in exposed banks, while many more are inferred from the presence of shells of pond snails here and there in the overlying loess. The pond snails presumably lived in shallow pools occupying depressions in the uneven surface left by the ice. Another reason for questioning whether the loess was formed at the end of an inter-glacial epoch is that this hypothesis does not provide a reasonable origin for the material which composes the loess. Near the Alps where the loess deposits are small and where glaciers probably persisted in the inter-glacial epochs and thus supplied flood plain material in large quantities, this does not appear important. In the broad upper Mississippi Basin, however, and also in the Black Earth region of Russia there seems to be no way to get the large body of material composing the loess except by assuming the existence of great deserts to windward. But there seems to be little or no evidence of such deserts where they could be effective. The mineralogical character of the loess of Iowan age proves that the material came from granitic rocks, such as formed a large part of the drift. The nearest extensive outcrops of granite are in the southwestern part of the United States, nearly a thousand miles from Iowa and Illinois. But the loess is thickest near the ice margin and thins toward the southwest and in other directions, whereas if its source were the southwestern desert, its maximum thickness would probably be near the margin of the desert.
The evidence cited above seems inconsistent not only with the hypothesis that the loess was formed at the end of an inter-glacial epoch, but also with the idea that it originated at times of maximum glaciation either from river-borne sediments or from any other source. A further and more convincing reason for this last conclusion is the probability and almost the certainty that
when the ice advanced, its front lay close to areas where the vegetation was not much thinner than that which today prevails under similar climatic conditions. If the average temperature of glacial maxima was only 6°C. lower than that of today, the conditions just beyond the ice front when it was in the loess region from southern Illinois to Minnesota would have been like those now prevailing in Canada from New Brunswick to Winnipeg. The vegetation there is quite different from the grassy, semi-arid vegetation of which evidence is found in the loess. The roots and stalks of such grassy vegetation are generally agreed to have helped produce the columnar structure which enables the loess to stand with almost vertical surfaces.
We are now ready to consider the probability that loess accumulated mainly during the retreat of the ice. Such a retreat exposed a zone of drift to the outflowing glacial winds. Most glacial hypotheses, such as that of uplift, or depleted carbon dioxide, call for a gradual retreat of the ice scarcely faster than the vegetation could advance into the abandoned area. Under the solar-cyclonic hypothesis, on the other hand, the climatic changes may have been sudden and hence the retreat of the ice may have been much more rapid than the advance of vegetation. Now wind-blown materials are derived from places where vegetation is scanty. Scanty vegetation on good soil, it is true, is usually due to aridity, but may also result because the time since the soil was exposed to the air has not been long enough for the soil to be sufficiently weathered to support vegetation. Even when weathering has had full opportunity, as when sand bars, mud flats, and flood plains are exposed, vegetation takes root only slowly. Moreover, storms and violent winds may prevent the spread of vegetation, as is seen on sandy beaches even
in distinctly humid regions like New Jersey and Denmark. Thus it appears that unless the retreat of the ice were as slow as the advance of vegetation, a barren area of more or less width must have bordered the retreating ice and formed an ideal source of loess.
Several other lines of evidence seemingly support the conclusion that the loess was formed during the retreat of the ice. For example, Shimek, who has made almost a lifelong study of the Iowan loess, emphasizes the fact that there is often an accumulation of stones and pebbles at its base. This suggests that the underlying till was eroded before the loess was deposited upon it. The first reaction of most students is to assume that of course this was due to running water. That is possible in many cases, but by no means in all. So widespread a sheet of gravel could not be deposited by streams without destroying the irregular basins and hollows of which we have seen evidence where the loess lies on glacial deposits. On the other hand, the wind is competent to produce a similar gravel pavement without disturbing the old topography. "Desert pavements" are a notable feature in most deserts. On the edges of an ice sheet, as Hobbs has made us realize, the commonest winds are outward. They often attain a velocity of eighty miles an hour in Antarctica and Greenland. Such winds, however, usually decline rapidly in velocity only a few score miles from the ice. Thus their effect would be to produce rapid erosion of the freshly bared surface near the retreating ice. The pebbles would be left behind as a pavement, while sand and then loess would be deposited farther from the ice where the winds were weaker and where vegetation was beginning to take root. Such a decrease in wind velocity may explain the occasional vertical gradation from gravel through sand to coarse loess and then to
normal fine loess. As the ice sheet retreated the wind in any given place would gradually become less violent. As the ice continued to retreat the area where loess was deposited would follow at a distance, and thus each part of the gravel pavement would in turn be covered with the loess.
The hypothesis that loess is deposited while the ice is retreating is in accord with many other lines of evidence. For example, it accords with the boreal character of the mammal remains as described above. Again, the advance of vegetation into the barren zone along the front of the ice would be delayed by the strong outblowing winds. The common pioneer plants depend largely on the wind for the distribution of their seeds, but the glacial winds would carry them away from the ice rather than toward it. The glacial winds discourage the advance of vegetation in another way, for they are drying winds, as are almost all winds blowing from a colder to a warmer region. The fact that remains of trees sometimes occur at the bottom of the loess probably means that the deposition of loess extended into the forests which almost certainly persisted not far from the ice. This seems more likely than that a period of severe aridity before the advance of the ice killed the trees and made a steppe or desert. Penck's chief argument in favor of the formation of loess before the advance of the ice rather than after, is that since loess is lacking upon the youngest drift sheet in Europe it must have been formed before rather than after the last or Würm advance of the ice. This breaks down on two counts. First, on the corresponding (Wisconsin) drift sheet in America, loess is present,—in small quantities to be sure, but unmistakably present. Second, there is no reason to assume that conditions were identical at each advance and retreat of the ice. Indeed, the
fact that in Europe, as in the United States, nearly all the loess was formed at one time, and only a little is associated with the other ice advances, points clearly against Penck's fundamental assumption that the accumulation of loess was due to the approach of a cold climate.
Having seen that the loess was probably formed during the retreat of the ice, we are now ready to inquire what conditions the cyclonic hypothesis would postulate in the loess areas during the various stages of a glacial cycle. Fig. 2, in Chapter IV, gives the best idea of what would apparently happen in North America, and events in Europe would presumably be similar. During the nine maximum years on which Fig. 2 is based the sunspot numbers averaged seventy, while during the nine minimum years they averaged less than five. It seems fair to suppose that the maximum years represent the average conditions which prevailed in the past at times when the sun was in a median stage between the full activity which led to glaciation and the mild activity of the minimum years which appear to represent inter-glacial conditions. This would mean that when a glacial period was approaching, but before an ice sheet had accumulated to any great extent, a crescent-shaped strip from Montana through Illinois to Maine would suffer a diminution in storminess ranging up to 60 per cent as compared with inter-glacial conditions. This is in strong contrast with an increase in storminess amounting to 75 or even 100 per cent both in the boreal storm belt in Canada and in the subtropical belt in the Southwest. Such a decrease in storminess in the central United States would apparently be most noticeable in summer, as is shown in Earth and Sun. Hence it would have a maximum effect in producing aridity. This would favor the formation of loess, but it is 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. doubtful whether the aridity would become extreme