Popular Science Monthly/Volume 40/January 1892/Remarkable Bowlders
By DAVID A. WELLS.
THE calling of attention, in The Popular Science Monthly for June, 1890, to the evidences of glacial action in southeastern Connecticut afforded by the number and great size of the bowlders in that section of the country, with accompanying illustrations from photographs, has been instrumental in creating no little popular interest on the subject, and in bringing to the attention of the public many other interesting examples of like glacial phenomena that have hitherto almost escaped notice.
Accepting reported measurements, the largest erratic block, or bowlder, as yet recognized in the United States, and probably in the world, is in the town of Madison, N. H., and, according to Prof. Crosby, of the Boston Institute of Technology, has the following maximum dimensions: Length, 83 feet; width, in excess of 45 feet; height, 30 to 37 feet; contents, 90,000 cubic feet; and probable weight, 15,300,000 pounds, or 7,650 tons.
Next to this in size is undoubtedly the great rock in the town of Montville, New London County, Connecticut, generally known by its Indian designation as "Sheegan," and also as "Mohegan" (Fig. 1). In the opinion of some, this rock is an isolated granite protuberance, and not a true "erratic" or bowlder; but recent examinations have seemed to completely negative the first supposition. Its approximate maximum dimensions are: Length, 75 feet; width, 58 feet; height, 60 feet; contents, 70,000 cubic feet; weight, 6,000 tons. If allowance be made for an immense fragment which has fallen from its northeast side, the dimensions and cubic contents of "Sheegan" would approximate more closely to those of the Madison bowlder. One point that goes far toward substantiating the claim on behalf of the "Sheegan" rock that it is a true bowlder, is the number of undoubted bowlders of an immense size and of the same granite which exist in comparative proximity. One, about a mile northwesterly, measures 21 feet high, 25 feet long, and 25 feet thick. Another, some three miles southeasterly, and but a short distance west of the Waterford station, on the New London and Northern Railroad (Fig. 2), and whose existence has
heretofore been only locally recognized, has almost the same dimensions; with the added peculiarity of a cavity, or rather tunnel, at its base, some five feet or more at the entrance, and extending with diminishing dimensions completely through the whole mass of the rock, which is about 25 feet in thickness. This cavity, which is somewhat imperfectly shown in the accompanying picture, is of such capacity that it has been fitted up with a cooking-stove, and has served a tramp family as a summer residence.
But one of the most curious and instructive examples of the disruptive and motor power of moving ice during the Glacial period to which attention has ever been called, occurs on the line of the New London and New Haven or "Shore Line" Railroad, about midway between Guilford and Leet's Island stations, and about a mile and a half from either place. Here, on the top of a narrow ledge of rock, which might almost be characterized as a pinnacle, rising (nearly perpendicularly from a salt marsh, or swamp, on one side) to a height of about GO feet, rests a rectangular, sarcophagus-looking block, 10 feet long, tapering from 7 feet 10 inches in width at one end to 5 feet 10 inches at the other, with an average thickness of 5 feet, and an approximative weight of about (>0 tons (see Fig. 3).
The peculiarities of this block, which invest it with unusual
interest, are: First, its apparent artificiality; second, the surface on which it rests is so narrow, smooth, and rounded, that, were it not for the blocking of a flat slab of rock (shown in Fig. 4), apparently artificially inserted underneath in exactly the proper
place, the block when released—i. e., by the melting of the ice—from the power that transported and placed it must have slid down and found a resting-place at the bottom of what is now a contiguous salt marsh; and, third, the circumstance that all the edges and angles of the block are as sharp and free from abrasion—which last is also true of its entire surface—as if it were but recently lifted from its original bed by the most modern and careful system of quarrying. It could not obviously, therefore, in its process of transportation have been rolled or tumbled about to any great extent; which conclusion in turn suggests that its movement after the first displacement was a lifting up to its present elevation, and that it was not subsequently transported to any great distance laterally. The extension of the ledge on which this great block rests having been largely broken up and removed through its use as a quarry, what might have been evidence confirmatory of this effect is now no longer obtainable. That it would have been perfectly practicable, with the requisite labor and machinery and large expenditure, to have quarried this block, and then have lifted it up and blocked it in its present position is not to be denied; but the idea that any such thing has been done, and for no practical purpose, is perfectly untenable. The surrounding country is very thinly populated, and the rock was in position long before any quarry (for the obtaining of rough stone for railroad construction) was worked in any immediate vicinity.
To travelers on the New London and New Haven Railroad this testimonial of the forces operative in a former geological age, by reason of its close proximity to the track, is clearly discernible on the right-hand side going west and the left-hand going east, and constitutes a most striking and picturesque object. Its obvious novelty, which has thus far undoubtedly saved it from destruction or displacement at the hands of workmen and vandals, may, it is to be hoped, continue to constitute its protection in the future, although as an object of attraction and interest to tourists and scientific men it is eminently worthy of care by the managers of the railroad company.
Figs. 5 and 6 are photographic reproductions of a huge bowlder, curiously disrupted on the land of Mr. Edward Atkinson, at Mattapoisett, on Buzzard's Bay, Mass., and having the following dimensions: Maximum height, 42 feet; measurement through the middle of the passage between the two fragments, from one side to the other in a straight line, 36 feet; average width of the crack between the two fragments at the level of the ground, 3½ feet; present surface area of the detached fragment, which has in part been quarried away, 462 feet.
To the trained geologist, the foregoing and all similar accounts
and representations of bowlders possess but little interest other than what pertains to peculiarities of size, shape, and location; while the agencies mainly concerned in the formation, movement, and distribution of the bowlder, as well as of the ordinary pebble, which is a miniature bowlder, have long ceased to be matters of controversy. With those not versed, however, in geological evidence and reasoning, the case is far different. To most of such, the attributing of the phenomena under consideration to the motor power of ice seems so fanciful and unnatural that the agency of the Indian (as has come within the experience of the writer) has appeared more reasonable. But if any one thus doubting will but acquaint himself with the present condition of Greenland, where we have a continental area covered with a sheet of ice of immense thickness—a mile or more, doubtless, in many places—continually accumulating through almost constant atmospheric precipitations, and moving, through the weight and pressure of such increments of snow and ice, with almost irresistible force from the center of such continent to its sea or coast line, and then in imagination transfer and reproduce such conditions (which are undoubted actualities) over the whole of the northern United States and Canada, he will be abundantly satisfied that the most striking of bowlder phenomena constitute but a very small measure of the forces that were concerned in their production and were concurrently exerted to modify the earth's surface—even to the extent of removing mountains.
It will also widen the sphere of interest in this subject to refer to the humbler but at the same time most instructive memorials of the Glacial period, which are, as it were, associated with the bowlders, and help to conceal the barrenness and desolation of the "drift"; namely, the pretty flowering plants like the "dandelion" and the "trailing arbutus," and others, which are believed to have come down in the Glacial period from their natural habitat in the far north to our present temperate zone, and to have remained, after the disappearance of the ice, with the bowlders as if to keep them company. Recent explorers of Greenland tell us that wherever in little sheltered nooks upon its dreary coast the ice and frost relax sufficiently in the brief summer to admit of any vegetation, these plants grow and flower most luxuriantly, while in their foreign homes they seem, as every one knows, to choose those times and temperatures for blooming and fruition—i. e., in the early spring—which are most in accordance with the conditions of their origin and primal existence; thus apparently reasserting their feræ naturæ, as did the old vikings when associated with the more delicate types of southern latitudes.