By William North Rice, Ph. D., Professor of Geology in Wesleyan University
The Metamorphic Rocks
THE AREA of Middlesex county may be divided geologically into two very well marked portions, which require separate description. The boundary between the two extends from a point in the north line of Portland, about a mile east of the Connecticut River, in a direction approximately south-southwest, to a point not far from the middle of the south line of Durham. The boundary crosses the river a short distance west of the range of hills called the White Rocks in Middletown.
The district east of this boundary consists entirely of highly crystalline rocks. The predominant rock is a micaceous rock, varying from a gneiss to a mica schist, according to the proportion of the mica to the quartz and feldspar, and the consequently varying degree of development of the schistose structure. Sometimes the gneiss becomes granitoid, almost losing its stratification. Sometimes the mica and feldspar disappear, so that the rock becomes a quartz-rock. A stratum of this quartz-rock forms the summit of the ridge called Great Hill, or Cobalt Mountain, on the boundary between Portland and Chatham. The extreme hardness of this rock, enabling it so effectively to resist the erosive action of water and ice, is doubtless the reason for the existence of the ridge, the softer rocks around it having been worn away. In other localities the mica gives place to hornblende, so that the rock becomes a hornblende schist. Such a hornblende schist is the rock in which the ores of cobalt and nickel are contained, which were formerly worked at Chatham. The granitoid gneisses of this formation afford good building stones, and have been quarried in various places for this purpose. The piers of the bridge over the Connecticut at Middletown are built of gneiss from Collins Hill, in Portland. A gneiss from Haddam Neck has been used in the building of some of the fortifications in New York Harbor. The more schistose strata afford an excellent material for curb-stones, and have been quarried for this purpose at various localities in Haddam and elsewhere. These schists have been used to some extent for flag-stones, but the surfaces of the layers are not usually smooth enough to adapt them well for that purpose.
The rocks above described have been entitled metamorphic rocks, and there can be no reasonable doubt that that name expresses truly their nature and history. They were originally deposited as sedimentary rocks, derived from the disintegration of older rocks. Subsequently, by the joint action of heat and moisture, they suffered a molecular re-arrangement by which they assumed their present crystalline texture. They may once have been fossiliferous; but whatever fossils they may formerly have contained, have been entirely obliterated by the process of metamorphism. At the time of their metamorphism the strata were subjected to extreme dislocation, being folded and broken in the most complicated ways. The evidence of these disturbances is seen in the extremely varying dips throughout the region. In some places, as at Arnold’s curb-stone quarry at Haddam, the strata are nearly vertical.
The region of metamorphic rocks in which the larger part of Middlesex county is included, occupies the greater part of New England, and extends southwestward, along the course of the Appalachian system of mountains, nearly the whole length of the eastern border of the United States. In New England this belt of metamorphic rocks lies immediately upon the coast, but southwardly it is separated from the sea by a strip of Tertiary and Quaternary deposits.
It was formerly the belief of geologists that all highly crystalline rocks must be of the greatest antiquity, and such rocks were formerly called primitive, or primary, with reference to that belief. It is, however, now well established, that rocks of the most highly crystalline character have been produced at various periods, so that the crystalline character of the rocks of the Appalachian region is in itself no proof of their great antiquity. All that is certainly known of the age of a large part of this belt of metamorphic rock, is that it is not later than the Carboniferous Period; the last great epoch of dislocation, with its usual accompaniment of metamorphism, in the Appalachian region, having been at the close of the Carboniferous. The opinion held by some geologists, that all these crystalline rocks of the Appalachian region are of Archaean age, is certainly not proved, and is probably not true. The lithological character of strata is of very little value as evidence of age. Fossils afford the only reliable criterion of age, and the age of a non-fossiliferous stratum can be determined only by reference to fossiliferous strata which it overlies or underlies. It is not at all unlikely that rocks of various ages, Archaean and Paleozoic, may be included in this region of metamorphic rock. The only way by which the problem of the age of these rocks can be solved, is by searching for the patches of rock, here and there, in which the metamorphism has been less complete than usual, and in which, therefore, traces of fossils have been preserved (as at Bernardston, Massachusetts, where Upper Silurian or Devonian fossils have been discovered), and then carefully tracing the relations of these patches of fossiliferous rock to the underlying and overlying masses of rock in which the fossils have been completely obliterated. The patches of fossiliferous rock appear to be so few and small, and the dislocations of the strata have been so complex, that it is doubtful whether it will ever be practicable to solve the problem completely; but confessed ignorance is better than imaginary knowledge.
Associated with these metamorphic rocks are numerous veins. Probably at the time of the dislocation and metamorphism of the strata numerous fissures were made, which were filled with crystalline material deposited from the hot waters which had held it in solution. These veins are sometimes very irregular, and cut across the strata in every direction; but often they coincide closely for considerable distances in dip and strike with the strata themselves. Some of the veins are very thin, resulting from the filling of mere cracks. Others are many yards in perpendicular thickness. Most of the larger veins are of a coarse granite. This granite has been quarried at numerous localities in Middletown, Portland, and Chatham, for the sake of the feldspar, which is used in the manufacture of porcelain. The mica in these granites occurs often in large sheets, but they are too irregular to have any commercial value. These granite veins are the chief repository of the minerals which have rendered the towns of Middletown, Haddam, Portland, and Chatham famous among mineralogists. The feldspar (chiefly orthoclase, but in part albite) often occurs crystallized; and the crystals are sometimes of very large size, occasionally two feet or more in dimensions. The mica (muscovite) is often in beautiful crystals. The quartz, though generally of a smoky gray, is sometimes of a fine rose color. The accessory minerals, occurring more or less abundantly in these granites, are very numerous. The following is probably not a complete list of the minerals which have been recognized in these granite veins: sphalerite, chrysoberyl, rhodonite, beryl, garnet, epidote, iolite (usually altered to fahlunite), lepidolite, oligoclase, tourmaline (black, green, and red), columbite, samarskite, apatite, monazite, torbernite, autunite. Besides these granite veins, there are numerous quartz veins, though the latter are generally of small size. In the southeastern part of Middletown is a large vein containing argentiferous galenite, associated with pyrite, chalcopyrite, and sphalerite, in a gangue consisting chiefly of quartz, with some calcite and fluorite. This vein was extensively worked for lead in colonial and Revolutionary times, and has been worked more recently for silver; but the workings have been abandoned.
The Connecticut Valley Sandstone
The northwestern portion of the county, including the towns of Cromwell and Middlefield, the larger part of Middletown and Durham, and a small part of Portland, is occupied by a group of rocks very different from the preceding. In the district now under consideration the predominant rock is a red sandstone. The rock varies much in texture, sometimes becoming coarser and passing into a conglomerate, sometimes becoming finer and passing into a shale. The color is usually a decidedly reddish brown, owing to the presence of ferric oxide, but some of the layers are gray rather than red. Here and there the percolation of waters charged with decomposing organic matter has effected a local deoxidation of the iron, and has thus produced spots and streaks of a greenish color. The sandstone proper (in distinction from the more shaly strata) is thick-bedded and massive, and can be quarried in large blocks of very uniform texture. It makes an excellent building stone, and has been quarried at various localities in the Connecticut Valley and elsewhere. Especially famous are the quarries at Portland, which have been worked for many years, and are still being worked on a most extensive scale. Great quantities of the stone are sent every year to New York and other cities, besides what is used in the immediate vicinity. Besides the red sandstone (including the red shale and conglomerate), two other rocks occur in small quantity in this formation. At several localities in Middletown, Middlefield, and Durham (the localities all lying nearly in one north and south line), may be observed outcrops of a black, highly carbonaceous shale, containing thin seams and small nodules of bituminous coal. Associated closely with the black shale is a stratum of dark gray impure limestone. A characteristic locality for these rocks is the little gorge of Laurel Brook, near the Middletown reservoir, in Middlefield. This black shale has unhappily proved a delusion and a snare to some of the farmers in the vicinity, who have expended considerable money in boring in search of coal. It is perfectly safe to say that no coal in workable quantities is to be found. A boring prosecuted with sufficient persistence will pass through various alternations of sandstone, conglomerate, and shale, with perhaps an occasional sheet of trap, and will eventually reach metamorphic rocks like those which have been already described. A very simple consideration will make this evident even to the non-geological reader. The strata of the sandstone formation, in most parts of the Connecticut Valley, dip pretty uniformly to the east, the average inclination being not far from twenty degrees. It is therefore evident that a stratum which is underground at any particular locality is likely to come to the surface further west. If a Durham farmer wishes to know what rocks underlie his farm, it will be much cheaper for him to take a walk through Wallingford and Cheshire, and examine the surface rocks, than to employ an adventurer with a diamond drill.
The formation now under consideration occupies a strip of territory extending from New Haven nearly to the northern boundary of Massachusetts, and varying from four miles to somewhat more than twenty miles in width. From the northern boundary of Massachusetts as far down as Middletown the course of the Connecticut River lies in this formation, but below Middletown the river has carved a channel for itself through the metamorphic rocks. There are several other basins at intervals along the Atlantic coast occupied by formations similar to that of the Connecticut Valley. One is in Nova Scotia; another, the most extensive, extends from the Palisades on the Hudson southwestward across New Jersey and Pennsylvania. Other basins occur in Virginia and North Carolina. All these localities present about the same variety of rocks. The rocks (with the exception of the limestone and coal) have evidently been derived from the disintegration of the older rocks outside of the basin, the strata of conglomerate often containing pebbles whose source can be recognized with some degree of definiteness. The beds appear to have been deposited in the brackish waters of shallow estuaries. The troughs in which these estuaries lay were probably formed at the time of the folding and dislocation of the older metamorphic rocks. The question is often asked whether the Connecticut River ever emptied into the Sound at New Haven. The old Connecticut estuary (as we have seen) communicated with the Sound at New Haven. But it is probable that, at the close of the period of the deposition of the sandstone and associated rocks, the region southwest of Middletown was so much elevated, that the waters of the upper part of the valley found a lower path to the eastward, and accordingly commenced cutting the valley in which they now flow through the metamorphic rocks. It is probable, therefore, that the Connecticut River, ever since it became a true river, has occupied substantially its present valley.
The rocks of the formation under consideration contain a variety of fossils, which serve as memorials of the life of the period in which the rocks were deposited. The black shales contain impressions of cycads and ferns, and more abundant remains of ganoid fishes. The cycads are a group of plants exceedingly abundant in the earlier part of the Mesozoic age, but at present very scantily represented. A familiar example is the beautiful plant commonly (though incorrectly) called the sago-palm, which is not infrequently seen in conservatories. The cycads superficially resemble palms and tree-ferns, but they are really much more closely related to the pines and other coniferous trees. The ganoid fishes are also a group now nearly extinct, though very abundant throughout the latter part of Paleozoic and the earlier part of Mesozoic times. One of the few modern examples of ganoid fishes is seen in the bony pike, or gar-fish, of the rivers of the Mississippi valley. The ganoids are generally, though not always, covered with an armor of bony scales or scutes; and the internal skeleton is generally less perfectly developed than in ordinary fishes. In the fossil specimens of ganoids, accordingly, little or nothing is usually preserved excepting the scales.
The red sandstones and shales have afforded few fossils except casts of trunks of trees and foot-prints of animals. The tracks are very abundant in certain layers, and are in great variety. Some of them indicate animals of very large size. One of the largest was a quadruped whose hind feet made a four-toed track eighteen inches in length. It is believed to have been an amphibian of the order of labyrinthodonts — an order now entirely extinct. The majority of the tracks are three-toed, and were apparently made by animals which at least ordinarily moved as bipeds, supporting themselves exclusively on their posterior limbs. Three-toed tracks of a biped naturally suggest to the mind the idea of a bird, and the tracks are popularly known as bird-tracks. Some eminent geologists have coincided with the popular opinion. It seems probable, however, that that opinion is erroneous. While the tracks are acknowledged to resemble exactly those of birds, it is now well known that there was in the Mesozoic age another order of animals to which the tracks might be referred — animals, in fact, whose tracks would be undistinguishable from those of birds. The animals referred to are the dinosaurs — an order of reptiles remarkable for their approximation to birds in many parts of the skeleton, and particularly in the structure of the pelvis and hind limb. The dinosaurs were not clothed with feathers, and did not have the anterior limbs developed as wings. But many of them were completely bipedal in their mode of progression, and their three-toed tracks would exactly resemble those of birds. So far as the appearance of the tracks goes, they might be referred with equal likelihood to birds or to dinosaurs. Two reasons, however, render the dinosaurian character of the animals much the more probable. First, it is certain that dinosaurs were in existence at the time of the deposition of the sandstones, while it is very doubtful whether there were any birds. It is still in doubt whether the age of these sandstones is Triassic or Jurassic. Now dinosaurs are known to have existed throughout these two periods, while the earliest skeletons of birds have been found in the beds of the later part of the Jurassic. Secondly, the colossal size of some of these tracks is strongly against their avian character; for all the unquestionable birds of the Mesozoic age were comparatively small animals, while among the dinosaurs were included the largest land animals that have ever lived. Of course, any determination of the affinities of the animals which made the tracks, must be regarded as merely provisional, in the absence of actual skeletons. But it is altogether probable that the three-toed tracks were those of dinosaurs.
No mammalian remains have been found in the sandstones of the Connecticut Valley; but a portion of a skeleton found in the corresponding formation in North Carolina has shown that at that period small marsupials, allied to the modern opossums, were already in existence.
As has already been remarked, the age of the Connecticut Valley sandstone and the associated rocks is either Triassic or Jurassic. They are certainly newer than the Carboniferous, for they overlie unconformably a system of folded rocks in which the Carboniferous is included. It is equally certain that these rocks are older than the Cretaceous, of which well characterized deposits are found in New Jersey. It is, however, at present impossible to fix the age more definitely. The characteristic fossils of the respective subdivisions of the Triassic and Jurassic periods, as recognized in other parts of the world, are chiefly remains of marine animals, the fossiliferous rocks being mostly marine. The Connecticut Valley sandstones and associated rocks contain no marine fossils whatsoever — scarcely any fossils, in fact, except fresh water fishes, impressions of land plants, and tracks of land animals. Hence it has been impossible to correlate these rocks exactly with any particular group of strata in other parts of the world. Lithologically the rocks much resemble the New Red Sandstone of England, and the Bunter Sandstein of Germany, which are of Triassic age. Lithological resemblance, however, in rocks of widely separated areas, is no reliable proof of contemporaneity.
The Trap Rocks
Closely connected with the Connecticut Valley sandstones are remarkable developments of igneous rock. The typical rock in the trap dikes and sheets is a dolerite or diabase, consisting chiefly of pyroxene and labradorite, but containing also more or less of magnetite and some other minerals. The presence of magnetite gives a remarkable magnetic property to much of the rock. If a compass be moved about upon a surface of the trap rock, it will often be found that at different points within an area of a square yard the needle will point in every possible direction. Even hand specimens of the rock often exhibit strikingly this magnetic property. Some of the trap rock has become hydrated by the penetration of water and aqueous vapor into the mass, more or less of the pyroxene being converted into chlorite. The hydrous traps are often amygdaloidal, the cavities being filled with datolite, prehnite, calcite, and other crystalline minerals. Fine specimens of datolite in the cavities of an amygdaloid were obtained from a cutting near Westfield, in the building of the Berlin Branch Railroad. The trap rocks of the Connecticut Valley often show, more or less distinctly, the columnar structure, resulting from contraction in cooling, which is so characteristic of igneous rocks. Very perfect examples of such columns may be seen at Mount Holyoke, in Massachusetts, and at Rabbit Rock, near New Haven. No very good examples have been observed within the limits of Middlesex county. The trap has been used very extensively for macadamizing roads, and to some extent as a building stone. For the former purpose it is exceedingly well adapted.
The trap has been spoken of as an igneous rock, and there can be no doubt that it came up in a melted state from the interior of the earth. The sandstone in many places shows, along the line of contact with the trap, the most unmistakable effects of heat, being sometimes strongly indurated, sometimes rendered vesicular and almost scoriaceous by the conversion into steam of the moisture present in the sandstone, sometimes impregnated with crystalline minerals. A remarkably fine example of this local metamorphism of the sandstone may be seen in Middlefield, at Rice’s Cut on the Air Line Railroad, about a mile northeast of Reed’s Gap.
The trap is sometimes seen to form unquestionable dikes cutting across the sandstone strata; but it more commonly occurs in sheets which coincide in dip and strike with the underlying and overlying sandstones. The latter mode of occurrence admits of two explanations. The trap may have been poured out on the surface as a lava overflow after the deposition of the underlying sandstone, and the overlying sandstone may have been subsequently deposited upon the cooled and hardened surface of the trap. Or, after the deposition of both the underlying and the overlying sandstone, some strain in the crust of the globe may have split them apart, forming a crack parallel with the planes of stratification, into which flowed the molten rock. In briefer technical language, the trap in intercalated sheets may be either contemporaneous or intrusive. A pretty good criterion to distinguish the two cases is afforded by the contact with the overlying sandstone, where that contact can be observed. For it is obvious that, in the case of contemporaneous trap, only the underlying sandstone should show the characteristic effects of heat; while, in the case of intrusive trap, the underlying and overlying sandstones should show those effects in about equal degree. Unfortunately, contacts between the trap and the overlying sandstone are seldom accessible, the overlying sandstone having been removed by erosion from the surface of the trap hills, and the lines of contact on lower ground being generally covered by Quaternary deposits and by vegetation. The most probable conclusion from the somewhat scanty evidence thus far collected is that some of the trap sheets are contemporaneous, and some of them intrusive. The trap was probably erupted, not all at once, but at intervals through a period of time commencing before, and continuing after, the close of the period of the deposition of the sandstones.
The intercalated sheets of trap are much harder than the associated sandstones, and this fact has produced a characteristic effect upon the topography of the district. The Connecticut Valley, since its elevation above the sea level, has suffered a great amount of erosion by the action of water and ice. The trap, owing to its greater hardness, has offered much greater resistance to erosion than the comparatively soft sandstones and shales. Hence, the trap sheets generally reveal themselves, in the topography of the district, as north-and-south ridges.
These ridges, which are remarkably uniform in character, present generally an almost precipitous face to the west; while the eastward slope is gentle, corresponding nearly with the dip of the strata. The summit of the ridge is formed by the sheet of trap, while the baked strata of the underlying sandstone may often be seen beneath the trap on the steep west face. The most extensive trap ridge of the Connecticut Valley is the one which extends from the Hanging Hills of Meriden to Mount Holyoke, in Massachusetts. A considerable ridge lies just on the western boundary of Middlesex county, extending from Paug Mountain, in the southwest corner of Durham, to Higby Mountain, on the western border of Middletown. Similar trap ridges are found in the sandstone basin of New Jersey; but in those the steep face is eastward, the dip of the strata being westward. The Palisades on the Hudson afford a classical example of such a ridge.
While the development of igneous rock in connection with the Connecticut Valley sandstones is so extensive, there is remarkably little exhibition of igneous rock in the metamorphic region which occupies the larger part of Middlesex county. There is, however, one remarkable dike of trap, which extends almost continuously across the metamorphic region of Connecticut, from Branford on the south, to Stafford on the north, and continues thence northward into Massachusetts. This dike crosses the towns of Killingworth, Haddam, and Chatham, in Middlesex county.
The Quaternary
No rocks of Cretaceous or Tertiary age occur in Middlesex county. The only geological phenomena, therefore, which remain for consideration, are those relating to the Quaternary age. In the earliest epoch of the Quaternary — the Glacial epoch — as is now well known, all the territory of the northeastern United States and Canada was covered by a vast glacier — a glacier such as those now existing in Greenland and in the Antarctic. The terminal moraine marking the southern boundary of the ice-sheet has been traced on Long Island, and westward across New Jersey, Pennsylvania, and Ohio. Middlesex county shows the same characteristic evidences of glacial action which are found in other glaciated regions. These evidences are twofold. One class of signs is seen in the peculiar forms and surfaces of rocks, resulting from glacial erosion — the rounded forms of roches moutonnées and the smoothed, polished and striated surfaces. The markings are often well preserved on the harder rocks, as the quartzite of Cobalt Mountain and the trap rocks. They may be seen even on the softer rocks, when a fresh surface is laid bare by the removal of the superficial drift; but of course on soft rocks the marks are speedily effaced by weathering. The other characteristic evidence of glacial action is the ubiquitous deposit of drift — the irregularly stratified or entirely unstratified superficial mass of clay, sand, and gravel, often containing large boulders. Sometimes isolated boulders are perched on the summits of hills composed of an entirely different kind of rock.
The melting of the continental glacier in the Champlain epoch produced, of course, great floods in all the rivers. There is no more interesting chapter in the geological history of Middlesex county than that which relates to the post-glacial flood in the Connecticut River.
Every one who has observed, at all attentively, the lower Connecticut (or the lower, non-torrential portion of almost any river), has learned to recognize the alluvial meadows or flood-plains by which the river is bordered. They are ordinarily dry, but in times of flood are covered by the water; and their elevation above the ordinary water level is an indication of the height of the floods. Now the valley of the Connecticut is bordered, in many places, by strips of plain elevated far above the modern flood-plains, but exhibiting the same characteristically level surface, and bearing indubitable testimony to the height of the water in the post-glacial floods. These ancient flood-plains, elevated above the modern flood-plains, are called terraces. The highest terrace, marking the maximum height of the flood, increases in altitude as we go northward. At the Shore Line Railroad bridge, at Saybrook, the highest terrace is 36 feet above mean tide level; at Essex, 58 feet; at Chester, 78 feet; at Goodspeeds, 94 feet; at Higganum, 125 feet; at Maromas, 152 feet; at Middletown, 195 feet; at Hartford, 210 feet; at Springfield, 240 feet.
A part of this enormous height of water is undoubtedly due to the subsidence of the land. Strata containing marine shells of recent species, now elevated above the sea level, prove that in the Champlain epoch the northern part of North America stood at a lower level than at present, and that the amount of the subsidence increased progressively northward. On the shore of Long Island Sound the amount of subsidence below the present level was about twenty-five feet; at Montreal, it was five hundred feet; and, in the Arctic regions, it was more than a thousand feet. As the amount of this subsidence can be indicated only by marine formations, we have no exact measure of the subsidence in districts remote from the coast. In the Connecticut Valley the subsidence undoubtedly increased northward; but whether at a uniform or at a varying rate we know not. Probably the amount of the subsidence at Middletown was not far from fifty feet, and at Springfield not far from one hundred feet.
Making allowance for the subsidence of the land, we should still have a flood at Middletown one hundred and forty feet or more above mean tide level. That amount of elevation may be assumed to be due to the increase in the volume of water by the melting of the glacier. The Connecticut River, at the maximum of the post-glacial flood, must have been indeed a colossal stream. From Hartford to Springfield and beyond, it averaged fifteen miles in width. Only a part of that vast flood found its way to the sea through the present channel of the lower Connecticut. In at least three places — the first north of Mount Tom, the second between Springfield and Westfield, Massachusetts, the third between Hartford and Meriden — the Connecticut overflowed westward into the valley now occupied in various parts by the Farmington, Quinnipiac, and Mill Rivers. A part of the waters of the Connecticut resumed, therefore, in the post-glacial flood, the position of the old Triassic estuary, and reached the Sound at New Haven.
The subsidence of the post-glacial floods, and the re-elevation of the land which had sunk below its present level, brought the region substantially into its present condition, and formed the conclusion of its geological history.
Note
In such an article as the foregoing, elaborate bibliographical references seem unnecessary. It may be well, however, to mention the principal authorities on this subject. Percival’s “Geology of Connecticut” gives a very full and accurate account of the distribution of the different rocks, and from his work the map (see p. 1), illustrating the present article has been taken. The main authority on the Quaternary Geology is Prof. J. D. Dana. His papers on the subject have been published in the “American Journal of Science” and the “Transactions of the Connecticut Academy.” Important papers on the trap rocks have been published by W. M. Davis and B. K. Emerson, in the “Bulletin of the Museum of Comparative Zoology,” and in the “American Journal of Science.” Information on many points bearing on the geology of our county may be obtained from Dana’s “Manual of Geology,” Dana’s “System of Mineralogy,” and Hitchcock’s “Geology of Massachusetts.”
Source
Whittemore, Henry, History of Middlesex county, Connecticut, with biographical sketches of its prominent men, New York : J. B. Beers & Co., 1884.
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