It has been urged against the criterion of differential weathering that superglacial material is or may be thoroughly oxidized before its deposition, and that a layer of oxidized drift between layers of till may be no more than superglacial debris deposited during a minor recession of the ice.[16] We believe that this attempt to eliminate the value of this criterion rests partly on an exaggerated idea concerning the amount of superglacial material, but more especially on a failure to apprehend the real meaning of the argument for the validity of the criterion, and upon a failure to note the limitations imposed upon it by its advocates. It is not affirmed that a layer of oxidized drift between beds of unoxidized drift is per se proof of two glacial epochs; but it is affirmed that if such layer of weathered drift can be shown to extend far below any possible superglacial till, into the subglacial till below, in such wise as to indicate that it is the result of subaërial exposure in a warm climate subsequent to its deposition and prior to the deposition of the overlying till, it constitutes the best possible evidence of an interglacial epoch, especially when accompanied by the corroborative testimony of other criteria. It is further affirmed that if the second sheet of drift failed to reach the limit of the first, and if the drift which was deposited by the first and never covered by the second ice-sheet, is more thoroughly and more deeply weathered than that deposited by the second, and especially if the two types of drift surface meet along a definite and readily traceable line, the argument becomes, in our judgment, irrefragable. In its application, this criterion would be infallible only in the hands of one who could distinguish between superglacial and superglacially oxidized material on the one hand, and material subaërially weathered after its deposition, on the other.
[16] This point was urged at the reading of the paper at Ottawa, by Prof. C. H. Hitchcock, Mr. Upham, and others.
In circumstances and relations where the weathering of the drift is not in itself conclusive, it might still have corroborative value in association with other lines of evidence.
The absence of an oxidized and disintegrated zone of drift below a superficial layer which is not oxidized, would be no proof that there were not distinct ice epochs, since the ice of any later epoch, if such there were, might have planed off the surface of the drift left by its predecessor to the depth of the weathering. The preservation of such surfaces after a second ice invasion must be regarded as the exception rather than as the rule. There is always the possibility, too, that an oxidized and weathered zone marking the surface of an older drift sheet exists, where excavations have not opened full sections of drift to view. The absence of weathered zones of drift beneath the surface, or the absence of knowledge of their existence, is therefore at best no more than negative evidence. The absence of greater weathering of the drift outside the limit of the drift supposed to belong to a later epoch, would be positive evidence against the reference of the two sheets of drift concerned to different epochs.
A specific part of the above line of evidence may be separately mentioned. One phase of weathering is the disintegration of boulders, and this is a point which can be readily applied even by those who are not geologists. If the boulders of one region are much more commonly disintegrated than those of another, and if the two regions are separated from each other by a well-marked boundary line, the inference lies close at hand that the boulders in the one case have been much longer exposed to disintegrating agencies than in the other. It is no answer to this argument to say that the materials lying at the very front of the drift deposits contain boulders which were derived from the disintegrated rock over which the ice has passed, and that they were therefore in a less firm state at the outset. In many cases these boulders have come from great distances, and coming from great distances they must have come in a firm and solid state, else they could not have suffered such extensive transportation, except indeed their position was superglacial throughout their whole journey. This argument has equal force when applied to the area covered by the two sheets of drift where two exist. If within the region of drift under investigation we find a surface layer of greater or less depth, the boulders of which are hard and fresh, and if beneath this we find another layer of drift, the stony material of which is largely disintegrated, at least in its upper parts, we have good evidence that the surface bearing the disintegrated boulders was exposed for a considerable length of time before the deposition of the overlying drift, which carries fresh boulders. Since the disintegration of boulders is only one phase of weathering, the limitations of this argument are identical with those already noted in connection with the general argument from differential weathering.
(7) Differential Subaërial Erosion. If the drift deposited by one ice-sheet were to be exposed for a considerable interval of time, and if the ice in its subsequent advance failed to reach the limit of its first invasion, the two areas should show different amounts of subaërial erosion, since the one has been exposed to the action of air and water much longer than the other. The line which marks the limit of the later ice invasion should be the line of more or less sudden transition from an area without, where stream erosion has been greater, to an area within, where stream erosion has been less.
The point here made can not be met by the suggestion that the greater erosion of the outer area was effected by the water issuing from the ice which had retreated to the position now marked by the border of the area of the lesser erosion. So far as we know, such waters would be depositing, not eroding. Furthermore, much of the erosion of the outer area would have such relation to drainage lines that waters issuing from the ice could never have reached the localities where it is shown.
If the outer and older drift be found to have suffered ten times as much stream erosion as the inner and newer, it is fair to assume that it has been exposed something like ten times as long, if the conditions for erosion are equally favorable in the two regions. The argument has especial weight if it can be found that beneath the newer drift the surface of the older is such as to indicate that it was deeply eroded before the newer was placed upon it. The argument is stronger the farther from the margin of the newer drift such erosion on the surface of the underlying older drift can be proved to have taken place. In other words, if, in addition to the greater surface erosion of the older drift sheet as now exposed outside the limit of the newer drift, we find a notable unconformity between the newer and the older drift, and especially if this unconformity lie far enough north of the margin of the newer drift, the argument becomes conclusive.
When differential erosion and drift unconformities are not in themselves conclusive, they may have great corroborative value in conjunction with differential weathering, forest beds, or other indications of separate ice epochs.