Abstracts of Symposium 55 and some papers from other related Symposia.

Geography of the Upper Quaternary Sediments in the Center of Russian Plain

Tatyana E. YAKUSHEVA and Alexander O. MAKEEV, Dokuchaev Soil Institute, 109017, Pyzhevsky lane, 7, Moscow, Russia.

The geography of soil forming rocks, that are in the most part of the Russian Plain represented by the upper layer of Quaternary strata have been established in the final stage of glacio-lithomorphogenesis, complicated by simultaneous or successive cryogenic processes. Microstratigraphy of sediments within the surface soil profile, as well as genetic types of glacial relief and glacial sediments are determined by paleoglacial zonality. Three areas may be outlined within the center of Russian Plain.

The main loess area comprising terrain with classic Chernozem and Gray Forest soils (most of the Ukraine and south-west part of Russia). It's main feature is vast interfluves, completely covered by universal mantle of loess, divided only by river valleys and fluvioglacial lowlands. Such a mantles could be formed in an area of a free glacial flow on a relatively flat terrain, slightly descending to the South. Vast fluvioglacial basins served as a main source of loess material. Swell-and-swale topography, that is a characteristic feature of such a terrain, have determined loess stratigraphy within surface soil profiles. In the bottom of depressions and on their slopes gradual sedimentation of an eolian dust was influenced by slope processes and intense freezing. Frost cracks on different levels, slope phenomena and humus horizons make it possible to distinguish separate stages of loess sedimentation, that is not visible in soils on interdepression surfaces.

An area of Moscow (Riss II) glaciation, comprising terrain with Podzolic soils (Alfisols), central part of European Russia. We can outline separate bodies of loess, ground moraine, areal sands, limnoglacial clays. Such a diversity is a result of a very complex glaciodynamic structure of glacial cover and high dissection of relief. There is a sequence of active and passive zones and numerous marginal and terminal forms due to local barriers and glacier oscillations. The key to understanding the formation of different lithomorphotypes in this area is the fact that day surfaces on different relief types here are heterochronous. The first surfaces that appeared bare were end-moraine chains and areas of moraine accumulation in the ice-shed zones. These surfaces served as a source of a heavy silty material that had formed loam mantles on some blocks of dead ice. Melting of this dead ice had caused over moistening of silty sediments and then its compaction and cracking. This initial cracking was then inherited by blocky structure of Bt horizon. When moraine surfaces were stabilised the source of wind-blown material had been changed - it was then fluvioglacial sands on surfaces, newly appeared after the decrease of water level. This final layer, windblown from sand areas looks more like typical loess - friable and light in texture. It is also more universally spread. Loess sediments in this area differ from those of a main loess area because they have regular two-layered stratification. These layers now constitute the texture-differentiated profile of a Podzolic soils (Alfisols).

An area of Valdai (Wurm) glaciation. The sediment cover was determined by a double barrier: from the South - the main ice-shed belt; and from the North - marine transgressions. Lack of a free flow had caused an abundance of limnoglacial clays and fluvial sands. High flooding had prevented eolian activity. That is why loess sediments are rare here. Wherever loess mantles occurs, their strata is characterised by slope and frost phenomena, similar to those in the main loess area.

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