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Causes Of Mild Geological Climates

Conclusion

Some Problems Of Glacial Periods

Hypotheses Of Climatic Change

The Climate Of History

The Solar Cyclonic Hypothesis

The Variability Of Climate

The Climatic Stress Of The Fourteenth Century

The Uniformity Of Climate

Glaciation According To The Solar-cyclonic Hypothesis[38]



Least Viewed

The Sun's Journey Through Space

The Changing Composition Of Oceans And Atmosphere

Terrestrial Causes Of Climatic Changes

The Earth's Crust And The Sun

The Effect Of Other Bodies On The Sun

Post-glacial Crustal Movements And Climatic Changes

The Origin Of Loess

The Uniformity Of Climate

Glaciation According To The Solar-cyclonic Hypothesis[38]

The Climatic Stress Of The Fourteenth Century






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.

FOOTNOTES:

[Footnote 56: Chamberlin and Salisbury: Geology, 1906, Vol. III, pp.
405-412.]

[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.]





Next: Causes Of Mild Geological Climates

Previous: Some Problems Of Glacial Periods



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