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Conclusion

Causes Of Mild Geological Climates

Hypotheses Of Climatic Change

Some Problems Of Glacial Periods

The Variability Of Climate

The Climate Of History

The Climatic Stress Of The Fourteenth Century

Glaciation According To The Solar-cyclonic Hypothesis[38]

The Uniformity Of Climate

The Solar Cyclonic Hypothesis



Least Viewed

The Changing Composition Of Oceans And Atmosphere

Terrestrial Causes Of Climatic Changes

The Sun's Journey Through Space

Post-glacial Crustal Movements And Climatic Changes

The Effect Of Other Bodies On The Sun

The Earth's Crust And The Sun

The Origin Of Loess

The Solar Cyclonic Hypothesis

The Uniformity Of Climate

The Climatic Stress Of The Fourteenth Century






Glaciation According To The Solar-cyclonic Hypothesis[38]








The remarkable phenomena of glacial periods afford perhaps the best
available test to which any climatic hypothesis can be subjected. In
this chapter and the two that follow, we shall apply this test. Since
much more is known about the recent Great Ice Age, or Pleistocene
glaciation, than about the more ancient glaciations, the problems of the
Pleistocene will receive especial attention. In the present chapter the
oncoming of glaciation and the subsequent disappearance of the ice will
be outlined in the light of what would be expected according to the
solar-cyclonic hypothesis. Then in the next chapter several problems of
especial climatic significance will be considered, such as the
localization of ice sheets, the succession of severe glacial and mild
inter-glacial epochs, the sudden commencement of glaciation and the
peculiar variations in the height of the snow line. Other topics to be
considered are the occurrence of pluvial or rainy climates in
non-glaciated regions, and glaciation near sea level in subtropical
latitudes during the Permian and Proterozoic. Then in Chapter IX we
shall consider the development and distribution of the remarkable
deposits of wind-blown material known as loess.

Facts not considered at the time of framing an hypothesis are especially
significant in testing it. In this particular case, the cyclonic
hypothesis was framed to explain the historic changes of climate
revealed by a study of ruins, tree rings, and the terraces of streams
and lakes, without special thought of glaciation or other geologic
changes. Indeed, the hypothesis had reached nearly its present form
before much attention was given to geological phases of the problem.
Nevertheless, it appears to meet even this severe test.

According to the solar-cyclonic hypothesis, the Pleistocene glacial
period was inaugurated at a time when certain terrestrial conditions
tended to make the earth especially favorable for glaciation. How these
conditions arose will be considered later. Here it is enough to state
what they were. Chief among them was the fact that the continents stood
unusually high and were unusually large. This, however, was not the
primary cause of glaciation, for many of the areas which were soon to be
glaciated were little above sea level. For example, it seems clear that
New England stood less than a thousand feet higher than now. Indeed,
Salisbury[39] estimates that eastern North America in general stood not
more than a few hundred feet higher than now, and W. B. Wright[40]
reaches the same conclusion in respect to the British Isles.
Nevertheless, widespread lands, even if they are not all high, lead to
climatic conditions which favor glaciation. For example, enlarged
continents cause low temperature in high latitudes because they
interfere with the ocean currents that carry heat polewards. Such
continents also cause relatively cold winters, for lands cool much
sooner than does the ocean. Another result is a diminution of water
vapor, not only because cold air cannot hold much vapor, but also
because the oceanic area from which evaporation takes place is reduced
by the emergence of the continents. Again, when the continents are
extensive the amount of carbonic acid gas in the atmosphere probably
decreases, for the augmented erosion due to uplift exposes much igneous
rock to the air, and weathering consumes the atmospheric carbon dioxide.
When the supply of water vapor and of atmospheric carbon dioxide is
small, an extreme type of climate usually prevails. The combined result
of all these conditions is that continental emergence causes the climate
to be somewhat cool and to be marked by relatively great contrasts from
season to season and from latitude to latitude.

When the terrestrial conditions thus permitted glaciation, unusual solar
activity is supposed to have greatly increased the number and severity
of storms and to have altered their location, just as now happens at
times of many sunspots. If such a change in storminess had occurred when
terrestrial conditions were unfavorable for glaciation, as, for example,
when the lands were low and there were widespread epicontinental seas in
middle and high latitudes, glaciation might not have resulted. In the
Pleistocene, however, terrestrial conditions permitted glaciation, and
therefore the supposed increase in storminess caused great ice sheets.

The conditions which prevail at times of increased storminess have been
discussed in detail in Earth and Sun. Those which apparently brought
on glaciation seem to have acted as follows: In the first place the
storminess lowered the temperature of the earth's surface in several
ways. The most important of these was the rapid upward convection in the
centers of cyclonic storms whereby abundant heat was carried to high
levels where most of it was radiated away into space. The marked
increase in the number of tropical cyclones which accompanies increased
solar activity was probably important in this respect. Such cyclones
carry vast quantities of heat and moisture out of the tropics. The
moisture, to be sure, liberates heat upon condensing, but as
condensation occurs above the earth's surface, much of the heat escapes
into space. Another reason for low temperature was that under the
influence of the supposedly numerous storms of Pleistocene times
evaporation over the oceans must have increased. This is largely because
the velocity of the winds is relatively great when storms are strong and
such winds are powerful agents of evaporation. But evaporation requires
heat, and hence the strong winds lower the temperature.[B]

The second great condition which enabled increased storminess to bring
on glaciation was the location of the storm tracks. Kullmer's maps, as
illustrated in Fig. 2, suggest that a great increase in solar activity,
such as is postulated in the Pleistocene, might shift the main storm
track poleward even more than it is shifted by the milder solar changes
during the twelve-year sunspot cycle. If this is so, the main track
would tend to cross North America through the middle of Canada instead
of near the southern border. Thus there would be an increase in
precipitation in about the latitude of the Keewatin and Labradorean
centers of glaciation. From what is known of storm tracks in Europe, the
main increase in the intensity of storms would probably center in
Scandinavia. Fig. 3 in Chapter V bears this out. That figure, it will be
recalled, shows what happens to precipitation when solar activity is
increasing. A high rate of precipitation is especially marked in the
boreal storm track, that is, in the northern United States, southern
Canada, and northwestern Europe.

Another important condition in bringing on glaciation would be the fact
that when storms are numerous the total precipitation appears to
increase in spite of the slightly lower temperature. This is largely
because of the greater evaporation. The excessive evaporation arises
partly from the rapidity of the winds, as already stated, and partly
from the fact that in areas where the air is clear the sun would
presumably be able to act more effectively than now. It would do so
because at times of abundant sunspots the sun in our own day has a
higher solar constant than at times of milder activity. Our whole
hypothesis is based on the supposition that what now happens at times of
many sunspots was intensified in glacial periods.

A fourth condition which would cause glaciation to result from great
solar activity would be the fact that the portion of the yearly
precipitation falling as snow would increase, while the proportion of
rain would diminish in the main storm track. This would arise partly
because the storms would be located farther north than now, and partly
because of the diminution in temperature due to the increased
convection. The snow in itself would still further lower the
temperature, for snow is an excellent reflector of sunlight. The
increased cloudiness which would accompany the more abundant storms
would also cause an unusually great reflection of the sunlight and still
further lower the temperature. Thus at times of many sunspots a strong
tendency toward the accumulation of snow would arise from the rapid
convection and consequent low temperature, from the northern location of
storms, from the increased evaporation and precipitation, from the
larger percentage of snowy rather than rainy precipitation, and from the
great loss of heat due to reflection from clouds and snow.

If events at the beginning of the last glacial period took place in
accordance with the cyclonic hypothesis, as outlined above, one of the
inevitable results would be the production of snowfields. The places
where snow would accumulate in special quantities would be central
Canada, the Labrador plateau, and Scandinavia, as well as certain
mountain regions. As soon as a snowfield became somewhat extensive, it
would begin to produce striking climatic alterations in addition to
those to which it owed its origin.[41] For example, within a snowfield
the summers remain relatively cold. Hence such a field is likely to be
an area of high pressure at all seasons. The fact that the snowfield is
always a place of relatively high pressure results in outblowing surface
winds except when these are temporarily overcome by the passage of
strong cyclonic storms. The storms, however, tend to be concentrated
near the margins of the ice throughout the year instead of following
different paths in each of the four seasons. This is partly because
cyclonic lows always avoid places of high pressure and are thus pushed
out of the areas where permanent snow has accumulated. On the other
hand, at times of many sunspots, as Kullmer has shown, the main storm
track tends to be drawn poleward, perhaps by electrical conditions.
Hence when a snowfield is present in the north, the lows, instead of
migrating much farther north in summer than in winter, as they now do,
would merely crowd on to the snowfield a little farther in summer than
in winter. Thus the heavy precipitation which is usual in humid climates
near the centers of lows would take place near the advancing margin of
the snowfield and cause the field to expand still farther southward.

The tendency toward the accumulation of snow on the margins of the
snowfields would be intensified not only by the actual storms
themselves, but by other conditions. For example, the coldness of the
snow would tend to cause prompt condensation of the moisture brought by
the winds that blow toward the storm centers from low latitudes. Again,
in spite of the general dryness of the air over a snowfield, the lower
air contains some moisture due to evaporation from the snow by day
during the clear sunny weather of anti-cyclones or highs. Where this is
sufficient, the cold surface of the snowfields tends to produce a frozen
fog whenever the snowfield is cooled by radiation, as happens at night
and during the passage of highs. Such a frozen fog is an effective
reflector of solar radiation. Moreover, because ice has only half the
specific heat of water, and is much more transparent to heat, such a
"radiation fog" composed of ice crystals is a much less effective
retainer of heat than clouds or fog made of unfrozen water particles.
Shallow fogs of this type are described by several polar expeditions.
They clearly retard the melting of the snow and thus help the icefield
to grow.

For all these reasons, so long as storminess remained great, the
Pleistocene snowfields, according to the solar hypothesis, must have
deepened and expanded. In due time some of the snow was converted into
glacial ice. When that occurred, the growth of the snowfield as well as
of the ice cap must have been accelerated by glacial movement. Under
such circumstances, as the ice crowded southward toward the source of
the moisture by which it grew, the area of high pressure produced by its
low temperature would expand. This would force the storm track southward
in spite of the contrary tendency due to the sun. When the ice sheet had
become very extensive, the track would be crowded relatively near to the
northern margin of the trade-wind belt. Indeed, the Pleistocene ice
sheets, at the time of their maximum extension, reached almost as far
south as the latitude now marking the northern limit of the trade-wind
belt in summer. As the storm track with its frequent low pressure and
the subtropical belt with its high pressure were forced nearer and
nearer together, the barometric gradient between the two presumably
became greater, winds became stronger, and the storms more intense.

This zonal crowding would be of special importance in summer, at which
time it would also be most pronounced. In the first place, the storms
would be crowded far upon the ice cap which would then be protected from
the sun by a cover of fog and cloud more fully than at any other season.
Furthermore, the close approach of the trade-wind belt to the storm belt
would result in a great increase in the amount of moisture drawn from
the belt of evaporation which the trade winds dominate. In the
trade-wind belt, clear skies and high temperature make evaporation
especially rapid. Indeed, in spite of the vast deserts it is probable
that more than three-fourths of the total evaporation now taking place
on the earth occurs in the belt of trades, an area which includes about
one-half of the earth's surface.

The agency which could produce this increased drawing northward of
moisture from the trade-wind belt would be the winds blowing into the
lows. According to the cyclonic hypothesis, many of these lows would be
so strong that they would temporarily break down the subtropical belt of
high pressure which now usually prevails between the trades and the zone
of westerly winds. This belt is even now often broken by tropical
cyclones. If the storms of more northerly regions temporarily destroyed
the subtropical high-pressure belt, even though they still remained on
its northern side, they would divert part of the trade winds. Hence the
air which now is carried obliquely equatorward by those winds would be
carried spirally northward into the cyclonic lows. Precipitation in the
storm track on the margin of the relatively cold ice sheet would thus be
much increased, for most winds from low latitudes carry abundant
moisture. Such a diversion of moisture from low latitudes probably
explains the deficiency of precipitation along the heat equator at times
of solar activity, as shown in Fig. 3. Taken as a whole, the summer
conditions, according to the cyclonic hypothesis, would be such that
increased evaporation in low latitudes would cooeperate with increased
storminess, cloudiness, and fog in higher latitudes to preserve and
increase the accumulation of ice upon the borders of the ice sheet. The
greater the storminess, the more this would be true and the more the ice
sheet would be able to hold its own against melting in summer. Such a
combination of precipitation and of protection from the sun is
especially important if an ice sheet is to grow.

The meteorologist needs no geologic evidence that the storm track was
shoved equatorward by the growth of the ice sheet, for he observes a
similar shifting whenever a winter's snow cap occupies part of the
normal storm tract. The geologist, however, may welcome geologic
evidence that such an extreme shift of the storm track actually occurred
during the Pleistocene. Harmer, in 1901, first pointed out the evidence
which was repeated with approval by Wright of the Ireland Geological
Survey in 1914.[42] According to these authorities, numerous boulders of
a distinctive chalk were deposited by Pleistocene icebergs along the
coast of Ireland. Their distribution shows that at the time of maximum
glaciation the strong winds along the south coast of Ireland were from
the northeast while today they are from the southwest. Such a reversal
could apparently be produced only by a southward shift of the center of
the main storm track from its present position in northern Ireland,
Scotland, and Norway to a position across northern France, central
Germany, and middle Russia. This would mean that while now the centers
of the lows commonly move northeastward a short distance north of
southern Ireland, they formerly moved eastward a short distance south of
Ireland. It will be recalled that in the northern hemisphere the winds
spiral into a low counter-clockwise and that they are strongest near the
center. When the centers pass not far north of a given point, the strong
winds therefore blow from the west or southwest, while when the centers
pass just south of that point, the strong winds come from the east or
northeast.

In addition to the consequences of the crowding of the storm track
toward the trade-wind belt, several other conditions presumably operated
to favor the growth of the ice sheet. For example, the lowering of the
sea level by the removal of water to form the snowfields and glaciers
interfered with warm currents. It also increased the rate of erosion,
for it was equivalent to an uplift of all the land. One consequence of
erosion and weathering was presumably a diminution of the carbon dioxide
in the atmosphere, for although the ice covered perhaps a tenth of the
lands and interfered with carbonation to that extent, the removal of
large quantities of soil by accelerated erosion on the other nine-tenths
perhaps more than counterbalanced the protective effect of the ice. At
the same time, the general lowering of the temperature of the ocean as
well as the lands increased the ocean's capacity for carbon dioxide and
thus facilitated absorption. At a temperature of 50 deg.F. water absorbs 32
per cent more carbon dioxide than at 68 deg.. The high waves produced by the
severe storms must have had a similar effect on a small scale. Thus the
percentage of carbon dioxide in the atmosphere was presumably
diminished. Of less significance than these changes in the lands and the
air, but perhaps not negligible, was the increased salinity of the ocean
which accompanied the removal of water to form snow, and the increase of
the dissolved mineral load of the rejuvenated streams. Increased
salinity slows up the deep-sea circulation, as we shall see in a later
chapter. This increases the contrasts from zone to zone.

At times of great solar activity the agencies mentioned above would
apparently cooeperate to cause an advance of ice sheets into lower
latitudes. The degree of solar activity would have much to do with the
final extent of the ice sheets. Nevertheless, certain terrestrial
conditions would tend to set limits beyond which the ice would not
greatly advance unless the storminess were extraordinarily severe. The
most obvious of these conditions is the location of oceans and of
deserts or semi-arid regions. The southwestward advance of the European
ice sheet and the southeastward advance of the Labradorean sheet in
America were stopped by the Atlantic. The semi-aridity of the Great
Plains, produced by their position in the lee of the Rocky Mountains,
stopped the advance of the Keewatin ice sheet toward the southwest. The
advance of the European ice sheet southeast seems to have been stopped
for similar reasons. The cessation of the advance would be brought about
in such an area not alone by the light precipitation and abundant
sunshine, but by the dryness of the air, and also by the power of dust
to absorb the sun's heat. Much dust would presumably be drawn in from
the dry regions by passing cyclonic storms and would be scattered over
the ice.

The advance of the ice is also slowed up by a rugged topography, as
among the Appalachians in northern Pennsylvania. Such a topography
besides opposing a physical obstruction to the movement of the ice
provides bare south-facing slopes which the sun warms effectively. Such
warm slopes are unfavorable to glacial advance. The rugged topography
was perhaps quite as effective as the altitude of the Appalachians in
causing the conspicuous northward dent in the glacial margin in
Pennsylvania. Where glaciers lie in mountain valleys the advance beyond
a certain point is often interfered with by the deployment of the ice at
the mouths of gorges. Evaporation and melting are more rapid where a
glacier is broad and thin than where it is narrow and thick, as in a
gorge. Again, where the topography or the location of oceans or dry
areas causes the glacial lobes to be long and narrow, the elongation of
the lobe is apparently checked in several ways. Toward the end of the
lobe, melting and evaporation increase rapidly because the planetary
westerly winds are more likely to overcome the glacial winds and sweep
across a long, narrow lobe than across a broad one. As they cross the
lobe, they accelerate evaporation, and probably lessen cloudiness, with
a consequent augmentation of melting. Moreover, although lows rarely
cross a broad ice sheet, they do cross a narrow lobe. For example,
Nansen records that strong lows occasionally cross the narrow southern
part of the Greenland ice sheet. The longer the lobe, the more likely it
is that lows will cross it, instead of following its margin. Lows which
cross a lobe do not yield so much snow to the tip as do those which
follow the margin. Hence elongation is retarded and finally stopped even
without a change in the earth's general climate.

Because of these various reasons the advances of the ice during the
several epochs of a glacial period might be approximately equal, even if
the durations of the periods of storminess and low temperature were
different. Indeed, they might be sub-equal, even if the periods differed
in intensity as well as length. Differences in the periods would
apparently be manifested less in the extent of the ice than in the depth
of glacial erosion and in the thickness of the terminal moraines,
outwash plains, and other glacial or glacio-fluvial formations.

Having completed the consideration of the conditions leading to the
advance of the ice, let us now consider the condition of North America
at the time of maximum glaciation.[43] Over an area of nearly four
million square miles, occupying practically all the northern half of the
continent and part of the southern half, as appears in Fig. 6, the
surface was a monotonous and almost level plain of ice covered with
snow. When viewed from a high altitude, all parts except the margins
must have presented a uniformly white and sparkling appearance. Along
the margins, however, except to the north, the whiteness was irregular,
for the view must have included not only fresh snow, but moving clouds
and dirty snow or ice. Along the borders where melting was in progress
there was presumably more or less spottedness due to morainal material
or glacial debris brought to the surface by ice shearage and wastage.
Along the dry southwestern border it is also possible that there were
numerous dark spots due to dust blown onto the ice by the wind.


(After Schuchert.)]

The great white sheet with its ragged border was roughly circular in
form, with its center in central Canada. Yet there were many departures
from a perfectly circular form. Some were due to the oceans, for, except
in northern Alaska, the ice extended into the ocean all the way from New
Jersey around by the north to Washington. On the south, topographic
conditions made the margin depart from a simple arc. From New Jersey to
Ohio it swung northward. In the Mississippi Valley it reached far south;
indeed most of the broad wedge between the Ohio and the Missouri rivers
was occupied by ice. From latitude 37 deg. near the junction of the Missouri
and the Mississippi, however, the ice margin extended almost due north
along the Missouri to central North Dakota. It then stretched westward
to the Rockies. Farther west lowland glaciation was abundant as far
south as western Washington. In the Rockies, the Cascades, and the
Sierra Nevadas glaciation was common as far south as Colorado and
southern California, respectively, and snowfields were doubtless
extensive enough to make these ranges ribbons of white. Between these
lofty ranges lay a great unglaciated region, but even in the Great Basin
itself, in spite of its present aridity, certain ranges carried
glaciers, while great lakes expanded widely.

In this vast field of snow the glacial ice slowly crept outward,
possibly at an average speed of half a foot a day, but varying from
almost nothing in winter at the north, to several feet a day in summer
at the south.[44] The force which caused the movement was the presence
of the ice piled up not far from the margins. Almost certainly, however,
there was no great dome from the center in Canada outward, as some early
writers assumed. Such a dome would require that the ice be many
thousands of feet thick near its center. This is impossible because of
the fact that ice is more voluminous than water (about 9 per cent near
the freezing point). Hence when subjected to sufficient pressure it
changes to the liquid form. As friction and internal heat tend to keep
the bottom of a glacier warm, even in cold regions, the probabilities
are that only under very special conditions was a continental ice sheet
much thicker than about 2500 feet. In Antarctica, where the temperature
is much lower than was probably attained in the United States, the ice
sheet is nearly level, several expeditions having traveled hundreds of
miles with practically no change in altitude. In Shackleton's trip
almost to the South Pole, he encountered a general rise of 3000 feet in
1200 miles. Mountains, however, projected through the ice even near the
pole and the geologists conclude that the ice is not very thick even at
the world's coldest point, the South Pole.

Along the margin of the ice there were two sorts of movement, much more
rapid than the slow creep of the ice. One was produced by the outward
drift of snow carried by the outblowing dry winds and the other and more
important was due to the passage of cyclonic storms. Along the border of
the ice sheet, except at the north, storm presumably closely followed
storm. Their movement, we judge, was relatively slow until near the
southern end of the Mississippi lobe, but when this point was passed
they moved much more rapidly, for then they could go toward instead of
away from the far northern path which the sun prescribes when solar
activity is great. The storms brought much snow to the icefield, perhaps
sometimes in favored places as much as the hundred feet a year which is
recorded for some winters in the Sierras at present. Even the
unglaciated intermontane Great Basin presumably received considerable
precipitation, perhaps twice as much as its present scanty supply. The
rainfall was enough to support many lakes, one of which was ten times as
large as Great Salt Lake; and grass was doubtless abundant upon many
slopes which are now dry and barren. The relatively heavy precipitation
in the Great Basin was probably due primarily to the increased number of
storms, but may also have been much influenced by their slow eastward
movement. The lows presumably moved slowly in that general region not
only because they were retarded and turned from their normal path by the
cold ice to the east, but because during the summer the area between the
Sierra snowfields on the west and the Rocky Mountain and Mississippi
Valley snowfields on the east was relatively warm. Hence it was normally
a place of low pressure and therefore of inblowing winds. Slow-moving
lows are much more effective than fast-moving ones in drawing moisture
northwestward from the Gulf of Mexico, for they give the moisture more
time to move spirally first northeast, under the influence of the normal
southwesterly winds, then northwest and finally southwest as it
approaches the storm center. In the case of the present lows, before
much moisture-laden air can describe such a circuit, first eastward and
then westward, the storm center has nearly always moved eastward across
the Rockies and even across the Great Plains. A result of this is the
regular decrease in precipitation northward, northwestward, and westward
from the Gulf of Mexico.

Along the part of the glacial margins where for more than 3000 miles the
North American ice entered the Atlantic and the Pacific oceans, myriads
of great blocks broke off and floated away as stately icebergs, to
scatter boulders far over the ocean floor and to melt in warmer climes.
Where the margin lay upon the lands numerous streams issued from beneath
the ice, milk-white with rock flour, and built up great outwash plains
and valley trains of gravel and sand. Here and there, just beyond the
ice, marginal lakes of strange shapes occupied valleys which had been
dammed by the advancing ice. In many of them the water level rose until
it reached some low point in the divide and then overflowed, forming
rapids and waterfalls. Indeed, many of the waterfalls of the eastern
United States and Canada were formed in just this way and not a few
streams now occupy courses through ridges instead of parallel to them,
as in pre-glacial times.

In the zone to the south of the continental ice sheet, the plant and
animal life of boreal, cool temperate, and warm temperate regions
commingled curiously. Heather and Arctic willow crowded out elm and oak;
musk ox, hairy mammoth, and marmot contested with deer, chipmunk, and
skunk for a chance to live. Near the ice on slopes exposed to the cold
glacial gales, the immigrant boreal species were dominant, but not far
away in more protected areas the species that had formerly lived there
held their own. In Europe during the last two advances of the great ice
sheet the caveman also struggled with fierce animals and a fiercer
climate to maintain life in an area whose habitability had long been
decreasing.

The next step in our history of glaciation is to outline the
disappearance of the ice sheets. When a decrease in solar activity
produced a corresponding decrease in storminess, several influences
presumably combined to cause the disappearance of the ice. Most of their
results are the reverse of those which brought on glaciation. A few
special aspects, however, some of which have been discussed in Earth
and Sun, ought to be brought to mind. A diminution in storminess
lessens upward convection, wind velocity, and evaporation, and these
changes, if they occurred, must have united to raise the temperature of
the lower air by reducing the escape of heat. Again a decrease in the
number and intensity of tropical cyclones presumably lessened the amount
of moisture carried into mid-latitudes, and thus diminished the
precipitation. The diminution of snowfall on the ice sheets when
storminess diminished was probably highly important. The amount of
precipitation on the sheets was presumably lessened still further by
changes in the storminess of middle latitudes. When storminess
diminishes, the lows follow a less definite path, as Kullmer's maps
show, and on the average a more southerly path. Thus, instead of all the
lows contributing snow to the ice sheet, a large fraction of the
relatively few remaining lows would bring rain to areas south of the ice
sheet. As storminess decreased, the trades and westerlies probably
became steadier, and thus carried to high latitudes more warm water than
when often interrupted by storms. Steadier southwesterly winds must have
produced a greater movement of atmospheric as well as oceanic heat to
high latitudes. The warming due to these two causes was probably the
chief reason for the disappearance of the European ice sheet and of
those on the Pacific coast of North America. The two greater American
ice sheets, however, and the glaciers elsewhere in the lee of high
mountain ranges, probably disappeared chiefly because of lessened
precipitation. If there were no cyclonic storms to draw moisture
northward from the Gulf of Mexico, most of North America east of the
Rocky Mountain barrier would be arid. Therefore a diminution of
storminess would be particularly effective in causing the disappearance
of ice sheets in these regions.

That evaporation was an especially important factor in causing the ice
from the Keewatin center to disappear, is suggested by the relatively
small amount of water-sorted material in its drift. In South Dakota, for
example, less than 10 per cent of the drift is stratified.[45] On the
other hand, Salisbury estimates that perhaps a third of the Labradorean
drift in eastern Wisconsin is crudely stratified, about half of that in
New Jersey, and more than half of the drift in western Europe.

When the sun's activity began to diminish, all these conditions, as well
as several others, would cooeperate to cause the ice sheets to disappear.
Step by step with their disappearance, the amelioration of the climate
would progress so long as the period of solar inactivity continued and
storms were rare. If the inactivity continued long enough, it would
result in a fairly mild climate in high latitudes, though so long as the
continents were emergent this mildness would not be of the extreme type.
The inauguration of another cycle of increased disturbance of the sun,
with a marked increase in storminess, would inaugurate another glacial
epoch. Thus a succession of glacial and inter-glacial epochs might
continue so long as the sun was repeatedly disturbed.

FOOTNOTES:

[Footnote 38: This chapter is an amplification and revision of the
sketch of the glacial period contained in The Solar Hypothesis of
Climatic Changes; Bull. Geol. Soc. Am., Vol. 25, 1914.]

[Footnote 39: R. D. Salisbury: Physical Geography of the Pleistocene, in
Outlines of Geologic History, by Willis, Salisbury, and others, 1910, p.
265.]

[Footnote 40: The Quaternary Ice Age, 1914, p. 364.]

[Footnote B: For fuller discussion of climatic controls see S. S.
Visher: Seventy Laws of Climate, Annals Assoc. Am. Geographers, 1922.]

[Footnote 41: Many of these alterations are implied or discussed in the
following papers:

1. F. W. Harmer: Influence of Winds upon the Climate of the Pleistocene;
Quart. Jour. Geol. Soc., Vol. 57, 1901, p. 405.

2. C. E. P. Brooks: Meteorological Conditions of an Ice Sheet; Quart.
Jour. Royal Meteorol. Soc., Vol. 40, 1914, pp. 53-70, and The Evolution
of Climate in Northwest Europe; op. cit., Vol. 47, 1921, pp. 173-194.

3. W. H. Hobbs: The Role of the Glacial Anticyclone in the Air
Circulation of the Globe; Proc. Am. Phil. Soc., Vol. 54, 1915, pp.
185-225.]

[Footnote 42: W. B. Wright: The Quaternary Ice Age, 1914, p. 100.]

[Footnote 43: The description of the distribution of the ice sheet is
based on T. C. Chamberlin's wall map of North America at the maximum of
glaciation, 1913.]

[Footnote 44: Chamberlin and Salisbury: Geology, 1906, Vol. 3, and W. H.
Hobbs: Characteristics of Existing Glaciers, 1911.]

[Footnote 45: S. S. Visher: The Geography of South Dakota; S. D. Geol.
Surv., 1918.]





Next: Some Problems Of Glacial Periods

Previous: The Climatic Stress Of The Fourteenth Century



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