Fig. 331.—Map to show the outcroppings of peculiar rock types in the region of the Great Lakes, and some of the localities where “float copper” has been collected (float copper localities after Salisbury).
The dispersion of the drift.—Long before the “ice age” had been conceived in the minds of Agassiz and his contemporaries, it had been remarked that scattered over the North German plain were rounded fragments of rock which could not possibly have been derived from their own neighborhood but which could be matched with the great masses of red granite in Sweden well known as the “Swedish granite.” Buckland, an English geologist, had in 1815 accounted for such “erratic” blocks of his own country, here of Scotch granite, by calling in the deluge of Noah; but in the late thirties of the nineteenth century, Sir Charles Lyell, with the results of English Arctic explorers in mind, claimed that such traveled blocks had been transported by icebergs emanating from the polar regions. A relic of Buckland’s earlier view we have in the word “diluvium” still occasionally used in Germany for glacier transported materials; while the term “drift” still remains in common use to recall Lyell’s iceberg hypothesis, even though the original meaning of the term has been abandoned. Drift is now a generic term and refers to all deposits directly or indirectly referable to the continental glaciers.
In general the place of derivation of the glacial drift may be said to be some point more distant from and within the former ice margin at the time when it was deposited; in other words, the dispersion of the drift was centrifugal with reference to the glacier.
Fig. 332.—Map of the “bowlder train” from Iron Hill, R. I. (based upon Shaler’s map, but with the directions of glacial striæ added).
Wherever rocks of unusual and therefore easily recognizable character can be shown to occur in place and with but limited areas, the dispersion of such material is easy to trace. The areas of red Swedish and Scotch granite have been used to follow out in a broad way the dispersion of drift over northern Europe. Within the region of the Great Lakes of North America are areas of limited size which are occupied by well marked rock types, so that the journeyings of their fragments with the continental glacier can be mapped with some care. Upon the northern shore of Georgian Bay occurs the beautiful jasper conglomerate, whose bright red pebbles in their white quartz field attract such general notice. At Ishpeming in the northern peninsula of Michigan is found the equally beautiful jaspilite composed of puckered alternating layers of black hematite and red jasper. On Keweenaw Peninsula, which protrudes into Lake Superior from its southern shore, is found that remarkable occurrence of native copper within a series of igneous rocks of varied types and colors. Fragments of this copper, some weighing several hundreds of pounds each and masked in a coat of green malachite, have under the name of “drift” or “float” copper been collected at many localities within a broad “fan” of dispersal extending almost to the very limits of glaciation ([Fig. 331]).
Some miles to the north of Providence in Rhode Island there is a hill known as Iron Hill composed in large part of black magnetite rock, the so-called Cumberlandite. From this hill as an apex there has been dispersed a great quantity of the rock distributed as a well marked “bowlder train” within which the size and the frequency of the dispersed bowlders is in inverse ratio to the distance from the parent ledge ([Fig. 332]). Similar though less perfect trains of bowlders are found on the lee side of most projecting masses of resistant rocks within the area of the drift.
Large bowlders when left upon a ledge of notably different appearance easily attract attention, and have been described as “perched bowlders.” Resting as they sometimes do upon a relatively small area, they may be nicely balanced and thus easily given a pendular or rocking motion. Such “rocking stones” are common enough, especially among the New England hills ([plate 17 B]). Many such bowlders have made somewhat remarkable peregrinations with many interruptions, having been carried first in one direction by an earlier glacier to be later transported in wholly different directions at the time of new ice invasions.
Plate 17.
