From a photograph kindly furnished by Professor F. W. Putnam, showing the Delaware and its bluff of gravel, where many of the rude implements have been found.
As some geologists were still inclined to insist upon the post-glacial character of the débris in which the implements were found, Dr. Abbott, admitting that the great terminal moraine of the northern ice-sheet does not approach nearer than forty miles to the bluff at Trenton, nevertheless insists that the character of the deposits there much more resembles a mass of material accumulated in the sea at the foot of the glacier than it does beds that have been subjected to the modifying arrangement of water. He finds an explanation of this condition of things in a prolongation of the glacier down the valley of the Delaware as far as Trenton, at a time when the lower portions of the State had suffered a considerable depression, and before the retreat of the ice-sheet. But besides the comparatively unmodified material of the bluff, in which the greater portion of the palæolithic implements has been found, there also occur limited areas of stratified drift, such as are to be seen in railway cuttings near Trenton, in which similar implements are also occasionally found. These, however, present a more worn appearance than the others. But it will be found that these tracts of clearly stratified material are so very limited in extent that they seem to imply some peculiar local condition of the glacier. This position is illustrated by certain remarkable effects once witnessed after a very severe rainfall, by which two palæolithic implements were brought into immediate contact with ordinary Indian relics such as are common on the surface. This leads to an examination of the question of the origin of this surface soil, and a discussion of the problem how true palæolithic implements sometimes occur in it. This soil is known to be a purely sedimentary deposit, consisting almost exclusively of sand, or of such finely comminuted gravels as would readily be transported by rapid currents of water. But imbedded in it and making a part of it are numerous huge boulders, too heavy to be moved by water. Dr. Abbott accounted for their presence from their having been dropped by ice-rafts, while the process of deposition of the soil was going on. The same sort of agency could not have put in place both the soil and the boulders contained in it, and the same force which transported the latter may equally well have brought along such implements as occur in the beds of clearly stratified origin. The wearing effect upon these of gravels swept along by post-glacial floods will account for that worn appearance which sometimes almost disguises their artificial origin.
In conclusion Dr. Abbott attempted to determine what was the early race which preceded the Indians in the occupation of this continent. From the peculiar nature and qualities of palæolithic implements he argues that they are adapted to the needs of a people “living in a country of vastly different character, and with a different fauna,” from the densely wooded regions of the Atlantic seaboard, where the red man found his home. The physical conditions of the glacial times much more nearly resembled those now prevailing in the extreme north. Accordingly he finds the descendants of the early race in the Eskimos of North America, driven northwards after contact with the invading Indian race. In this he is following the opinion of Professor William Boyd Dawkins, who considers that people to be of the same blood as the palæolithic cave-dwellers of southern France, and that of Mr. Dall and Dr. Rink, who believed that they once occupied this continent as far south as New Jersey. In confirmation of this view he asserts that the Eskimos “until recently used stone implements of the rudest patterns.” But unfortunately for this theory the implements of the Eskimos bear no greater resemblance to palæolithic implements than do those of any other people in the later stone age; and subsequent discoveries of human crania in the Trenton gravels have led Dr. Abbott to question its soundness.[1490]
These discoveries of Dr. Abbott are not liable to the imputation of possible errors of observation or record, as would be the case if they rested upon the testimony of a single person only. As has been already stated, in September, 1876, Professor Putnam was present at the finding in place of two palæolithic implements, and in all has taken five with his own hands from the gravel at various depths.[1491] Mr. Lucien Carr also visited the locality in company with Professor J. D. Whitney, in September, 1878, and found several in place.[1492] Since then Professors Shaler, Dawkins, Wright, Lewis, and others, including the writer, have all succeeded in finding specimens either in place or in the talus along the face of the bluff, from which they had washed out from freshly exposed surfaces of the gravel.[1493] The whole number thus far discovered by Dr. Abbott amounts to about four hundred specimens.[1494] Meanwhile, the problem of the conditions under which the Trenton gravels had been accumulated was made the subject of careful study by other competent geologists, besides Professor Shaler, to whose opinion reference has already been made. In October, 1877, the late Thomas Belt, F. G. S., visited the locality, and shortly afterwards published an account of Dr. Abbott’s discoveries, illustrated by several geological sections of the gravel. His conclusion is, “that after the land-ice retired, or whilst it was retiring, and before the coast was submerged to such a depth as to permit the flotation of icebergs from the north, the upper pebble-beds containing the stone implements were formed.”[1495] The geologists of the New Jersey Survey had already recognized the distinction between the drift gravels of Trenton and the earlier yellow marine gravels which cover the lower part of the State. But it was the late Professor Henry Carvill Lewis, of Philadelphia, who first accurately described the character and limits of the Trenton gravels.[1496] This he had carefully mapped before he was informed of Dr. Abbott’s discoveries, and it has been found (with only one possible very recent exception) that the implements occur solely in these newer gravels of the glacial period.
Professor Lewis’s matured conclusions in regard to the geological character and the age of the Trenton gravel cliff are thus expressed: “The presence of large boulders in the bluff at Trenton, and the extent and depth of the gravel at this place, have led to the supposition that there was here the extremity of a glacial moraine. Yet the absence of ‘till’ and of scratched boulders, the absence of glacial striæ upon the rocks of the valley, and the stratified character of the gravel, all point to water action alone as the agent of deposition. The depth of the gravel and the presence of the bluff at this point are explained by the peculiar position that Trenton occupies relatively to the river, ... in a position where naturally the largest amount of a river gravel would be deposited, and where its best exposures would be exhibited.... Any drift material which the flooded river swept down its channel would here, upon meeting tide-water, be in great part deposited. Boulders which had been rolled down the inclined floor of the upper valley would here stop in their course, and all be heaped up with the coarser gravel in the more slowly flowing water, except such as cakes of floating ice could carry oceanward.... Having heaped up a mass of detritus in the old river channel as an obstruction at the mouth of the gorge, the river, so soon as its volume diminished, would immediately begin wearing away a new channel for itself down to ocean level. This would be readily accomplished through the loose material, and would be stopped only when rock was reached.... It has been thought that to account for the high bank at Trenton an elevation of the land must have occurred.... An increase in the volume of the river will explain all the facts. The accompanying diagram will render this more clear.
Section of bluff two miles south of Trenton, New Jersey. a b, Trenton gravel; Implements—a, fine gray sand (boulder); b, coarse sandy gravel; c, red gravel; d, yellow gravel (pre-glacial); e, plastic clay (Wealden); f, fine yellow sand (Hastings?); g, gneiss; h, alluvial mud; i, Delaware River.
A From a cut in Primitive Industry, p. 535.
“The Trenton gravel, now confined to the sandy flat borders of the river, corresponds to the ‘intervale’ of New England rivers, ... and exhibits a topography peculiar to a true river gravel. Frequently instead of forming a flat plain it forms higher ground close to the present river channel than it does near its ancient bank. Moreover, not only does the ground thus slope downward on retreating from the river, but the boulders become smaller and less abundant. Both of these facts are in accordance with the facts of river deposits. In time of flood the rapidly flowing water in the main channel, bearing detritus, is checked by the more quiet waters at the side of the river, and is forced to deposit its gravel and boulders as a kind of bank.... Having shown that the Trenton gravel is a true river gravel of comparatively recent age, it remains to point out the relation it bears to the glacial epoch.... Two hypotheses only can be applied to the Trenton gravel. It is either post-glacial, or it belongs to the very last portion of the glacial period. The view held by the late Thomas Belt can no longer be maintained.... He fails to recognize any distinction between the gravels. As we have seen, the Trenton gravel is truly post-glacial. It only remains to define more strictly the meaning of that term. There is evidence to support both of these hypotheses.”[1497]
After discussing them both at considerable length, he concludes as follows: “A second glacial period in Europe, known as the ‘Reindeer Period,’ has long been recognized. It appears to have followed that in which the clays were deposited and the terraces formed, and may therefore correspond with the period of the Trenton gravel. If there have been two glacial epochs in this country, the Trenton gravel cannot be earlier than the close of the later one. If there has been but one, traces of the glacier must have continued into comparatively recent times, or long after the period of submergence. The Trenton gravel, whether made by long-continued floods which followed a first or second glacial epoch,—whether separated from all true glacial action or the result of the glacier’s final melting,—is truly a post-glacial deposit, but still a phenomenon of essentially glacial times,—times more nearly related to the Great Ice Age than to the present.”