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The Origin Of Loess

One of the most remarkable formations associated 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 accumulated 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 associated 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 associated 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 accumulated 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 quantities 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, Shimek 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 shells 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 accumulated

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

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 deg.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 Wuerm 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 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 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.[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.


[Footnote 56: Chamberlin and Salisbury: Geology, 1906, Vol. III, pp.


[Footnote 57: It may have retreated soon after reaching its maximum. If

so, the general lack of thick terminal moraines would be explained. See

page 122.]