Part 8 (2/2)
Ellsworth Huntington: The Basins of Eastern Persia and Seistan, in Explorations in Turkestan.]
[Footnote 55: Griffith Taylor: Australian Meteorology, 1920, p. 189.]
CHAPTER IX
THE ORIGIN OF LOESS
One of the most remarkable formations a.s.sociated with glacial deposits consists of vast sheets of the fine-grained, yellowish, wind-blown material called loess. Somewhat peculiar climatic conditions evidently prevailed when it was formed. At present similar deposits are being laid down only near the leeward margin of great deserts. The famous loess deposits of China in the lee of the Desert of Gobi are examples. During the Pleistocene period, however, loess acc.u.mulated in a broad zone along the margin of the ice sheet at its maximum extent. In the Old World it extended from France across Germany and through the Black Earth region of Russia into Siberia. In the New World a still larger area is loess-covered. In the Mississippi Valley, tens of thousands of square miles are mantled by a layer exceeding twenty feet in thickness and in many places approaching a hundred feet. Neither the North American nor the European deposits are a.s.sociated with a desert. Indeed, loess is lacking in the western and drier parts of the great plains and is best developed in the well-watered states of Iowa, Illinois, and Missouri.
Part of the loess overlies the non-glacial materials of the great central plain, but the northern portions overlie the drift deposits of the first three glaciations. A few traces of loess are a.s.sociated with the Kansan and Illinoian, the second and third glaciations, but most of the America loess appears to have been formed at approximately the time of the Iowan or fourth glaciation, while only a little overlies the drift sheets of the Wisconsin age. The loess is thickest near the margin of the Iowan till sheet and thins progressively both north and south.
The thinning southward is abrupt along the stream divides, but very gradual along the larger valleys. Indeed, loess is abundant along the bluffs of the Mississippi, especially the east bluff, almost to the Gulf of Mexico.[56]
It is now generally agreed that all typical loess is wind blown. There is still much question, however, as to its time of origin, and thus indirectly as to its climatic implications. Several American and European students have thought that the loess dates from inter-glacial times. On the other hand, Penck has concluded that the loess was formed shortly before the commencement of the glacial epochs; while many American geologists hold that the loess acc.u.mulated while the ice sheets were at approximately their maximum size. W. J. McGee, Chamberlin and Salisbury, Keyes, and others lean toward this view. In this chapter the hypothesis is advanced that it was formed at the one other possible time, namely, immediately following the retreat of the ice.
These four hypotheses as to the time of origin of loess imply the following differences in its climatic relations. If loess was formed during typical inter-glacial epochs, or toward the close of such epochs, profound general aridity must seemingly have prevailed in order to kill off the vegetation and thus enable the wind to pick up sufficient dust.
If the loess was formed during times of extreme glaciation when the glaciers were supplying large quant.i.ties of fine material to outflowing streams, less aridity would be required, but there must have been sharp contrasts between wet seasons in summer when the snow was melting and dry seasons in winter when the storms were forced far south by the glacial high pressure. Alternate floods and droughts would thus affect broad areas along the streams. Hence arises the hypothesis that the wind obtained the loess from the flood plains of streams at times of maximum glaciation. If the loess was formed during the rapid retreat of the ice, alternate summer floods and winter droughts would still prevail, but much material could also be obtained by the winds not only from flood plains, but also from the deposits exposed by the melting of the ice and not yet covered by vegetation.
The evidence for and against the several hypotheses may be stated briefly. In support of the hypothesis of the inter-glacial origin of loess, s.h.i.+mek and others state that the glacial drift which lies beneath the loess commonly gives evidence that some time elapsed between the disappearance of the ice and the deposition of the loess. For example, abundant sh.e.l.ls of land snails in the loess are not of the sort now found in colder regions, but resemble those found in the drier regions.
It is probable that if they represented a glacial epoch they would be depauperated by the cold as are the snails of far northern regions. The gravel pavement discussed below seems to be strong evidence of erosion between the retreat of the ice and the deposition of the loess.
Turning to the second hypothesis, namely, that the loess acc.u.mulated near the close of the inter-glacial epoch rather than in the midst of it, we may follow Penck. The mammalian fossils seem to him to prove that the loess was formed while boreal animals occupied the region, for they include remains of the hairy mammoth, woolly rhinoceros, and reindeer.
On the other hand, the typical inter-glacial beds not far away yield remains of species characteristic of milder climates, such as the elephant, the smaller rhinoceros, and the deer. In connection with these facts it should be noted that occasional remains of tundra vegetation and of trees are found beneath the loess, while in the loess itself certain steppe animals, such as the common gopher or spermaphyl, are found. Penck interprets this as indicating a progressive desiccation culminating just before the oncoming of the next ice sheet.
The evidence advanced in favor of the hypothesis that the loess was formed when glaciation was near its maximum includes the fact that if the loess does not represent the outwash from the Iowan ice, there is little else that does, and presumably there must have been outwash. Also the distribution of loess along the margins of streams suggests that much of the material came from the flood plains of overloaded streams flowing from the melting ice.
Although there are some points in favor of the hypothesis that the loess originated (1) in strictly inter-glacial times, (2) at the end of inter-glacial epochs, and (3) at times of full glaciation, each hypothesis is much weakened by evidence that supports the others. The evidence of boreal animals seems to disprove the hypothesis that the loess was formed in the middle of a mild inter-glacial epoch. On the other hand, Penck's hypothesis as to loess at the end of inter-glacial times fails to account for certain characteristics of the lowest part of the loess deposits and of the underlying topography. Instead of normal valleys and consequent prompt drainage such as ought to have developed before the end of a long inter-glacial epoch, the surface on which the loess lies shows many undrained depressions. Some of these can be seen in exposed banks, while many more are inferred from the presence of sh.e.l.ls 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 quant.i.ties, 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 a.s.suming 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 6C. 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 gra.s.sy, semi-arid vegetation of which evidence is found in the loess. The roots and stalks of such gra.s.sy 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 acc.u.mulated 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, s.h.i.+mek, who has made almost a lifelong study of the Iowan loess, emphasizes the fact that there is often an acc.u.mulation 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 a.s.sume 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 coa.r.s.e 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 Wurm advance of the ice. This breaks down on two counts. First, on the corresponding (Wisconsin) drift sheet in America, loess is present,--in small quant.i.ties to be sure, but unmistakably present. Second, there is no reason to a.s.sume 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 a.s.sociated with the other ice advances, points clearly against Penck's fundamental a.s.sumption that the acc.u.mulation 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 acc.u.mulated 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 doubtful whether the aridity would become extreme 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 pus.h.i.+ng 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 diminis.h.i.+ng 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 gra.s.sy vegetation had made a start.
The cyclonic hypothesis also seems to offer a satisfactory explanation of variations in the amount of loess a.s.sociated 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.[57] 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 debris. 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.
FOOTNOTES:
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