Popular Science Monthly/Volume 40/December 1891/The Lost Volcanoes of Connecticut
|THE LOST VOLCANOES OF CONNECTICUT.|
By Prof. WILLIAM MORRIS DAVIS.
SEVERAL years ago, while walking down the lower Connecticut valley with a party of students, we chanced upon a curious ledge of rock surmounting a low ridge by the road that runs from Berlin to Meriden, about half-way between Hartford and New Haven. A scramble up the slope through a bushy growth of young trees led to the foot of the ledge—a thick bed of gray-greenish rock, not in layers like limestone or sandstone, not crystalline like granite or gneiss, but of a loose, structureless texture, here and there carrying roughly rounded blocks of a dense, dark rock which we knew to be an old lava, from its resemblance to the rocks ejected from modern volcanoes. Although a ledge of this kind is not of ordinary occurrence, its features were so well marked that there could be little doubt of its nature and origin; it was a bed of volcanic ashes, interspersed with blocks or bombs of lava that must have been thrown from some neighboring vent long ago in the ancient time when the rocks of the valley were made. The ash-bed lay upon a series of muddy sandstones that
had evidently been formed under water, for they were deposited in layers, just as sand and mud are now when they are washed into a pond; and to all appearances the eruption of the ashes and bombs had taken place during the accumulation of the sandstones. The ashes had fallen into the water and settled down gently on the soft, sandy mud at the bottom; one of the dense lava blocks was seen to have indented itself in the sandy layers, bending them down on either side of it, just as if it had been an early product of the eruption, arriving here before the ashes, plunging down after its lofty flight through the air, and sinking into the mud at the bottom of the water. In this it recalls the reptilian footprints that have made the sandstones of the valley famous. The old reptiles walked over the mud-flats and left their heavy prints on the surface to be buried under the next layer of mud; the lava block fell into the soft sandy mud and made its print, where it still lies. Long may it rest undisturbed! A poor indication of it is presented in Fig. 1, copied from a photograph by a friend in New Britain, Conn. All this was much more evident and more easily interpreted than those who try to learn geology from books are disposed to believe. Indeed, one of the students with me exclaimed: "This is the most realistic thing I ever saw; I had no idea that it could be so plainly made out." The ledge has been visited by hundreds of persons from Meriden and the surrounding towns, and a well-beaten path now leads up to it from the road. I have taken parties of students there every summer since then, and hope to do my share toward beating down that path for many years to come. But although the meaning of the ash-bed is plain enough, there is a question suggested by it that is not so easily answered. Where is the volcano from which the ashes and bombs were blown out?
The same question has arisen in other countries. For example, in central France, in Auvergne, there are chalky beds that were once a soft white mud, and in these lie bombs of lava, bending down the layers on either side; manifestly again the result of a bombardment from some adjacent volcano. In the same district there are beds of ashes and flows of lava, all indicating volcanic outbursts in their vicinity; but when the question is there asked—Where are the volcanoes from which these products came?—it is easily answered, for many volcanic cones still stand up in plain sight near by; the lava-flows may be traced up to their bases, the craters are still visible at the summits, and although no record exists of their eruptions, it is manifest that at a relatively recent prehistoric period these cones exhibited a brisk activity. I walked over them a dozen years ago; they make a delightful strolling and sketching ground, and I remember well lunching with a shepherd on one of their sunny slopes, and answering his questions about distant America (Fig. 2).
We may look in vain for volcanic cones in the neighborhood of our Meriden ash-bed bluff. There are hills and ridges all around, but nowhere can we see the smooth and characteristic concave slopes of a volcanic cone. To the south, there are several symmetrically rounded hills, but they are convex, not concave, on the side, and an examination of the road-cuts made in their slopes shows them to be of anything but volcanic origin. They are "drumlins," hills of rubbish that were left there and given their even form when the whole of New England was buried in a deep sheet of moving ice, as Greenland is now (Fig. 3.) They give no clew to the source of the bombs and ashes. If we go west or east of the ash-bed ledge, there are high ridges, six or seven hundred feet above the valley, with gentle slopes on the east, and bold, rocky cliffs, descending to a long stony talus on the west. The one next east of us is Mount Lamentation; it may be well seen eastward from the railroad between Hartford and Meriden while the train is passing a pond. The ash-bed ledge can be seen at the same time under the southern end of Lamentation, but it is not a conspicuous object a mile away. Lamentation and its fellows are not the least like volcanoes, and yet they confirm the belief that volcanoes must have once existed hereabouts; for these high ridges are of lava, the edges of great tilted lava-flows that were poured out at intervals during the deposition of hundreds and thousands of feet of sandstones. Our ash-bluff is indeed only a part of one of these parallel lava ridges; when traced north and south lava may be found lying on the ash-bed. Lamentation is higher, because its lava-flow is much thicker than that in the ash-bed ridge, and therefore has not been worn down so low. On the back of these flows, at one point and another, may be seen the slaggy, bubbly surface of the lava, like that poured out of Vesuvius or any other modern volcano; but these ancient lavas have been deeply buried in sands and muds, and tilted up and worn down, during the evolution of their present form. There is a quarry at Meriden where one lava-sheet may be seen lying directly upon the scoriaceous, ropy surface of an older one. Evidently, the region has witnessed volcanic action, as the ash-bed implied. Perhaps we fail to recognize the cone at the point of outburst because it has been partly worn away. There are many volcanic regions where the eruptive action is not so recent as in Auvergne, and where the cones are consequently somewhat out of repair; deep gulleys furrow their sides and destroy their symmetrical form. Something of this may, indeed, be seen in Auvergne, for the volcanoes there are not all of the same age. Some are sadly wasted, and are recognized as volcanoes only because their remnants of lava-flows and ash-beds all slope away from a central lava-mass, which marks the place of the vent. It is chiefly in this way that the Madeira Islands differ from the Azores; the latter possess many cones of regular form, but the older volcanoes on the former are deeply dissected; so much so that it is difficult to reconstruct the original cones from which the present rugged hills and ridges have been carved out. The same contrast may be seen on a grand scale in the Hawaiian Islands, as described by Dana. The most southeastern of the group is the most recent. It is the largest, and is in the best repair; not a volcanic cone of the usual steep-sided form, indeed, but of long, smooth, gentle slopes, because its lavas were too liquid when erupted to stand on steep slopes such as are formed by heaps of ashes and cinders. Other islands farther to the northwest in the same group are mere wrecks; their edges are cut off by the waves, forming great sea-cliffs, their slopes are scored by deep ravines and canons, and their once even profiles are replaced now by sharply notched outlines. Yet nothing of even those angular forms is to be found about Meriden. If the absence of the cone from which the ashes came is due to wearing away, it must truly have been worn out.
There is, however, another method of disposing of volcanoes that has been practiced in Italy. The cone has either been blown to pieces and scattered by violent eruptions, or has been allowed to sink down by the withdrawal of lava from beneath its foundations. In either case, a great basin, often holding a lake, marks the site of the lost cone. There are several lakes of this kind in Italy—Trasimeno, Bolsena, Bracciano, and others; Sumatra possesses some huge basins of the same pattern; but there are no such basins in Connecticut. There are no lakes at all near Meriden, and the lakes in the back country are only old valleys obstructed by glacial drift.
There is an account of an old volcanic region out in New Mexico that may, perhaps, guide our search. In the district of the Zuñi plateaus, Dutton describes numerous relatively small isolated buttes or sharply conical hills, steeper sided than volcanic cones, of a different profile, and without the crater at the top. They consist of dense lava, not in layers spread out from a central vent upon the surrounding surface, but in a solid mass with columnar structure; and at their bases it is sometimes possible to see that they are inclosed on all sides by the country rock. It is believed that these buttes are nothing more than lava-plugs, frozen solid in the pipes up through which the lava rose at the time of eruption from its deep source to the surface where it overflowed; but that the time of eruption is so long ago that the cones and all the surface outpourings are worn away, and only the stumps of the plugs remain to tell the tale. Fig. 6 attempts to show the early and late forms, one below the other. Structures of the same kind are known in the Black Hills, in Scotland, and elsewhere. Perhaps this hint will help us in understanding Connecticut.
There is one thing about the ash-bed and lava-sheets in Connecticut that is certainly favorable to the suggestion given by the Zuñi buttes. The lava-sheets are not now level, as they undoubtedly were when they were poured out; but all the series of sandstones, ash-beds, lava-sheets, and the rest have been lifted up together on the western side of the valley, so that they slant down or dip to the eastward at a moderate angle. Standing on the bluff of the ash-bed, it is easy to trace its edge north and south, and to perceive that it is continued slanting underground on the east, and to imagine that it was once continued upward into the air on the west; for on this side the uplifting exposed it to the patient, persistent attack of the weather, by which in the course of ages it may have been greatly worn away. In the same way, other lava-ridges in the neighborhood, such as Mount Lamentation and the beautiful Hanging Hills, are simply the worn edges of lava-sheets that still plunge underground eastward, and that once rose high into the air westward.
It follows from this new understanding that if the vent, from which the ashes were blown and the lavas poured, lay to the east of the ash-bed ridge, it must be still underground and not discoverable at present. It may be revealed to distant future ages, but to us it is buried. But if the vent lay to the west of the ridge, it may be discovered, not as the cone for which we looked at first, but as a pipe or neck of lava. Indeed, it must in this case be discoverable, for the lava and ashes must somewhere have risen from a deep subterranean reservoir, through the country rocks, up to the surface; and if their point of escape lie west of the ash-bed ridge, it must be in sight somewhere. We may not now hope to find the cone where the lavas rose and burst out through the body of water in which the muddy sandstones were accumulating; we can not now hope to discover the crater from which the ashes and bombs were scattered far and wide, and from whose flanks the lava-floods were poured over the low grounds around about it; but we may hope to find a knob or hill where the lava-pipe has been worn down to an undetermined depth beneath the surface on which its cone was built.
This seems to be the fact. Some ten miles southwest of Meriden lie the rugged Blue Hills, one of which is known as Mount Carmel. These may be seen to the west of Wallingford, on the railroad between New Haven and Hartford, or east of Mount Carmel station on the New Haven and Northampton Railroad. They consist of a network of thick necks and dikes of lava; not of loose texture like the ashes, not slaggy like the backs of the lava-sheets, but dense and solid, as if they had been driven there under great pressure. Mount Carmel and its fellows have not the simple outline of the Zuñi buttes; they are of irregular form, corresponding to their complicated structure, as if a compound fracture had been opened to give passage to the ascending lavas, or as if repeated eruptions had forced their way surfaceward at this point, every one increasing the size and complexity of the lava pipes and cracks. There is no other vent of the kind to be found so near to the ash-bed and lava ridges of the Meriden district as Mount Carmel; and while it is entirely possible that a vent may exist at a less distance on the east, concealed beneath the overlying strata in that direction, it is at least permissible and plausible to regard Mount Carmel and the Blue Hills as the source of the ashes and bombs and lava-sheets over by Meriden and up and down the valley.
The Blue Hills have rough slopes to climb, but the view from their tops and the suggestion of past history that one gains there pay for the labor of the scramble. It is easily understood that the rocks are lavas and that they have ascended through the surrounding rocks from some deep source. It is manifest that they did not rise from below when the surface of the country had its present form, for in that case they must have flowed down into the low lands on all sides, and they must have had the slaggy and scoriaceous texture characteristic of surface lavas. One can not doubt that when the lavas of the Blue Hills were placed in their present relation to their surroundings they were deep underground, inclosed by rocky walls on all sides, and heavily pressed upon by the mass above. They forced their way upward from some deep reservoir of molten lava because the push upon them was even greater than the heavy resistance from above. They reached the surface at last, hundreds or thousands of feet above the present summit of the Blue Hills, and there burst out in true volcanic eruption, forming a conical island in the great estuary in which the valley sandstones were formed. We can hardly suppose that they built a grand cone, like Fujiyama, in Japan, twelve thousand feet above sea-level; perhaps they only formed a small mound, like the little temporary volcanic island that appeared in the middle Mediterranean in 1831, called Graham Island, Isle Julia, and Nerita, by its various discoverers. But the Blue Hills were undoubtedly in eruption more than once. This may be safely inferred from the complex network of their pipes and dikes, as well as from the repeated occurrence of lava flows among the series of bedded rocks in the Meriden district. In this respect, as in others, the Blue Hills were like volcanoes of our times. Some of their outpourings were more plentiful than others. Mount Lamentation is part of a lava-sheet whose thickness must be from three to four hundred feet, and whose total original area must have been at least two or three hundred square miles. But the other sheets are not so massive as this one; they indicate eruptions of less energy. While the eruptions were going on there must have been a great scurrying about of the old reptiles whose tracks are found on the sandstone beds at various points in the valley; perhaps the patient searcher may some day find one of their skeletons buried under the ashes of an eruption, just as the old Pompeians have been found buried under the mud and ashes from the outburst of Vesuvius that destroyed their city. During the intervals of rest between the eruptions a luxuriant growth of tree-ferns may have clothed the slopes of the volcanic island, for leaves of cycads are found in the neighboring beds of shales. And yet all this is gone. The volcanoes are only things of the imagination. The Blue Hills mark the conduits through which they were fed with lavas, but the cones are lost in the empty air above; only the deep roots of the structure are now preserved for us.
Perhaps the accompanying diagrams may aid the reader in gaining a fuller understanding of the geological history of the region. They are drawn from a wooden model that was prepared for exhibition before the Geological Society of America at its last winter meeting in Washington. The first (Fig. 7) represents a block of the Triassic formation, lying horizontally on its deep crystalline foundation, the whole representing a cube of about ten miles on a side, and hence showing a hundred square miles of upper sur—face. The oblique lines across the top need not be considered for the present. The horizontal lines around the sides near the top are the interbedded lava-sheets, and all these, with the sandstones and shales, lie on the upturned eroded edges of the foundation of old crystalline rocks. The bedded rocks were spread out in the old sinking estuary in deposits of great volume, aggregating ten or twelve thousand feet in thickness at least, but always in shallow water, for they frequently show cross-bedding and ripple marks, and sometimes mud-cracks and rain-drops, and occasionally even foot-prints of various kinds. The famous Hitchcock collection, in the Amherst College Museum, illustrates all these features in great variety. During the period of accumulation of the bedded rocks there were at least three epochs of considerable volcanic activity. About half of the total thickness of the strata had been deposited when the first outburst took place, and this is the one that yielded the ashes and bombs at Meriden. Its lava-flows spread many miles north and south, but gained only a moderate measure of thickness, generally not more than a hundred feet. These correspond to the bed marked A in Fig. 8, which represents a magnified view of a corner of the block seen in Fig. 7. When this first volcanic disturbance was over, the accumulation of sandstones went on again, the sands were washed in from the shores of the estuary and crept out over the back of the lava-sheet; the finer sediments settled down into the irregular crevices in the surface of the flow, even filling little half-open vesicles. A microscopic examination of specimens from these contacts of lava and overlying sandstones brings back vividly the condition of their deposition. Loose fragments of the lava, carried a little way by the waves and more or less water-worn, were mixed with the sands for a few feet above the lava, but they were soon all buried. Then things went on for a long time about as before the eruption. The supply of sediments seems to have become finer after a while, for a bed of black shale is found, with numerous impressions of fossil fishes and plants, one of the few traceable fossiliferous layers of the entire formation. Then came more barren sandy shales again. It is impossible to measure the time of this quiet work in years, but after three or four hundred feet of strata had been formed, another outburst of lava (M) took place, and on a greater scale than the first. The lava-sheet formed by this eruption is three or four hundred feet thick—thick enough to have in all probability filled the shallow estuary wherever it ran, transforming it into a level lava plain, like the plain of the Shoshone River of to-day Bat the depression of the estuary trough continued; if the lava surface was at first above water level, it was soon submerged and buried in sands and mud, repeating all the significant phenomena of contact that have been mentioned above. Then came another long period of quiet, broken by a third lava outpouring (P); and after that, still more sandstones and shales, until aqueous and igneous rocks had accumulated to a thickness of perhaps two miles. At some time during this long history a sheet of lava was driven in or intruded between the sandstones near the bottom of the formation (marked I in Fig. 8); it is easily known to be an intrusion by the dense texture of its upper surface, and by the occasional branches that rise from it into the overlying beds, and by various other features in which it differs distinctly from the overflow sheets or extrusions. But it need not be further considered now.
In order to exhibit these relations of the igneous rocks to the stratified deposits in a clearer manner, the model is constructed so as to open on a diagonal section ( as in Fig. 9), and disclose the
pipe or chimney up through which the lavas rose from their deep source. The volcanic cones, presumably formed at the surface where the chimney opened at the three times of eruption, are here placed in their proper positions in the series of stratified deposits; but even the topmost cone is supposed to have been entirely buried by gradual submergence and by the accumulation of sands and muds upon it. The intrusive sheet is shown near the bottom of the stratified series. The whole series may then be named as follows. First, a moderate thickness of bottom sandstones, often conglomeratic; then, the intrusive sheet; next, the great series of lower sandstones and shales, also sometimes conglomeratic; then, the three extrusive sheets, with their intervening sandstones and shales. The first of the extrusions will be called the anterior sheet, the middle one is the main sheet, the third is the(for reasons that will appear more clearly further on), and they are separated by the anterior and posterior shales respectively. On the top of all come the upper sandstones and shales. The whole series is probably two miles thick, as already stated.
We may imagine in a general way that in time the estuary was filled with the detritus that was washed into it, and thus transformed into a lowland plain, like that of the Po, between the Alps and the Apennines; or like the plain of California, between the Sierra Nevada and the coast range. If it was not ultimately filled up so as to form n land area, it was at least a subaqueous plain of very even and level surface. The deeper layers of the formation may have sagged a little toward the middle of the estuary on account of the progressive depression that the region had suffered during the accumulation of the entire mass, but their departure from horizontality was moderate. Yet at present the whole series, with but trifling exceptions, inclines at an angle of twelve, fifteen, or twenty degrees to the eastward. Evidently a serious disturbance has affected the original attitude of the beds. The eastward slant or dip of the series might be imitated by tilting the model over bodily, so that its upper surface should be inclined to the east; but this fails to represent the dislocations by which the mass is known to be traversed. The model was therefore made in several parts, each of which could be tilted independently of its neighbors, as shown in Fig. 10, the observer looking southeast. It is here made clear that while the dip of the beds is to the eastward, the course of the fractures by which they are dislocated is northeastward; this relation prevailing in a very constant manner in the region of the Meriden ash-bed. The blocks into which the mass is thus divided, five of which are shown in the model, have been moved by moderate amounts on one another; the movement varies from a few feet up to two thousand. This is called faulting, and its effect in this case is manifestly to break up the continuous surface of the inclined plane that would have been formed by simple tilting, and produce a discontinuous surface, with steps from one part to another. If we may judge by the angle at which the beds lie, the elevated edges of these dislocated blocks must have once risen high into the air, producing mountainous ridges of no insignificant relief. Yet at present nothing of this ancient constructional form is apparent. The tilting and faulting were both done so long ago that no part of the original surface remains. It has all been worn away. The best evidence of the antiquity of the dislocations is found in another State.
Down in New Jersey, the corresponding red sandstone formation is unconformably overlain by the Cretaceous strata of the coastal plain, proving that the sandstones were not only tilted but deeply eroded before the Cretaceous beds were laid upon them. The formations in New Jersey and Connecticut are so much alike that we may safely conclude that the period of dislocation was the same in both; hence we shall suppose that the Meriden sandstones and lava-sheets were tilted and faulted into the position illustrated in Fig. 10 during the interval between Triassic and Cretaceous time—that is, in the Jurassic period. From that time to now their history is concerned chiefly with the erosion by which their original constructional inclined planes have been reduced to their present surface of varied topography.
There is good reason to think that the history of the erosion is a double one, comprehending first a longer cycle, and second a shorter cycle of time. During the first cycle, the great relief of the uptilted beds was reduced to a lowland of denudation, a surface of a moderate relief close to the base-level of erosion, an almost plane surface, a "peneplain"—the evidence of this being found in the even uplands of the crystalline plateaus which now inclose the Triassic valley on the east and west. No explanation for the evenness of these plateaus can be found save the one which regards them as having been reduced from some greater mass by a long-continued process of erosion, at a time when the region stood somewhat lower than now—low enough to place the present plateau-like uplands close to sea-level; and the sandstones, shales, and lava-sheets between the two plateaus undoubtedly suffered the same denudation. This is indicated in Fig. 11, in which all the upper part of the model as shown in Fig. 10 has been removed; the obliquely beveled surface of the beds now represents the lowland of denudation, or peneplain, to which they were reduced. The effect of the oblique faulting is now rendered apparent by the dislocations in the belts of the different outcrops. The main sheet of lava, for example, is seen in each of the blocks into which the formation is divided by the faults; so is the belt of shales lying under it, and so on with every member of the series. Indeed, the reader must perceive that it is only because the actual facts of observation are thus arranged that the existence of the faults is inferred. Most of the faults are of moderate displacement; but just north of Meriden there is one whose movement amounted to two thousand feet; it cuts off the northern end of the main lava-sheet in Lamentation and the southern end of the same in the Hanging Hills group of lava-ridges. In following along the line between these two dislocated portions of the sheet, every ridge formed by the more resistant sandstones or conglomerates is cut off in a most systematic manner, precisely according to the pattern shown in the beveled surface of the model. The railroad crosses this great fault about a mile above Meriden, but the traveler will see nothing there to indicate the dislocation; its constructional effects have all been worn out.
But the region is not now a plain. It is a rolling lowland with occasional ridges formed on the resistant edges of the lava-sheets. The cause of this is found in a moderate uplift of the whole country since it was reduced to a peneplain, introducing the second chapter in the history of its erosion. After this uplift a new cycle of erosive work was undertaken, and we now find ourselves at a moderate advance in this division of the valley's history. The softer beds have wasted away into lowlands, the harder ones still stand up as ridges. In the adjoining crystalline areas on the east and west, where most of the rocks are hard, the erosion of this cycle has made comparatively little progress; there the valleys are narrow and the interstream spaces are rolling uplands. In the Triassic belt, where most of the rocks are soft, the erosion of the same cycle has made much greater progress and reduced the area nearly to a second peneplain, except where the edges of the hard lava-sheets still hold up their crest lines to give some indication of the elevation that the whole surface once had. Here the valleys are broad and the interstream highlands are reduced to narrow ridges. This stage is indicated for our ten-mile-square area in Fig. 12, produced by removing from the previous form of the model certain little slips by which it is transformed from a peneplain to a broken country. It is practically in this stage that the region now stands. It has suffered certain slight changes by glaciation, and by small variations of level; but its main features are explained in accordance with the scheme thus presented; and from this general sketch we may return to the more especial consideration of the lost volcanoes.
Fig. 13 presents a partial dissection of the tilted and faulted mass, in order to show the relation of the peneplain, produced at the end of the first cycle of erosion, to the volcanoes from which the lavas were poured out. The near corner block is stripped down to the present stage of topographic form; the second represents the peneplain stage; the other three retain their constructional form. It is here made apparent that by reason of the tilting, the volcanic cones were raised above the old base-level of erosion, and were hence doomed to destruction in the process of base-leveling. The further edges of their flows remain; the stump of the long chimney up through which their lavas rose to the surface is still discoverable, but the cones, where the chimney rose to the surface and gave forth the flows, are lost. Fig. 11, which represents the completed peneplain, has no trace of them, although the edges of the flows and the stump of the chimney can be identified. Fig. 13. illustrating the present form of the surface in a general way, shows no volcanoes, but it shows the edges of the flows and the stump of the chimney better than before, because they, being hard rocks, have held up their edges, while the surrounding weaker sandstones and shales have wasted away. Thus the Blue Hills have been developed; not by lifting up their heavy summits above the surrounding surface, but by holding hard to the form that they had at the end of the previous cycle, while the surrounding rocks have lost it. Denudation has not yet progressed deep enough to reveal the connection that very likely exists between the chimney and the lower intrusive sheet; this is still buried. Fig. 14 tells the same sequence of events, but in very diagrammatic style.
The wooden working model from which several of these figures are taken is a very wooden affair; it is rigid and straightlined, instead of varying in irregular curves after a natural fashion; yet it may serve to present concrete illustrations of the successive stages through which the Meriden district has passed; and when thus viewed, the interest of the place grows wonderfully.
Its scenery is not grand or magnificent; many other regions exceed it in height of mountains or depth of valleys; but it has a fine story to tell about its lost volcanoes, and it tells the story with great distinctness and emphasis when the listener passes by.