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SEDIMENTARY ENVIRONMENTS/Alluvial Fans, Alluvial Sediments and Settings 493 The same situation for determining datings and the sequence of processes and events is valid regarding the stratigraphy of alluvions in more extended riversides and main valleys The detailed regional examination and the comparability of the results may be exemplified by the studies described in the further reading section at the end of this chapter Sometimes the archaeological datings of single layers within alluvial strata evidence the covering of ancient sites of human presence and settlement by subsequent floods, as is the case of urban centres connected with the medieval Slavonic state of Greater Moravia (actually in Czechia: Poha´ nsko, Mikulcˇ ice, ´ herske´ Hradisˇ teˇ ) Stare´ Me˘ sto, and U Other chances of achieving more specific and precise dating of the sequence of Holocene events and processes in the stratigraphy of alluvions, are provided by trunks of trees engulfed by the fluvial deposits Trunks have been found frequently, for instance, in many central European valleys, as Elbe and Spree, Rhine and Main, or the Danube with its tributaries In the German literature, they are called ‘Rannen’ On a regional scale, there are numerous trunks within the sediment In the brown coal seams of central Germany (Saxonia, Saxony-Anhalt, Lusatia) the respective fluvial deposits are frequently well-exposed However, independent of the conditions of exposure, these trunks have provided the opportunity of dendrochronological datings Using this method can be applied to examination of the valley development in many cases This may be exemplified for some catchment areas in central Europe as that of the Danube and its tributaries, or that of the River Main The regional development of valley-net systems is reflected by the stratigraphy of alluvions A detailed documentation of varying stratigraphical observations in different parts of one and the same valley-net illustrates the regional differentiation in the course of events This differentiation has been exemplified carefully in the case of the River Vistula (Weichsel) with its tributaries in Poland The chronological foundation of this investigation has been completed by radiocarbon datings and pollen analysis However, in other parts of temperate Europe, careful and detailed stratigraphical examinations have covered different sections of the same catchment area Such investigations may be exemplified by the valley system of the River Severn in the British Isles (counties of Shropshire and Worcestershire, England) Alternating layers in the stratigraphy of alluvions reflect temporally changing conditions of accumulation, as in single valleys or valley segments, such as completely examined valley-net systems These changes record climatic consequences, with the varying amount of precipitation in the respective catchment area and its temporal peaks caused by floods They also reflect the changing intensity of human impact influencing the sedimentary yield of the respective river or creek This yield comprises effects resulting from the extension of open grounds, forest clearance, and utilisation of arable land in the respective catchment area The same causes have ruled the origin, development, and stratigraphy of alluvions in the Mediterranean Evidence has been provided previously for the Mediterranean and more recently, in particular, from the Iberian Peninsula In all the investigated catchment areas, the effect of human impacts depends on the respective regional course of settlement and land-use intensity The climatic causes take effect in extended regions of the European continent in a comparable way However, the detection of palaeoclimatic fluctuations, and especially of oscillations of the water balance in the course of the Holocene, is based on other stratigraphic observations as in exposures of freshwater lime deposits Among geostratigraphical, palaeontological (especially palynological and palaeomalacological), and archaeological evidence supporting the resulting concept concerning the palaeoclimatic development during Holocene, the stratigraphy of alluvions claims an important but not essential role Related to the youngest alluvions, historical record archives especially support the linking of distinct stratigraphical facts with particular palaeofloods However, irrespective of all other sources clarifying the palaeoenvironmental development as a whole and on the valley bottoms in particular, significant contributions to the reconstruction of previous conditions during the Holocene are provided by the alluvions and their stratigraphy See Also Europe: Holocene Sedimentary Processes: Fluvial Geomorphology Sedimentary Rocks: Rudaceous Rocks Further Reading Andrzejewski L and Jus´ kiewicz W (2003) Lithofacies diversification of the alluvia in the area of Kepa Dzikowska, Kepa Bazarowa, Kepa Stron´ ska and of the Vistulian floodplain near Torun´ Prace Geograficzne 189 (Warszawa): 159 176 Becker B (1982) Dendrochronologie und Palaăooăkologie subfossiler Baumstaămme aus Flubablagerungen Ein Beitrag zur nacheiszeitlichen Auenentwicklung im suădli chen Mitteleuropa Wien Mitt Komm Quartaăr ă sterr Akad d Wiss No forschung d O
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