Man and the Glacial Period

by G. Frederick Wright

To some extent, however, the kettle-holes may have been formed by the irregular deposition of streams of water whose courses have crossed each other, or where eddies of considerable force have been produced in any way. The ordinary formation of kettle-holes can be observed in progress on the foot of almost any glacier, or, indeed, on a small scale, during the melting away of almost any winter’s snow. Where, from any cause, a stratum of dirt has accumulated upon a mass of compact snow or ice, it will be found to settle down in an irregular manner; furrows will be formed in various directions by currents of water, so that the melting will proceed irregularly, and produce upon a miniature scale exactly what I have seen on a large scale over whole square miles of the decaying foot of the great Muir Glacier in Alaska. The effects of similar causes and conditions we can see on a most enormous scale in the ten thousand lakes and ponds and peat-bogs of the whole glaciated area both in North America and in Europe.

In addition to these two lines of evidence of glacial action in New England, we should mention also the innumerable glacial grooves and scratches upon the rocks which can be found on almost any freshly uncovered surface. In New England the direction of these grooves is ordinarily a little east of south. Upon the east coast of Massachusetts and New Hampshire the scratches trend much more to the east than they do over most of the interior. This is as it should be on the glacial theory, since the ice would naturally move outwards in the line of least resistance, which would, of course, be towards the open sea wherever that is near. In the interior of New England the scratches upon the rocks indicate a more southerly movement in the Connecticut Valley than upon the mountains in the western part of Massachusetts. This also is as it should be upon the glacial theory. The scratches upon the mountains were made when the ice was at its greatest depth and when it moved over the country in comparative disregard of minor irregularities of surface, while in the valleys, at least in the later portion of the Ice age, the movement would be obstructed except in one direction. In the interpretation of the glacial grooves and scratches it should be borne in mind that they often represent the work done during the closing stages of the period. Just as the last shove of the carpenter’s plane removes the marks of the previous work, so the last rasping of a glacial movement wears away the surfaces which have been previously polished and striated.

In various places of New England it is interesting as well as instructive to trace the direction of the ice-movement by the distribution of boulders. My own attention was early attracted to numerous fragments of gneiss in eastern Massachusetts containing beautiful crystals of feldspar, which proved to be peculiar to the region of Lake Winnepesaukee, a hundred miles to the north, and to a narrow belt stretching thence to the southwestward. In ascending almost any of the lower summits of the White Mountains one’s attention can scarcely fail of being directed to the difference between the material of which the mountains are composed and that of the numerous boulders which lie scattered over the surface. The local geologist readily recognises these boulders as pilgrims that have wandered far from their homes to the northward.

Trains of boulders, such as those already described in Rhode Island, can frequently be traced to some prominent outcrop of the rock in a hill or mountain-peak from which they have been derived. One of the earliest of these to attract attention occurs in the towns of Richmond, Lenox, and Stockbridge, in the western part of Massachusetts. Here a belt of peculiar boulders about four hundred feet wide is found to originate in the town of Lebanon, N. Y., and to run continuously to the southeast for a distance of nine miles. West of Fry’s Hill, where the outcrop occurs, no boulders of this variety of rock are to be found, while to the southeast the boulders gradually diminish in size as their distance from the outcrop increases. Near the outcrop boulders of thirty feet in diameter occur, while nine miles away two feet is the largest diameter observed.

Sir Charles Lyell endeavoured to explain this train of boulders by the action of icebergs during a period of submergence—supposing that, as icebergs floated past or away from this hill in Lebanon, N. Y., they were the means of the regular distribution described. It is needless to repeat the difficulties arising in connection with such a theory, since now both by observation and experiment we have become more familiar with the movement of glacial ice. What we have already said about the transportation of boulders over Switzerland by the Alpine glaciers, and what is open to observation at the present time upon the large glaciers of Alaska, closely agree with the facts concerning this Richmond train of boulders, and we have no occasion to look further for a cause.

Indeed, trains of boulders ought to appear almost everywhere over the glaciated area; and so they do where all the circumstances are favourable. But, readily to identify the train, requires that to furnish the boulders there should be in the line of the ice-movement a projecting mass of rock hard enough to offer considerable resistance to the abrading agency of the ice and characteristic enough in its composition to be readily recognised. Ship Rock, in Peabody, Mass., weighing about eleven hundred tons, and Mohegan Rock, in Montville, Conn., weighing about ten thousand tons, have ordinarily been pointed to as boulders illustrating the power of ice-action. Their glacial character, however, has been challenged from the fact that the variety of granite to which they belong occurs in the neighbourhood, and indeed constitutes the bed-rock upon which they rest.[AV] Some would therefore consider them, like some of which we have already spoken, to be boulders which have originated through the disintegration of great masses of rock, of which these were harder nuclei that have longer resisted the ravages of the tooth of time. It must be admitted that possibly this explanation is correct; but it is scarcely probable that, in a region where there are so many other evidences of glacial action, these boulders could have remained immovable in presence of the onward progress of the ice-current that certainly passed over them.

[AV] Popular Science Monthly, vol. xxxvii, pp. 196-201.

However, as already seen, we are not left to doubt as to the movement of some boulders of great size. That which now claims the reputation of being the largest in New England is in Madison, N. H., and measures thirty by forty by seventy-five feet. This can be traced to ledges of Conway granite, about two miles away.[AW] Many boulders in the vicinity of New Haven, Conn., can be identified, as from well-known trap-dykes, sixteen miles or more to the north. The so-called Judge’s Cave, on West Rock, 365 feet above the adjoining valley and weighing a thousand tons, is one of these. Professor Edward Orton[AX] describes a mass of Clinton limestone near Freeport, Warren County, Ohio, as covering an area of three-fourths of an acre, and as sixteen feet in thickness. It overlies glacial clays and gravels, and must have been transported bodily from the elevations containing this rock several miles to the northwest.

[AW] See W. 0. Crosby’s paper in Appalachia, vol. vi, pp. 59-70.