Geology Reference
In-Depth Information
either an interglacial or interstadial period, north- and east-facing slopes would have
experienced freeze-thaw conditions fi rst. But, during the height of a cold period, south-
and west-facing slopes would probably have experienced greater frequencies of freeze-
thaw because north- and east-facing slopes would have remained frozen for a longer period
of time. Therefore, depending upon the severity of the temperature drop, frost-action
processes might be favored either on (i) south- and west-facing slopes (being “warmer”)
or (ii) north- and east-facing slopes (being “colder”). This has obvious implications for
the effectiveness of mass-wasting processes and slope evolution.
Related to solar radiation inputs is the question of permafrost. In high latitudes the
long Arctic “night” enables the intense cooling of the ground surface that favors perma-
frost formation. However, the relatively short duration of the mid-latitude winter and the
fact that, even in winter, temperatures would have followed a diurnal pattern, means that
the extremes of winter temperatures, currently experienced by high latitudes today, may
not have existed. This would have been especially true for oceanic environments, less so
for continental environments. For example, it has been suggested that temperature depres-
sions as great as 10-12 °C might have existed in the central European lowlands. Permafrost
would certainly have formed. However, in more oceanic locations of western England and
southwest Ireland, it is unreasonable to envisage the formation of thick permafrost. Snow
cover would also have been an important factor infl uencing the ground-thermal regime.
In all likelihood, permafrost was a transitory phenomenon, forming only during the
coldest periods of the cold-climate oscillations. Deep seasonal frost, rather than perma-
frost, was probably more typical for much of the cold periods.
Another major difference between the mid-latitudes of the Pleistocene and the high
latitudes today arises from the fact that global atmospheric circulation patterns were dif-
ferent during glacial times when large ice sheets formed in the mid- and high-latitudes. In
all probability, there was an increased intensity of the climatic gradients equator-wards
away from the ice margins. The mid-latitude westerlies would not only have been displaced
towards the equator but they would also have been strong (see earlier). At the same time,
anticyclonic (high pressure) conditions would have developed over the ice sheets and over
ice-free continental lowlands. In Central Europe, for example, the development of strong
high-pressure systems, especially in winter, would have led to the “blocking” of traveling
disturbances in the westerly winds. Disturbances would have been diverted either north-
wards towards Iceland, Svalbard, and northern Scandinavia or southwards towards the
Mediterranean. Under these conditions, central and eastern Europe would have experi-
enced below-average precipitation while western France, northwestern Spain, southwest
England, and southern Ireland would have experienced above-average amounts. Thus,
one might expect only 40% of today's amount to have fallen in parts of Central and
Eastern Europe. In Poland, for example, where current precipitation amounts are approxi-
mately 400-500 mm per annum, this would mean less than 250 mm per annum, an amount
comparable to many High-Arctic regions today. It must also be remembered that the
Atlantic Ocean would have been several hundred kilometers further to the west, the North
Sea would not have existed, and the eastern half of England would have been connected
to the European continent.
Similar paleo-geographic considerations are relevant in other areas of the mid-
latitudes. For example, in northeastern China, a complicating factor was the continuing
uplift of the Tibetan Plateau throughout the Quaternary. This caused major realignment
of the monsoon winds.
There are many reasons to suggest that wind action was important in mid-latitudes
during the Pleistocene. First, as mentioned earlier, wind and pressure gradients
would have been high due to the contraction of the various zonal climate belts south of
