2. The Rhine delta in the Roman Period
In recent years, Van Dinter (in press) has analyzed in detail LIDAR-data and geo(archaeo)logical, geomorphological and soil data of the Lower Rhine delta between Vechten and Katwijk. This has resulted in a palaeogeographical map for the Roman period which covers an area of more than 1,500 km2 (fig. 2). This research has revealed that the Roman defence system, situated on the southern side of the Lower Rhine, was built in three different types of landscape. Each type has its own possibilities and limitations for living grounds, food production and the occurrence of wood.
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Figure 2 Palaeogeographical map of the western Lower Rhine delta during the Roman period. (After Van Dinter in press).
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The eastern part, with the forts Vechten, Utrecht and De Meern, the so-called river region, was part of the Dutch River Area. The river Vecht branched off in a northerly direction near the fort at Utrecht. In the Roman period, the Dutch River Area was characterised by active rivers flanked by levees, older alluvial ridges (levees formed by former rivers together with their residual channels) and flood basins (Berendsen 1982; Berendsen & Stouthamer 2001). Height differences were minimal in the Dutch River Area and the substratum was soft. The alluvial ridges and the levees of active rivers consisted of relatively fertile sandy to silty, clayey soils. They formed the highest parts of the landscape, which rarely flooded (fig. 3a). When levees and alluvial ridges were not used by man, mixed deciduous woodland developed. The composition of this woodland depended on the flooding frequency (Van Beurden 2008). The majority of the alluvial ridges and levees were already deforested before the Roman period, because these areas were the most suitable as living grounds and these woodlands delivered the best quality timber. The alluvial ridges and levees were also in use for arable farming and animal husbandry (Groot & Kooistra 2009). From the relict woodlands timber and wood for fuel could be collected. The flood basins were the lowest areas in the Roman riverine landscape. During every flood, flood waters brought fertile clay into the flood basins. This explains the nature of flood basin soils: fertile but wet and heavy. Water levels varied between different parts of the flood basins, and throughout the year. The highest water levels occurred during winter and in springtime. In a natural situation reed and sedge marshes covered the lower-lying areas. In drier places wetland woodlands occurred in which alder and willow dominated (Groot & Kooistra 2009; Van Beurden 2008). Due to the heavy clays and overall wet conditions, the flood basins were not suitable for arable farming, but were perfectly suited for pasture and hay meadows for cattle, sheep and horses. The wetland woodlands could be used to collect timber and wood for fuel.
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Figure 3 Cross-sections through the three types of landscape in which the Roman defence system of the Lower Rhine was built, a. river region, b. peat region, c. coastal region.
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The central part of the line of defence, with the forts Woerden, Bodegraven, Zwammerdam and Alphen aan den Rijn, was located on the southern levees of the Lower Rhine, which formed a narrow corridor of accessible terrain through extensive wetlands with active peat development (Van Dinter in press; fig. 3b). As in the river area, the levees in this peat region consisted of fertile sandy and silty, clayey soils and the low-lying flood basins of fertile, but heavy clays. In a natural situation, the levees were covered with mixed deciduous woodland and parts of the flood basins with wet alder woodlands (Van Rijn in prep.). It is likely that the low-lying parts of the levees and flood basins were covered with reed and sedges. Behind the flood basins Van Dinter (in press) reconstructed extensive eutrophic fen woodlands, mostly consisting of alder carrs. Further away from the river, the fen woodlands gave way to mesotrophic reed and sedge fields, followed by huge, dome-shaped, nutrient-poor Sphagnum peat bogs. Although these peat bogs were the highest places in the area (fig. 3b), they were very wetland not accessible. A complex, interconnected network of small watercourses received the drainage water of these domes and transported it to the rivers Rhine and Vecht. The human activities were concentrated on the levees and flood basins in the same way as in the river region. The fen woodlands in the peat area were in use extensively, mainly for obtaining wood, as will be argued below.
The coastal region in the west forms the third type of landscape. The defence system with the forts Leiden Roomburg, Valkenburg and Katwijk was constructed there. This region includes a freshwater tidal district and the estuary of the Lower Rhine, which interrupted a series of parallel dune ridges and barrier plains (fig. 3c). In the estuary fresh water of the Lower Rhine was mixed with salt seawater. The extent of the reach of salt or brackish water lay just to the east of the fort at Leiden Roomburg (Van Dinter in press). The highest places were situated on the levees of the Lower Rhine, with a mix of fertile sandy and clayey soils, and the parallel dune ridges, which consisted of poor aeolian sand. In a natural situation, the dune ridges and the highest parts of the levees were covered with mixed deciduous woodlands of slightly different compositions. The dunes nearest to the sea and the estuary were free of trees, because of salt spray and flooding by brackish water. Various kinds of salt marsh vegetations were found in the flood basins and low-lying parts of the levees in the estuary. Reed and sedge marshes prevailed in the fresh-water tidal district. Peat accumulated in low-lying barrier plains, which existed in between the parallel dune ridges. These peat areas were normally covered with alder carrs (Kooistra 2008). The land use possibilities were more or less the same as in the other two regions. The dune ridges could have been used for the same activities as the levees and the salt marshes were excellent grazing grounds.