Man and the Glacial Period

by G. Frederick Wright

[BA] Proceedings of the Boston Society of Natural History, vol. xxv (1890), pp. 115-140.

The results of Mr. Crosby’s investigations show that “not more than one-third of the detritus composing the till of the Boston Basin was in existence before the Ice age, and that the remaining two-thirds must be attributed to the mechanical action of the ice-sheet and its accompanying torrents of water. In other words, if we assume the average thickness of the drift as thirty feet, the amount of glacial erosion can scarcely fall below twenty feet. After scraping away the residuary clays and half-decomposed material, the ice-sheet has cut more than an equal depth into the solid rocks.”

Mr. Crosby’s investigations also convinced him that the movement of the till, or ground moraine, underneath the ice was not en masse, but that “it must have experienced differential horizontal movements or flowing, in which, normally, every particle or fragment slipped or was squeezed forward with reference to those immediately below it, the velocity diminishing downward through the friction of the underlying ledges.... The glaciation was not limited to masses which were firmly caught between the ice and the solid ledges, and it was in every case essentially a slipping and not a rolling movement.... These differential horizontal movements mean that the till acted as a lubricant for the ice-sheet; and the clayey element, especially, co-operating in many cases with the pent-up subglacial waters, must have greatly facilitated the onward progress of the ice.” He concludes, therefore, that the onward movement of the vast ice-sheet greatly exceeded that of the main part of the ground moraine, the ice-sheet slipping over the till, the whole being in some degree analogous to that of a great land-slip. “In both cases the progress of a somewhat yielding and mobile mass is facilitated by an underlying clayey layer saturated with water.”

New York, New Jersey, and Pennsylvania.

West of New England the glacial phenomena over the northern part of the United States are equally marked all the way to the Missouri River, and the boundary-line of the glaciated region can be traced with little difficulty. It emerges from New York Bay on Staten Island and enters New Jersey at Perth Amboy. A well-formed moraine covers the northern part of Staten Island, and upon the mainland marks the boundary from Perth Amboy, around through Raritan, Plainfield, Chatham, Morris, and Hanover, to Rockaway, and thence in a southwesterly direction to Belvidere, on the Delaware River. That portion of New Jersey lying north of this serpentine line of moraine hills is characterised by the presence of transported boulders, by numerous lakes of evident glacial origin, and by every other sign of glacial action, while south of it all these peculiar characteristics are absent. The observant passenger upon the railroad trains between New York and Philadelphia can easily recognise the moraine as it is passed through on the Pennsylvania Railroad at Metuchen and on the Bound Brook Railroad at Plainfield. Near Drakestown, in Morris County, there is a mass of blue limestone measuring, as exposed, thirty-six by thirty feet, and which was quarried for years before discovering that it was a boulder brought with other drift material from many miles to the northwest and lodged here a thousand feet above the sea.

Across Pennsylvania the glacial boundary passes through Northampton, Monroe, Luzerne, Columbia, Sullivan, Lycoming, Tioga, and Potter Counties, where it enters the State of New York, running still in a northwest direction through Allegany and Cattaraugus Counties to the vicinity of Salamanca. Here it turns to the south nearly at a right angle, running southwestward to Chautauqua County and re-entering Pennsylvania in Warren County, and thence passing onward in the same general direction through Crawford, Venango, Mercer, Butler, and Lawrence Counties to the Ohio line in Columbiana County, about ten miles north of the Ohio River.

The occurrence of a well-defined terminal moraine to mark the glacial boundary eastward from Pennsylvania led Professor Lewis and myself, who made the survey of that State in 1880, to be rather too sanguine in our expectations of finding an equally well-marked moraine everywhere along the southern margin of the glaciated area; still, the results are even more interesting than would have been the exact fulfilment of our expectations, since they more fully revealed to us the great complexity of effect which is capable of being brought about by ice-action. Before proceeding farther with the details, therefore, it will be profitable at this point to pause in the narrative and briefly record a few generalisations that have forced themselves into prominence during the years in which field-work has been in progress.

Previous to our explorations in Pennsylvania it had been thought that the indications of ice-action would extend much farther south in the valleys than on the mountains, and this indeed would have been the case if the glaciers in northern Pennsylvania had been of local origin; but our experience very soon demonstrated that the great gathering-place of the snows which produced the glacial movement in northern Pennsylvania could not have been local, but that over the northern part of that State there was distinct evidence of a continental movement of ice whose centre was far beyond the Alleghanies.

For example, we found that the evidences of direct glacial action extended farther south upon the hills and plateaus than they did in the narrow valleys, while everywhere on the very southern border of glacial indications we found boulders that had been brought from the granitic region of northern New York or central Canada. In eastern Pennsylvania we found indeed a terminal moraine more or less distinctly marking the southern border over the highlands. This was more specially true in Northampton and Monroe Counties.

In Northampton County it was very interesting to see long lines of hills, a hundred or more feet in height and lying several hundred feet above the Delaware River, composed entirely of glacial débris, much of which had been brought bodily over the sharp summit of the Blue Ridge, or Kittatinny Mountain, which rises as a continuous wall to the northwest and is everywhere several hundred feet higher than the moraine in Northampton County. The summit of Blue Ridge, also, as far south as the glacial movement extended, shows evident signs of glacial abrasion, some hundreds of feet evidently having been removed by that means, leaving a well-defined shoulder, marking the limits of its southwestern border. Resting upon the summit of the glaciated portion of the Blue Ridge, there are also numerous boulders of Helderberg limestone, which must have been brought from ledges at least five hundred feet lower than the places upon which they now lie.