Loess deposition

One of the most conspicuous geological effects of periods of glacial climate in the northern zones of Europe was the formation of thick deposits of wind-blown loess. The precise origins and mode of formation of these deposits are still open to some debate (Butzer 1972; Bowen 1978; Wintle 1990). The deposits were evidently laid down by heavy wind action and were presumably derived from areas where extensive spreads of finegrained sand or silt deposits were exposed to the effects of wind erosion without the protection of a continuous vegetation cover. But how far they were derived from river or beach deposits and how far from various forms of glacial outwash deposits is more debatable, and no doubt varied between different locations.

In western Europe, thick deposits of loess can be traced in a broad arc from the western coasts of Brittany and Normandy through Picardy and the Paris basin into Belgium and northern Germany. Southwards, occasional deposits of loess can be traced into the Loire valley and into at least the northern parts of Burgundy (Fig. 2.18). The thickness attained can be impressive; in parts of northern France, for example, it is estimated that up to 10-15 metres of loess may have built up during the various stages of the last glaciation.

The major problem has always been precise dating of loess deposits. Wintle et al (1984) have suggested that the formation of loess may be a rare event during the Pleistocene period as a whole, perhaps occupying only 10 percent or less of Pleistocene time. The only form of absolute dating which can be applied directly to the loess deposits is that of ther-moluminescence (TL) dating - based on the presumed bleaching of the TL signal in quartz grains by the action of sunlight as particles were transported through the air (Wintle 1990). By applying this technique to loess profiles in Normandy, Wintle et al. (1984) have suggested that the principal

Figure 2.18 Distribution of loess and related wind-blown deposits in western Europe. The deposits are likely to have formed during a number of different glacial episodes over the past 500,000 years. After Flint 1957.

Figure 2.18 Distribution of loess and related wind-blown deposits in western Europe. The deposits are likely to have formed during a number of different glacial episodes over the past 500,000 years. After Flint 1957.

phases of loess deposition during the last glaciation were concentrated mainly during isotope stages 2 and 4, with perhaps an earlier, brief period of accumulation during stages 5b or 5d (see also Parks & Rendell 1992). A similar pattern has been suggested recently by Van Vliet-Lanoe (1989, 1990) based on the overall stratigraphy of the loess deposits and associated soil profiles (Fig. 2.19). The intervening, warmer episodes of the Pleistocene were characterized by heavy weathering of the existing loess deposits and the formation of a series of complex soil profiles, such as the well known Warneton or Rocourt soil com-

Climate / Humidity Isotope _stage

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