Geoscience Reference
In-Depth Information
that it was existing from the earlier glaciation and this last glaciation continued only
within the earlier ice accumulation. Many warm periods even with higher mean
temperatures existed during the past hundreds of thousands of years when glaciers,
surviving up to now, partly melted and shrank and then grew again in cool periods.
Their dynamics of growth and shrinking is traceable by moraines. When a glacier
partly melted, the space between the moraine and the receding tail of glacier was
entirely adequate to hold the water originating from the melted ice. Hence, many
mountain lakes evolved and grew. The number and size of those past lakes prove
that the regress of glaciers in some of the earlier interglacials was more distinctive
than now. Considering the above, we conclude that the present-day worries of some
climatologists about the complete melting of alpine glaciers due to recent warming
are groundless.
We have mentioned earlier that the majority of recent soils did not originate
directly from weathered rock. They developed in place on unconsolidated sedi-
ments that were deposited by fl oodwaters or on deposits initially weathered at
higher elevations and further eroded and moved downhill by water. Even if both
processes appear identical, they are not the same because the fl oodwaters trans-
ported the material from great distances and deposited them on large areas, while
local erosion was restricted to small distances.
Another alternative was the transport of the weathered material by wind during
dust storms occurring in the past. Dust storms were very frequent and extremely
strong during as many as 11 major glaciations in the last 2.5 million years. Here, we
limit our discussion to the last two glaciations. The earlier one called either Riss in
Central Europe (Saalian in northern Europe) or Illinoian in the USA lasted roughly
from 200,000 to 130,000 years BP. The next new glaciation denoted as Würm in
Europe (Weichselian in northern Europe) or Wisconsin in the USA was closer to
recent times, again roughly in the span from 110,000 to 11,500 years BP. These two
glacials were separated by the warmer Eemian interglacial (Sangamonian in the
USA). Actually, we are now living in the interglacial in the Holocene that started
11,700 years ago and shall end in a near future. Here, referring to a near future, we
are speaking in geologic terms where the time scale is at least a thousand times
greater than our contemporary scale of human beings. Moreover, our indication of
time for glacial periods may change slightly owing to the region.
Because the soil surface was not suffi ciently covered by vegetation in the zone
south of the great glaciers of the Northern Hemisphere during the glacial periods, it
was not protected from blowing winds. Fine soil particles were sucked into the air
by hurricane-like winds and deposited hundreds and thousands of kilometers away
from the place of their origin. Moreover, with huge volumes of seawater being fro-
zen and kept in glaciers, the sea level sank by about 100-150 m. The muddy sea
bottoms emerging along continental shelves exposed to dry, cold atmosphere started
to be the probable source of the majority of dust particles in storms. The dust was
deposited as loess in thicknesses up to several tens of meters. Today, the largest
areas of loess occur in China and have thicknesses greater than 100 m, mantling
hillsides and forming extensive loess plateaus. Large loess layers also exist in
Central Europe, Ukraine, and a portion of US prairies.
