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

onco ing 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.]