enough to explain such vast deposits as are found throughout large parts of the Mississippi Basin. That would demand that hundreds of thousands of square miles should become almost absolute desert, and it is not probable that any such thing occurred. Nevertheless, according to the cyclonic hypothesis the period immediately before the advent of the ice would be relatively dry in the central United States, and to that extent favorable to the work of the wind.
As the climatic conditions became more severe and the ice sheet expanded, the dryness and lack of storms would apparently diminish. The reason, as has been explained, would be the gradual pushing of the storms southward by the high-pressure area which would develop over the ice sheet. Thus at the height of a glacial epoch there would apparently be great storminess in the area where the loess is found, especially in summer. Hence the cyclonic hypothesis does not accord with the idea of great deposition of loess at the time of maximum glaciation.
Finally we come to the time when the ice was retreating. We have already seen that not only the river flood plains, but also vast areas of fresh glacial deposits would be exposed to the winds, and would remain without vegetation for a long time. At that very time the retreat of the ice sheet would tend to permit the storms to follow paths determined by the degree of solar activity, in place of the far southerly paths to which the high atmospheric pressure over the expanded ice sheet had previously forced them. In other words, the conditions shown in Fig. 2 would tend to reappear when the sun's activity was diminishing and the ice sheet was retreating, just as they had appeared when the sun was becoming more active and the ice sheet was advancing. This time, however, the semi-arid conditions arising from the scarcity
of storms would prevail in a region of glacial deposits and widely spreading river deposits, few or none of which would be covered with vegetation. The conditions would be almost ideal for eolian erosion and for the transportation of loess by the wind to areas a little more remote from the ice where grassy vegetation had made a start.
The cyclonic hypothesis also seems to offer a satisfactory explanation of variations in the amount of loess associated with the several glacial epochs. It attributes these to differences in the rate of disappearance of the ice, which in turn varied with the rate of decline of solar activity and storminess. This is supposed to be the reason why the Iowan loess deposits are much more extensive than those of the other epochs, for the Iowan ice sheet presumably accomplished part of its retreat much more suddenly than the other ice sheets.[59] The more sudden the retreat, the greater the barren area where the winds could gather fine bits of dust. Temporary readvances may also have been so distributed and of such intensity that they frequently accentuated the condition shown in Fig. 2, thus making the central United States dry soon after the exposure of great amounts of glacial débris. The closeness with which the cyclonic hypothesis accords with the facts as to the loess is one of the pleasant surprises of the hypothesis. The first draft of Fig. 2 and the first outlines of the hypothesis were framed without thought of the loess. Yet so far as can now be seen, both agree closely with the conditions of loess formation.
[CHAPTER X]
CAUSES OF MILD GEOLOGICAL CLIMATES
In discussions of climate, as of most subjects, a peculiar psychological phenomenon is observable. Everyone sees the necessity of explaining conditions different from those that now exist, but few realize that present conditions may be abnormal, and that they need explanation just as much as do others. Because of this tendency glaciation has been discussed with the greatest fullness, while there has been much neglect not only of the periods when the climate of the earth resembled that of the present, but also of the vastly longer periods when it was even milder than now.
How important the periods of mild climate have been in geological times may be judged from the relative length of glacial compared with inter-glacial epochs, and still more from the far greater relative length of the mild parts of periods and eras when compared with the severe parts. Recent estimates by R. T. Chamberlin[60] indicate that according to the consensus of opinion among geologists the average inter-glacial epoch during the Pleistocene was about five times as long as the average glacial epoch, while the whole of a given glacial epoch averaged five times as long as the period when the ice was at a maximum. Climatic periods far milder, longer, and more monotonous than any inter-glacial epoch appear repeatedly
during the course of geological history. Our task in this chapter is to explain them.