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The influence of anthropopressure..

elevational drop in lowland areas resulted in a rise in water levels and turned their lowest parts into marshes. The increased irregularity of river flows was another conseąuence of the phenomenon.

Freshets were increasingly high, while the duration of Iow stages lengthened with a simultaneous decrease in water level. Increased surface run-off contributed to intensified erosion of soil resulting in the increased presence of rubble which in turn accelerated the process of reversion of rivers to their virgin state. This is by no means a complete list of the conseąuences of deforestation in Wielkopolska.

According to rough estimates, in the lOth century, forests, swamps and surface waters occupied between 70% and 80% of the total area of Poland (Kaniecki 1993). The oldest chronicles and accounts present the Wielkopolska Lowland not only as a land of forests, but also as a region of marshes, difficult to travel across. In order to determine the size of the ancient wetlands, the surface areas of hydrogenic soils were calculated on the basis of the existing soil maps at the scalę of 1:100,000 (Kuczyńska 1995). The types of soils formed on wetlands were taken into account. These included: black-earth, silt-peat soil, peat and muck-peat, muck-mineral and fen soils. One can assume that in the Middle Ages the area of wetlands slightly exceeded the present area of hydrogenic soils, sińce as a result of dewatering carried out in the 19th and 20th centuries, soils in some areas were transformed and dried out to such an extent that now it is difficult to find traces of their marshy origin. In the six provinces under study the total area of the above listed types of soil amounts to 6040 km², that is slightly morę than 22% of their area.

Adding the area of surface waters, i.e. rivers and lakes, which in the Middle Ages was much larger than nowadays (the present area of lakes in the Wielkopolska-Kujawy Lakę Districtis 1.23% of the total area; see Choiński 1995), to the wetlands area, one can assume that they together occupied around 25% of the area of the Wielkopolska Lowland.

Numerous town names, freąuently originating in the early Middle Ages, refer to the past marshy character of the area (e.g. Kalisz, Babiebloto, Trzęsawiska, Bielawy, Błotno, Mokrzec, Bagienko, etc.). The vastness of the wetlands created the need for the construction of dikes, roads embankments and bridges to maintain communication between towns and settlements. Numerous mentions referring to that fact can be found in source materials. Probably the earliest piece of information about a "bridge" over the marshes comes from Ibrahim ibn Jacob’s relation from the journey to Slavic countries in the year 996, translated by Al-Bekri. The author described a one mile long platform or road madę of logs and round timber laid across a marsh in the land of prince Nakon, south of the present town of Perleberg (The Ibrahim ... 1946).

Archaeological research has revealed numerous traces of such medieval fords. Inhabitants of towns or villages adjacent to such crossings over rivers and swamps were obliged to repair them. In the Middle Ages the construction of