Popular Science Monthly/Volume 37/June 1890/Evidences of Glacial Action in South-Eastern Connecticut
|EVIDENCES OF GLACIAL ACTION IN SOUTH-EASTERN CONNECTICUT|
REMARKABLE evidences of glacial action in southeastern Connecticut seem thus far to have almost entirely escaped the attention of geologists. In fact, the most superficial survey of the section of country bordering on Long Island and Fisher's Island Sounds, and extending from Connecticut River on the west to Watch Hill, and perhaps to a point farther east, in Rhode Island, can hardly fail to produce a conviction that it was in this region that one, at least, of the great New England glaciers debouched into the waters of the Atlantic; unloading or dropping,
as its progress was arrested by the ocean, or as it subsequently gradually wasted and receded by change of climate, a vast multitude of bowlders, of which a very large proportion are of uncommon magnitude. There would also seem some reasons for believing that the central or medium line of this glacier is now indicated by the course of the so-called Thames River—which is more properly an arm of the sea rather than a river— and represents a deep but comparatively narrow cut in the underlying hard granitic rocks; and which, certainly near its mouth, to a depth of fifty feet or more beneath. the present river-bottom, as was shown by the recent borings in connection with the construction of the Shore Line Railroad Bridge at New London, is now filled up with mud or coarser detritus. East of the mouth of the Thames River the shores of the mainland, and the surface of the numerous little adjoining islands, are strewed with bowlders—many of large size, and often resting on a highly smoothed basis of bed-rock without the intervention of any surface soil whatever; as is illustrated by Fig. 1, which represents (from a photograph) a bowlder (and the changes in the way of destruction which such masses of rock are undergoing), between Groton and Noank, on the line of the New London and Providence Railroad, and which is a very conspicuous object as seen from the cars, on the left hand side of the track going east.
The number and size of the bowlders that are strewed over the bottom of Fisher's Island Sound are also a matter of interest and
wonderment to even those least acquainted with the subject, who sail over and fish in its shallow waters; while Fisher's Island itself is little other than a mass of bowlders covered in great part by sand, and probably marks the terminal line where a heavy ocean surf arrested the further progress of the glacier by breaking in upon its structure, floating off its ice-fragments in the form of bergs or floes, and, by releasing at the same time its heavier rock and gravel constituents, built up a breakwater which, as an island, now forms what is known as "Fisher's Island Sound." Fig. 2 represents a not unfrequent example of the character
of the materials which enter into the construction of this natural breakwater, as seen from the western side of this island.
But it is in the region to the east and west of the line of the Thames River, and which it has been suggested may have been the axis of the ancient glacier, and not very far removed from this line, that bowlders of extraordinary size occur most numerously; and among them is a rock which until very recently has been regarded as one of the largest, if not the very largest, bowlder that has thus far been recognized in this or any other country. This rock—of coarse crystalline granite—is situated in the town of Montville, New London County, about six miles south of Norwich, and about a mile west of the Montville Station on the New London and Northern Railroad; and, under the Indian name of "Sheegan," has almost from the first settlement of the country been recognized as a great natural curiosity. Its position is on the edge of a gentle mound or knoll, on the northeast slope of a little valley; and its dimensions, according to recent measurements by Prof. Crosby, of the Boston Society of Natural History, are as follows: northwest side, forty-six feet; northeast, fifty-eight; southeast, forty-five; southwest, seventy. Maximum height, reckoning from the lower or down-hill side, to the highest point on the upper side, approximately, sixty feet; approximate cubic contents, seventy thousand cubic feet; approximate weight, about six thousand tons. Other and former reported measurements of this rock indicate much larger dimensions than those reported by Prof. Crosby; and, although the determinations of an expert observer like the latter are entitled to the greatest confidence, it is nevertheless true that the form of the rock is so irregular as to render an exact estimate of its size, cubical contents, and weight a matter of no little difficulty. Figs. 3 and 4 give an idea of the position, size, and appearance of the "Sheegan" Rock, as seen from the valley beneath it, looking west. The introduction into the picture of the horse and wagon beneath the rock affords in some degree a standard for estimating its height. The cavity or recess beneath the rock, which is said to have been occupied, at the time of the first settlement of the country, by a
Mohegan Indian (from whom the rock undoubtedly derived its name) as a dwelling-place, is sufficiently capacious to admit of being used as a place of shelter for the sleds and other farm implements of the farmer proprietor. A rude ladder on the southern side of the rock affords facilities for reaching its top and obtaining a somewhat extensive view of the surrounding country.
It will probably have been noticed in the above description that the expression, "has been regarded" as a bowlder, has been employed. The reason of this is, that a recent examination of this rock (in March, 1890) has led Prof. Crosby to the somewhat startling conclusion that it is not a bowlder, but "simply an angular and prominent remnant of a large granite vein, still undisturbed in its original position upon beds of gneiss; and that its chief geological interest is found in the fact that, notwithstanding its
exposed position, it has survived the disintegrating influence of the elements and successfully resisted the pressure of the great ice-sheet." Prof. Crosby also states that, "through the undercutting action of the frost, forming quite an extensive rock-shelter" (i. e., the cavity or recess on the lower or valley side), "is afforded an opportunity to observe the actual contact of the massive granite and the finely laminated micaceous gneiss" upon which the granite rests.
For one of very limited experience to dispute the conclusions of such a trained observer as Prof. Crosby would be presumptuous; and yet it would not seem unreasonable to ask that they should not be considered as entirely determinative without a further careful examination of the problem on the part of experts. The question as to whether the contact of the granite of the assumed bowlder and the underlying gneiss is one of situation or of composition is not an easy one for decision, without a very clear opportunity for examination. The fact that such a huge mass of granite should have resisted the pressure of a great ice-sheet, and remained so prominently in place as part of a vein, when such pressure and an accompanying movement and grinding were sufficient to not only round off and obliterate everything like angularity from the granite surface, but also remove or reduce down to a much lower level and over a large proximate area the whole vast mass of rock on which the granite protuberance, if it be a portion of a vein, must have been as it were originally imbedded, is, as Prof. Crosby admits, a result not a little singular. There is certainly nothing analogous to such a phenomenon in the vicinity, and it may well be questioned whether there is anything similar anywhere.
Furthermore, as throwing some light on this subject, there are, as before stated, in comparative proximity to the "Sheegan" Rock, a large number of undoubted bowlders of the same granite, which, though not comparable as regards size, may yet be regarded as extraordinary, and as clearly involving the exercise of an enormous disrupting and transporting power within a rather limited area. One of these bowlders in the same township of Montville, which is also an object of public curiosity, and known as the "Goal" Rock, is, according to measurements made for the writer, twenty-one feet high, twenty-five long, and twenty-five thick. Another, in the vicinity of Gardner's Lake, from which nearly one fourth of the original mass has been detached in fragments, is reported as eighteen feet six inches high, thirty-five feet long, and twenty feet thick. A third, on the east side of the Thames River, in the town of Preston, is fourteen feet high, twenty feet long, and seventeen feet thick; and at least three or four others in the same region, of similar dimensions, might be enumerated. Above a mile east of "Allen's Point," and on one of the highest of the elevations bordering the river, an area of several acres is so covered with huge bowlders that in places it is difficult to find a path through them; while the southern slope of the same elevation, not far removed, is so strewed with such a multitude of rounded, small bowlders that they have the appearance of having been planted artificially.
Fig. 5 represents an extremely picturesque though not a very large bowlder, on the road between Norwich and Taftville, on the lands of the Ponemah Manufacturing Company, and almost in the center of the village that has within a comparatively few years grown up about it; and which, most fortunately, has thus far been carefully protected by the company against the Vandalic spirit which is so often prompted to mutilate or destroy everything in the nature of a public curiosity.
- All the illustrations accompanying this paper are reproduced from photographic pictures.