Factors of Pedogenesis - Major Soil Groups of the World - page 39

Table 2.4 Glacial chronology according to Wikipedia.
Thousands of yrs. North America Alps Northern Europe

11-present Present interglacial period
110-11 Wisconsinan Würm Weichsel
128-110 Sangamonian Riss/Würm interglacial Eemian
200-128 Illinoian Riss Saale
300/380-200 Yarmouth Mindel/Riss interglacial Holstein
455-380/300 Kansan Mindel Elsterian
620-455 Aftonian Günz/Mindel interglacial Cromerian
680-620 Nebraskan Günz Menapian
2000-680 At least 15 to 20 glacial advances during this period
The glacial periods are in white, the much shorter interglacial periods are in grey. The table is very
approximate because it does not properly take into account all the finer climatic fluctuations. It is
introduced here to define the ideas and furnish the terminology found in the literature.

For glaciation to start in the temperate zone of the Northern Hemisphere,
it is not necessary that the winter be particularly cold since it is already
snowing on the peaks. It is enough that the summer is cooler so that
the snow does not melt completely but is accumulated century after
century. Three cosmographic situations favour this process by reducing
the amounts of energy received from the Sun:
￿ when the Earth's axis is less inclined on the ecliptic (21º to 24.5º),
the solar rays are a little more oblique in summer; this happens
with a cycle of 41,000 years;
￿ when a year is exactly over, the Earth has not made a complete
circuit of its elliptical orbit since the previous year; this is
precession of the equinoxes; the cycle is about 22,000 years long;
the summer is cooler if it occurs when the Earth is farthest from
the Sun in its orbit (aphelion);
￿ the Earth's orbit has varying eccentricity; thus, it is more or
less elliptical or circular; when it is in the phase of maximum
stretching it cools down the summer if it occurs when the Earth
is at aphelion; this cycle is of 100,000 years.
These phenomena are independent. When they occur at the same
time the reduction in received energy can be as much as 20 per cent in
the higher latitudes. Then the Northern Hemisphere enters a glaciation.
Many feedback effects reinforce this phenomenon. Firstly, the glaciers
reflect the light of the Sun, thereby accelerating overall cooling. Secondly,
when it becomes cold, biological life is less active in the soils of the
Northern Hemisphere; the CO 2 content of the atmosphere drops; the
natural greenhouse effect is less marked. The Southern Hemisphere is