents the same features, but with this variation: It is superimposed through a thin layer of drift upon the sandstone, into which it has cut a deep, picturesque gorge. The Hockanum and Farmington are also “locally superimposed” in a few places. The Connecticut, also, north of Middletown, although following its pre-glacial valley, has departed in numerous places from its former bed, and has cut down through the valley-filling onto ledges of rock beneath. The water-power at Enfield, Conn., and at Turner’s Falls and Bellows Falls, Mass., is the result of this superimposed position.

_Abandoned gaps._ Many abandoned water-gaps must exist among the hills of the state. Cook’s Gap, through which the New York and New England Railroad crosses the trap ridge, three miles west of New Britain, has already been discussed. It must not be confounded with the majority of the other gaps in the trap ridge, which are oblique, break the alignment of the ridge, and are due to faults.

The New York and New England Railroad in ascending to the eastern plateau passes through Bolton Notch, a few miles east of Manchester. This notch, also, is an abandoned river bed but, as it seems, abandoned at a later date and for another reason than that assigned for Cook’s Gap. The drift is very heavy in this region, and the most probable explanation is that the post-glacial streams do not altogether follow pre-glacial valleys. This gap, used by turnpike and railroad, testifies of another and older drainage system.

That in this brief article all the problems connected with the Connecticut rivers have been solved, or even noted, is not to be expected. It is hoped, however, that the work done may prove a help to further study of the same regions, and that the tentative conclusions advanced may be substantiated by further investigation.

HENRY B. KUMMEL.

FOOTNOTES

[33] The author desires to express his obligation to Professor
W. M. Davis for aid in the preparation of this article. It
was first written under his direction and with the help of
his suggestions when the author was in the graduate school of
Harvard University. Prof. Davis is not responsible, however,
for the statement of the views herein advanced, although in
general it is believed that he is in accord with them.

[34] Amer. Jour. Sci. 3d ser., vol. xxxvii, 1889, p. 423. Bull.
Geol. Soc. Amer., vol. ii, p. 545.

[35] The rough diagrams accompanying this paper may aid the
reader who is unacquainted with the details of the region
under discussion. The abbreviations on the above figure are
as follows: C. The Connecticut. Cr. Pl. Crystalline plateau
(the shaded area). F. The Farmington. H. Hartford. Ho. The
Housatonic. Lm. Limestone area. M. Meriden. Mi. Mill River. Mt.
Middletown. N. The Naugatuck. N. H. New Haven. No. The Norwalk.
Q. The Quinnipiac. Qg. The Quinnebaug. S. The Scantic. Sa. The
Saugatuck. T. Tariffville. Th. The Thames. The unshaded area
is the Triassic sandstone lowland, and the blackened areas
represent the ridges of the faulted trap sheets.

[36] Am. Jour. of Sci., 3d ser., vol. xxxvii, p. 430.

[37] Bulletin of Geol. Soc. of Amer., vol. ii, p. 554.

[38] It is not desired to affirm that these periods of erosion and
elevation began and ended promptly with the beginning or end
of a period. The time statements must be considered as only
approximate.

[39] An exaggerated idea must not be had of the steepness and
narrowness of these crystalline valleys. The valley of the
Farmington, five miles up from where it opens into the Triassic
sandstone, is 400 to 500 feet deep, and a mile and a half wide
at the top. The Connecticut valley, just below Middletown, is
about 400 feet deep and two miles wide at the top. These are
fair representatives of the valleys in the crystalline rocks in
the central part of the state.

[40] Amer. Jour. of Sci., vol. x, 3d ser., 1875, p. 506.

[41] McGee. Amer. Jour. Sci., 3d ser., vol. xxxv, p. 376.

[42] Davis and Wood, Geographic Development of Northern New Jersey,
pp. 413, 414.

[43] “Rivers and Valleys of Pennsylvania,” Davis, W. M., published
in The National Geographic Magazine, in 1889.

[44] Davis, W. M., “Geographic Development of Northern New Jersey,”
p. 397–8.

[45] Davis, W. M., Amer. Jour. of Sci., 3d ser. vol. xxxvii., 1889,
p. 432.

[46] Geog. Devel. of Northern New Jersey, p. 404 et seq. Proc. Bos.
Soc. Nat. Hist. Also Rivers of Northern New Jersey, p. 11 et
seq. National Geographic Magazine, vol. ii, p. 93.

[47] Faults in the Triassic Formation near Meriden, Conn. Bulletin
of the Mus. Comp. Zoöl. Harvard Univ. vol. xvi. No. 4, p. 82.

[48] J. D. Dana, Amer. Jour. Sci. 3d. ser., vol. xv, p. 506.

[49] J. D. Dana, Amer. Jour. of Sci. 3d ser. vol. xxv, p 446.

[50] J. D. Dana, Amer. Jour. of Sci., 3d ser., vol. xxiii, p. 198.

[51] J. D. Dana. Amer. Jour. Sci., 3d ser., vol. xxv, p. 441.

_STUDIES FOR STUDENTS._

GEOLOGICAL HISTORY OF THE LAURENTIAN BASIN.

The study of the Pleistocene history of the basin drained by the St. Lawrence has been fragmentary and is still far from being complete. There is a lack of agreement in the interpretation of observations already made, due in part to the comparatively limited portion of the field examined even by those who have given the subject most attention, and in part to lack of uniformity in the standards of comparison used. It is with the hope of assisting in reaching more harmonious results that attention is here invited to methods of study.

In the present treatment of the subject it may be advantageously subdivided, and the facts and hypotheses relating to each division separately considered. Of the divisions that may be suggested the following seem the most important:

1. Character of the sub-morainal or hard-rock topography in the Laurentian basin.

2. Origin of the basin.

3. Sedimentary deposits.

4. Shore markings left by former water-bodies.

5. Fossils in ancient sediments, shore ridges, terraces, etc.

6. Fauna of the present lakes.

7. Changes in elevations of the land.

8. Former outlets.

9. Probable effects of an ice sheet on drainage.

10. Probable effects of a subsidence which would make the basin an arm of the sea.

1. _Character of the hard-rock topography._ In order to learn the character of the Laurentian basin it is necessary to examine the rock surface beneath the general covering of glacial débris and stratified sediments which partially fill it. To do this, those areas in which rock in place forms the surface require to be mapped and their elevations noted; the records of wells and other excavations which pass through the superficial deposits should also be obtained and the character of the underlying rock ascertained, as far as is practicable. When sufficient data of this nature shall have been recorded, a contour map of the basin can be drawn that will reveal the shape of the depression with which the student has to deal. The depth of the present lakes plus an estimated thickness of clay and morainal material covering their bottoms, will probably furnish the only means of sketching contours over the deeper portions of the basin. Even an approximately accurate map of this character cannot be constructed for a long time to come, but every advance towards it will serve to make the problems to be studied more and more definite.

Something of the form of the rock-basin is already known and several deep channels in its borders, now filled with drift, have been discovered. The courses of buried channels connecting the basins of some of the present lakes have also been approximately determined. It is not necessary at this time to refer specifically to the discoveries that have been made, but it may be stated that enough is known to assure us that the basin is a depression in solid rock, the bottom of which is below sea level.

2. _Origin of the basin._ The rocks in which the Laurentian basin is situated are, with the exception of the Lake Superior region, nearly horizontal and belong almost wholly to the Paleozoic. The basin is essentially a depression in undisturbed strata, and all who have considered its origin seem agreed that it has been formed by excavation. A vast mass of horizontal strata has been removed, leaving an irregular rim of undisturbed rocks on all sides. The form of the depression is now obscured by drift; the deeper portions contain stratified sediments which have been deposited within it and it has been warped somewhat by orographic movement.

The manner in which the excavation was formed has been explained principally in two ways. One hypothesis is that it owes its origin to a time of subaërial denudation preceding the Glacial epoch, during which a valley, or series of valleys, was worn out by stream erosion; and that the depression thus produced has been but slightly modified by ice action. The closing of the ancient valley has been referred to orographic movements and to the filling of its outlet by glacial débris. Another hypothesis is to the effect that the excavation is mainly due to ice erosion during the Glacial epoch, without special reference to previous topographic relief. A warping of the earth’s crust so as to produce a true orographic basin does not seem to require consideration, for the same reason as already stated, that the rocks in which the basin lies have been but little disturbed from their original horizontal position. Future study of the region must determine which of the two hypotheses outlined above best suits the facts; or if each hypothesis has something in its favor, what combination of the two may be accepted as the final explanation.

It is a suggestive fact in connection with the first of these hypotheses, that the youngest rocks in the region antedating the Pleistocene belong to the Carboniferous. This seems to show that the land has not been submerged since at least the close of the Paleozoic. If not a region of sedimentation during this vast interval, it must have been subjected to erosion. The erosion of an ancient land surface might result in the production of topographic forms of diverse character, depending on its altitude, on the length of time it was exposed to atmospheric agencies during various stages of elevation, and on climatic and other conditions. The study of topographic forms is now sufficiently advanced to enable one to predict somewhat definitely what features would appear under certain conditions. We also know the characteristics of topographic forms due to glacial erosion. It seems evident, therefore, that a knowledge of the hard-rock topography in the Laurentian basin, would enable one to draw definite conclusions in reference to the part that ice and water each had in shaping the forms now found there.

The conclusion that the region under consideration has been glaciated is well established; it remains, therefore, to determine what topographic forms, if any, due to pre-glacial stream erosion can be recognized. As an example of this kind of evidence desired, attention may be directed to the northward facing rock escarpments which follow the southern shores of lakes Erie and Ontario for a large part of their courses and at varying distances up to several miles. These escarpments are composed of the edges of nearly horizontal strata, mostly of Paleozoic limestone, and their bases are buried beneath glacial débris and stratified clays so deeply that in some instances, at least, they do not reveal half of their actual height. These escarpments not only have Pleistocene deposits banked against them, but their faces and summits are polished and grooved, showing how stubbornly they resisted the invasion of the ice which impinged against them from the north. South of lake Ontario especially, the trend of the escarpment referred to is directly athwart the course of the ancient glaciers. The entire history of these escarpments cannot be discussed here, as my desire is simply to call attention to the fact that they existed before the Glacial epoch, and are relics of a strongly accented pre-glacial topography. They are within the southern border of the Laurentian basin, and hence afford means of determining, in part, what was the form of that basin before it was modified by ice action. Other similar escarpments exist in the northern and western portions of the same great basin, and as this study progresses it is to be expected that still other features of the pre-glacial land will be revealed. It is perhaps too early to decide what were the special topographic forms which gave character and expression to the St. Lawrence basin before the ice invasion, but the Erie and Ontario escarpments and some other similar features now recognized, suggest that in Tertiary times it resembled the present condition of the upper portion of the Mississippi valley, where bold, rock escarpments border wide stream-worn depressions.

Deep drift-filled channels are known to cut across the Erie and Ontario escarpments. These seem to have been formed by streams tributary to the main drainage line to the north. If this conclusion is well founded, a study of the hard-rock topography should reveal other similar channels and finally indicate a well matured drainage system. If even the broader and stronger features of the pre-glacial surface can be determined, then the modifications due to glacial abrasion will become conspicuous, and the amount that glaciers have broadened and deepened the basin be determinable.

A study of the lithological character of the drift south of the present lakes should show, at least in a rough way, what portion of it was derived from the waste of rocks within the Laurentian basin. This inquiry has already been undertaken by at least two geologists, and estimates of the quantity of material removed from the basins of lakes Michigan and Erie respectively, have been made. This method may be extended so as to embrace a larger area, or some special portion of the great depression best suited for the trial may be selected. If the material removed from the basin or re-distributed within it by glacial action can be shown to be approximately equivalent in volume to the amount of rock excavated in order to form the depression, it would evidently tend to support the hypothesis of glacial erosion. If, on the contrary, the amount of débris derived from the basin should fall far short of what would be requisite to refill it, no very definite conclusion would seem to be indicated unless account could also be taken of the fine material carried away by glacial streams.

As the case stands at present it appears that there is evidence of a pre-glacial valley or series of valleys as has been claimed by several geologists, and that all but the boldest features of the old topography have been obliterated or greatly modified by glacial erosion followed by glacial and other sedimentation. Additional observations should show somewhat definitely the amount of work assignable to particular portions of the history. How far the results of subaërial and of glacial erosion have been modified by other agencies, more especially by orographic movements, has also to be considered.

If the St. Lawrence basin shall be shown to be largely the result of subaërial erosion it will follow, unless it is found that the deeper portions are the result of glacial action, that the land at the time the streams did their work, must have stood higher than at present, for the reason that the bottom of the depression is now below sea level. Some idea of the smallest amount of elevation necessitated by this hypothesis might be obtained by estimating the gradients of the ancient streams and the amount of elevation required to bring the bottom of the depression up to sea level.

A study of the hard-rock topography in the valleys of the Ottawa and St. Lawrence and of the present submerged Atlantic border of the continent would also be instructive in this connection. The strict correlation of the topographic history of the interior and of the continent’s margin may be difficult, but as the two regions are directly connected, valuable results should follow their comparative study.

The hypothesis that the Laurentian basin is due largely to pre-glacial erosion, necessitates also that the ancient system of river valleys should have been closed in some way so as to form the basins of the present and of former lakes. The closing has been referred to several agencies. An unequal subsidence following the period of stream erosion has been postulated. During the Glacial epoch the entire region was ice-covered and only glacial streams of one kind or another could have existed. On the retreat of the ice, when portions of the basin were abandoned, the drainage is supposed to have been obstructed by the ice itself, as will be noticed below. When the glaciers melted, a vast sheet of débris was left which in many instances filled or obstructed previous drainage lines. Old channels, now deeply buried, have been reported to connect the basins of the various existing lakes, as has already been mentioned, but no similar channel which could have afforded an escape for the waters of the entire basin has been discovered. Here again an acquaintance with the hard-rock topography should give assistance and indicate either that such a channel existed or that orographic movements have taken place which have obstructed the former drainage system. The glacial hypothesis assumes that the basin was excavated mainly by glacial abrasion and does not require that the land should be either higher or lower than at present. The study in this direction merges with that of the general glaciation of the northeastern part of the continent, and cannot be treated at this time.

3. _Sediments._--Regularly stratified deposits of clay and sand occur along many portions of the borders of the present Laurentian lakes. These were clearly formed in water bodies which formerly existed within the Laurentian basin, and which in certain directions, at least, were of wider extent than the present lakes. The areas occupied by these deposits have been partially mapped, but much remains to be done in this direction. Fresh sections, particularly of the stratified clays, are exposed from time to time by artificial excavations, in which much of their history may be learned. Not only should records be made of the facts noted at special excavations, but the extent and character of the stratified deposits in one area should be determined and compared with similar data obtained in other areas. For example: the clays covering large tracts on the west shore of Lake Michigan and on the southern and western border of Lake Superior are of a red color, while other areas bordering Lake Erie are covered with blue clay. These two deposits have been supposed to have been laid down at the same time and in the same lake. The definite correlation of the clays of these two areas by direct contact, however, does not seem to have been made, and there are reasons for thinking that they may be quite distinct and that they originated in separate lakes.

The outer limits of the deposits of clay and sand here referred to are known in some instances to be determined by ancient beaches and terraces. Such associations of deep and of shallow water deposits require special attention, as the study of one may assist in interpreting the significance of the other. The fine, evenly stratified clays frequently contain large angular bowlders, which appear to have been dropped from floating ice and to show an intimate connection between the ancient lakes and neighboring glaciers. The possibility, however, of the bowlders having been brought into the ancient water bodies by rivers, or floated outwards from the shore by lake ice, should also be considered. Huge angular masses of limestone have been reported as occurring in southern Michigan especially, which rest on superficial deposits and are thought to have been carried northward by lake ice. The relations of these masses to well defined shore lines have never been determined. If it should be found that they are above all former shores, it is evident that they must have been carried by some other agency than the one mentioned.

A chemical examination of the clays, or of their contained water, may indicate whether or not the basin was formerly in direct communication with the ocean. Analyses of the clays of the Champlain valley and of the similar clays in the Ontario and Erie basins might indicate whether or not they were deposited under similar conditions.

4. _Shore records._ Beaches and terraces have been studied at many localities about the borders of the present lakes, sometimes at a distance of more than twenty miles from their margins and at various elevations up to several hundred feet above their surfaces. In some instances these ancient shore records have been followed continuously for scores of miles. The tracing and mapping of individual beaches is one of the most important parts of the study here outlined, and is already well advanced. Confusion has unfortunately arisen, however, for the reason that topographic features, due to shore action, have, in some instances, been confounded with somewhat similar features due to other causes. Moraines and gravel ridges, formed by glacial streams, have been mistaken for beach ridges, and terraces of various origin have not been clearly discriminated.

In order not to be led astray by topographic forms that simulate shore phenomena, the student should examine the shores of existing lakes and learn what records are there being made. In the study of topography, “the present is the key to the past,” just as definitely as in any other branch of geology. The topography of lake shores has already received attention from one skilled in reading geological history in the relief of the land[52] and the study of existing shores in the light of what has already been done in that direction should enable even the beginner to avoid falling into serious error in interpreting ancient records of the same nature.

To be able to discriminate clearly between shore features and somewhat similar glacial phenomena, it is necessary to become familiar also with the topography of glacial deposits. Fortunately in this study also a guide is at hand[53] which, in connection with field observations, should soon train the eye to discriminate the shapes assumed by moraines and the deposits of glacial streams from all other topographic forms.

In examining the records of former lakes it will soon be observed that, in many instances, where the highest of a series of ancient beaches is obscure and indefinite, the topographic expression above and below a certain horizon, and also the character of the surface material, whether of the nature of lacustral clays and sands or of glacial débris, residual clay, etc., above and below the same level, are significant, and enable one to map the outline of a former water body with considerable accuracy.

In tracing ancient beaches and terraces, their forms and internal structure need to be recorded, so that the fact of their being true shore records may be made plain to others. The elevations of various well-defined points throughout the extent of an ancient shore should be carefully measured, for, as will be noticed below, although originally horizontal, they have, in many instances, been elevated or depressed, owing to broad general movements of the earth’s crust. The continuous tracing of individual shore lines for as great a distance as possible is highly desirable, especially in a wooded country, in order to be positive as to which ridge or terrace measurements of elevation relate, and also for the purpose of observing the nature of the changes that occur when a shore line gives place to other records. For example: some of the ancient beach ridges about the west end of Lake Erie have been found to be continuations of moraines. In other instances shore ridges have been reported to end indefinitely and to be replaced at the same general horizon by glacial records of various character. The correct interpretation of phenomena of this nature is especially important.