Geology Reference
In-Depth Information
the high-pressure systems that would have formed over the continental ice sheets. Second,
these anticyclonic conditions would have promoted strong gravity-induced (katabatic)
winds. Third, an abundant supply of fi ne material suitable for abrasion and eolian trans-
port was readily available on the outwash plains located at the margins of the ice sheets.
Fourth, the general decrease in precipitation resulting from the colder conditions of glacial
times and the anticyclonic blocking of traveling disturbances in the zonal westerly winds
would have favored aridity and defl ation.
The nature of mid-latitude Pleistocene rivers also needs brief mention. In all probabil-
ity, fl uvial activity was greater than conventionally imagined. Because of the absence of
the Arctic “night,” running water would have been a year-round phenomenon, and cer-
tainly more than just the two- or three-month activity that is typical of many high latitudes
today (see Chapter 10). The spring fl ood (nival freshet) would not have been so marked
because some winter snowfall would have entered the drainage system during the winter
period. Finally, if permafrost were either discontinuous or absent, there would have been
more infi ltration into the ground and less direct runoff. The large Pleistocene drainage
systems of the cold periods are discussed more fully in Chapter 12.
11.5.2. Mammals and Ecosystems
The most reliable criterion for identifying the extent of the mid-latitude periglacial envi-
ronments of the Pleistocene is the distribution of plants, and the previous extent of the
tundra, steppe tundra, and forest-tundra ecozones. Palynological studies, together with
those involving steppe and tundra faunal remains, help determine environmental
conditions.
For our purposes, faunal and insect remains, pollen taxa, and even the drawings of
ancient cave men (Sutcliffe, 1985) provide the most convincing and dramatic evidence.
Certain insects, such as beetles (coleoptera) and land snails (non-marine mollusca) survive
only under limiting temperature and moisture ranges. Many species are only found today
in sub-arctic and arctic environments. Both beetles and land snails have been used suc-
cessfully in paleo-reconstruction (Keen, 1985; Kerney, 1963; Morgan and Morgan, 1980;
Morgan et al., 1983; Sher et al., 1979). Because they live at or near the ground surface,
they provide good indications of ground temperatures. The same is true for pollen with
regards to July air temperatures (Kolstrup, 1980). The larger mammals, with their greater
mobility, have less rigid environmental controls, but even here the evidence is clear. For
example, caves in the Dordogne region of southern France were occupied by early man
∼
20-30 ka ago at the height of the last glacial maximum (Delluc et al., 2001). Crude wall
drawings depict a variety of now-extinct animals, including woolly mammoths, bears,
woolly rhinoceros, and reindeer (Figure 11.11). Likewise, in the mid-latitude prairies of
North America, the Middle- and Late-Quaternary stratigraphic records reveal mammal
bones from a variety of animals that, now, are either extinct or live in sub-arctic and arctic
environments (Stalker, 1984; Stalker and Churcher, 1982).
11.5.3. Perennial or Seasonal Frost?
In the context of mid-latitudes, it is important to distinguish, without ambiguity, between
perennially-frozen and seasonally-frozen ground. This is because the warm limits of the
mid-latitude periglacial domain merged with the non-periglacial environments of lower
latitudes. It is probable that deep seasonal frost, rather than permafrost, characterized
major areas of the mid-latitudes during much of the Pleistocene.
