Reconstruction of selected paleoenvironmental components of medieval Toruń, Poland, and its close suburbs

ABSTRACT This article presents maps of reconstructions of selected primary (i.e. prior to the thirteenth-century founding of the town) abiotic components of the natural environment of medieval Toruń (northern Poland, Kuyavian-Pomeranian Voivodeship) and its close suburbs. The reconstructions were carried out on the basis of topographical, geological and historical data. Despite their many limitations, the procedures employed represent the first attempt at a quantitative reconstruction of the primary relief, surface geology and hydrology of the analysed area. The result of the reconstruction presented on the maps could be used as the basis for both a reconstruction of biotic components of the natural environment and verification of hypotheses regarding the conditions of the founding and spatial development of the town.


Introduction
To date, descriptions of the primary traits ofand changes inthe natural environment of Toruń (northern Poland, Kuyavian-Pomeranian Voivodeship) since its founding in the thirteenth century have been almost entirely qualitative in nature. They relate to the entire town and its surroundings (Fedorowicz, 1993;Niewiarowski & Tomczak, 1969;Niewiarowski & Weckwerth, 2006;Podgórski, 1996;Rayzacher, 1989;Tomczak, 1971aTomczak, , 1999. Only a few of them contain quantitative indicators presented on maps (Molewski, 2011;Podgórski, 2005). In these works, environmental changes are considered primarily in terms of human activity being the principle driver of landscape development.

Objective
The aim of this study was to attempt a reconstruction of selected primary (i.e. prior to the founding of the town in the thirteenth century) abiotic components of the natural environment of medieval Toruń and its close suburbs, i.e. relief, surface geology and hydrology (Main Map). The reconstructions, in the form of digital maps, were created using archival topographical, geological (hydrogeological) and historic (archaeological) data which were integrated, analysed and visualised on the GIS platform ( Figure 1).

Study area
Old Toruń and its close suburbs are located in the Toruń Basin (Kondracki, 1998) on the right bank of the Vistula River.
The geomorphology and the surface geology of the study area is the result of complex processes (i.e. glacial, fluvioglacial, fluvial, denudational, Aeolian and biogenic processes) towards the end of the Pleistocene (the Weichselian glaciation) and in the Holocene. Its geological structure is dominated by fluvioglacial and fluvial deposits associated with the formation of the terraces in the Toruń Basin during the recession of the Scandinavian ice sheet and after its disappearance. The action of the ice sheet and its meltwaters resulted in the occurrence of tills and glacilacustrine clays in the structure of the terraces. The sandy deposits in the terraces became dunes, and in the former river channels, biogenic deposits accumulated. As a result of centuries of human activity in the study area, the natural deposits were covered by embankments of up to a dozen-orso metres thick.
The designated area of study is delimited by a 3km square. Within its boundaries are the Old Town, of 0.49 km 2 , surrounded to the west, north and east by parts of the suburbs, and the waters of the Vistula and a fragment of the river's left bank known as Kępa Bazarowa to the south. Excluding the waters of the Vistula, the contemporary surface area of the terrain under analysis amounts to 7.67 km 2 . measurements (data from 2012) and a photogrammetric model (data from 2005) (Molewski & Juśkiewicz, 2014).
A digital geological (hydrogeological) database was created. The sources of the geological data were archives of geological engineering drillings, hydrological drillings and archaeological excavations. Data were taken from 1076 geological drillings (drilling performed in 1881-2012) and 104 archaeological excavations (archaeological excavations performed in 1983-2010). From the descriptions of drilling profiles, information was gained on the lithology (including the thickness of embankments), the stratigraphy of the geological substrate and the depth of the water table. The archaeological excavations provided data on the combined thickness of the cultural layers and modern embankments, as well as the lithology of the natural substrate.
Archival queries of existing historical plans and maps of Toruń were made ( Figure 2). Of particular use were cartographic sources from the turn of the eighteenth to the nineteenth century, still depicting a feudal town, unchanged from the Middle Ages. Later cartographic sources were also used, because, in the fortress that was Toruń until the beginning of the twentieth century, investments in the suburbs were very limited (Kwiatkowska, 1969). As a result, a large part of the close suburbs of the medieval town kept their almost natural character until the middle of the twentieth century. Based on the modern geodesic plan of the town, selected historical plans and maps were rectified.
The research procedure employed to reconstruct the primary relief of the analysed area included the following stages (Molewski & Juśkiewicz, 2014): (1) Gathering of information on the primary relief of the land and its transformation, based on historical plans and maps of Toruń and on textual sources.
(2) Inventory of relief features (or parts thereof) which have been transformed, levelled or covered by earthworks, and marking them onto the raster digital elevation model of the contemporary analysed area.
(3) Plotting of maps of embankment thicknesses on the studied area based on geostatic analysis of the gathered geological data. (4) Generation of a raster digital elevation model of the studied area, after deletion of embankments based on map algebra (operation in a Geographic Information System (GIS) which allows two or more raster layers ('maps') of similar dimensions to be used to produce a new raster layer (map) using algebraic operations such as addition, subtraction, etc.).  For the reconstruction of the primary surface geology, the following stages were adopted (Molewski, 2016): (1) Identification of the surface geology of the analysed area, based on contemporary geological data.
(2) Deletion of embankments from the map of the contemporary surface geology. (3) Plotting of a map of the primary surface geology of the analysed area by the chorochromatic (also known as area-class) method. Due to the large vertical variability of deposits in the geological profiles and archaeological excavations, and in the interests of the readability of the map, the boundaries of characteristic deposits were marked (e.g. biogenic deposits) or those that predominate to a depth of 2 m.
For the reconstruction of the water table of the analysed area, the following stages were adopted (Molewski, 2016): (1) Gathering of information on changes in the hydrology in the analysed area, based on historical plans and maps of Toruń and on textual sources.
(2) Plotting of a map of contemporary hydrographic networks and the depth of occurrence of the water table in the studied area. The research procedures adopted in the reconstruction are limited by: (1) Lack of geological and hydrological data for part of the analysed area.
(2) The inability to determine the relationship of the primary relief to the fossil surface revealed by the removal of embankments, i.e. whether the fossil surface was initially levelled (cut) and to what extent (Molewski & Juśkiewicz, 2014). (3) Limited information regarding the primary relief of the analysed area from archival sources, including historical plans and maps. (4) The inability to determine either the number and courses of the primary (original) waterways of the analysed area (due to a lack of cartographic sources) or the original depth of the water table (which doubtless experienced a differentiated lowering from the thirteenth century). Additionally, data relating to this depth are from a long period extending from the end of the nineteenth century to the modern day.

Results
The oldest anthropogenic transformations of the relief of the analysed area probably took place on the banks of the Vistula at the site of the Teutonic castle, which was built between the thirteenth and fourteenth centuries. That location has been inhabited since the end of the Bronze Age (Chudziakowa & Kola, 1974).
As a result of generally anthropogenic processes, terrace edges were rounded and shallow valleys dissecting their slopes disappeared. Dunes were levelled or transformed. In the surface, the predominant anthropogenic relief features are settlement-related levelled planes. They were created by levelling of the ground, or the creation of embankments (Fedorowicz, 1993).
Significant transformations of the primary relief of the analysed area are associated with transport systems. A network of roads and a railway cross the study area, along with part of the infrastructure of a railway station situated on the left bank of the Vistula. The greatest density of streets occurs in the Old Town. Other than levelled planes, there are linear forms, i.e. road and rail trenches/embankments, roadway cuttings and Figure 2. Continued drainage channels associated with the transport networks. Due to the inconsiderable depths and heights of these features, most are not represented on the digital elevation model.
As previously mentioned, one characteristic feature of the anthropogenic relief of Toruń is the remains of fortifications. These include: traces of a moat which existed as early as the end of the thirteenth century and which was in-filled in the second half of the nineteenth century; the remains of earthen bastions built from the beginning of the seventeenth century onwards and repeatedly modified; and features built into the natural relief, i.e. terrace and dune slopes, and Prussian fortifications covered with earthen embankments (Fedorowicz, 1993;Niewiarowski & Tomczak, 1969;Podgórski & Chechłowska, 2011;Tomczak, 1999;Tomczak, 1975).
Changes in the extent and shape of the Vistula flood plain can also be considered to be partly anthropogenic. Their courses ran naturally until almost the second half of the nineteenth century when the Vistula began to be regulated along the Toruń stretch. As a result, the width of the river by the Old Town was half that of the end of the eighteenth century. It is supposed that, until the beginning of the nineteenth century, the Vistula was relatively stable and the extent of its bed had not significantly changed since the end of the early Middle Ages (Tomczak, 1971a).
At the end of the nineteenth century, to the west of the Old Town at the site of an oxbow lake in the floodplain, the 'Winter Port' (Pol. Port Zimowy) was established (Tomczak, 1971a). The thirteenth century saw the excavation of the waterway known as the Toruń Stream (Struga Toruńska). Waterways within the medieval town and its close suburbs were exploited for industry (e.g. by water mills) or for filling the moat. One of the preserved relief features visible on the digital elevation model associated with the industrial exploitation of the Toruń Stream is the repeatedly rebuilt millpond, known as the 'kaszownik'.
The anthropogenic relief features also include flood protection embankments built at the turn of the nineteenth to the twentieth century (Fedorowicz, 1993); a fragment of them is located in the very southern part of the analysed area.
Areas of former clay, sand and gravel quarries, most of which are now filled in and developed over, were once found to both the east and west of the medieval town (Fedorowicz, 1993). The receded edgeand loweringof the river terrace at the zoo and botanical gardens is a remnant of one such large quarry (Tomczak, 1999).

Relief reconstruction
In the study area, the thickest surface embankments mainly overlap with the areas of the eighteenth-and nineteenth-century town fortifications, i.e. the earthen bastions and moat, and the locations of former quarries now filled in. These embankments are 3-6 m in thickness, and up to a maximum in excess of 10 m in places. In the Old Town and suburbs directly adjacent to the former fortifications, the thickness of embankments ranges from 1 to 3 m. In the more distant surroundings of the medieval town, they exceed 1 m only in places. The areas where embankments practically do not occur, or are slight, are Kępa Bazarowa and those close suburbs which feature town parks and green areas.
After the removal of embankments from the digital elevation model of the analysed area, the surface of the river terraces within the limits of the Old Town and the adjacent areas to the north were lowered. A very clear edge to the ice-marginal valley terrace appeared to the east of the medieval town. To the southeast of the Old Town, the edge of the flood terrace receded to the north. This appears to be the result of the eastern part of the medieval town being set back from the banks of the Vistula as a precaution against flooding.
The course of the edge of the river terrace in the southern part of the Old Town clearly shows a quasipeninsular terrace, on which the Teutonic castle was built. That location, as previously mentioned, has been inhabited since the end of the Bronze Age (Chudziakowa & Kola, 1974). It was favoured for its safety from flooding and defensive value.
The dunes in the northern and western parts of the reconstructed relief, which partly survive today, are more extensive. They were originally surrounded by plains of zeolitised sands whose extent cannot currently be determined. Aeolian accumulations led to a partial masking of the edges of the higher terraces. Additionally, presently non-existent dunes were revealed, mainly to the west of the medieval town.
The image of the primary relief was supplemented by the extents of wetland areas, as identified from geological drilling profiles which indicated the occurrence of organic depositsmainly peatsin the shallow substrate.

Lithology of natural surface deposits
The natural surface deposits which originally predominated over the analysed area were silts, sands and gravels of the river and ice-marginal valley terraces. Sands in the surface-most layer of the higher terraces are usually zeolitised. The action of aeolian processes is attested to by, among others, the occurrence of biogenic deposits under dune sands. After the felling of the forests surrounding the medieval town, these processes were reactivated (as witnessed by Georg Friedrich Steiner's depictions of Toruń in the first half of the eighteenth century (Biskup, 1998) (Figure 3).
These biogenic deposits, mainly in the form of peats, are concentrated in the northern part of the described area, and along the course of the Toruń Stream around the 'kaszownik' millpond. They often lie beneath the sands of levelled dunes. Their occurrence is associated with the original level of groundwaters from subsoil (subsurface) waters, which were higher than at present, and the uneven configuration of the low-porosity substrate.
The deposits which characterise the surface geology of the terraces are shallowly occurring clays and tills. The different height of their roof results from erosion processes (Wilczyński, 1969) and also probably from glaciotectonic disturbances. Their occurrence around the medieval town correlates with the location of the oldest brickworks, which supplied bricks for the town's construction (i.a. Fedorowicz, 1993;Gąsiorowska & Gąsiorowski, 1963;Mikulski, 1998).

Reconstruction of the primary hydrology
Apart from the course of the Vistula riverbed, which, as previously mentioned, did not significantly change from the thirteenth to the beginning of the nineteenth century, little is known of the other watercourses in the study area. There is a lack of cartographic sources, and it is not possible to divine any trace of them in the contemporary topography of the town or the geological record, due to many centuries of fortification and hydrotechnical works.
The town moat was connected to the primary watercourses. One of them was the now non-existent Small Stream (Pol. Mała Struga, Ger. Kleine Bache), which flowed from the northeast and fed the moat surrounding the earthen bastions. To the east of the Old Town ran another (nameless) watercourse, carrying excess water from the moat. The courses of these two streams suggest that they may have originally formed a single watercourse flowing into the Vistula to the east of the medieval town (Gąsiorowska & Gąsiorowski, 1963;Molewski, 2016;Tomczak, 1971b). The third and longest watercourse, flowing to the east of the Small Stream the so-called Toruń Stream (Ger. Grosse Bache)had its original source in the wetlands to the north of the medieval town, and probably issued into the Vistula to the east of the Teutonic castle. Based on thirteenth-century sources, it is assumed that this primary (original) watercourse was the Postolsk (Gąsiorowska & Gąsiorowski, 1963). In the second half of the thirteenth century it was fedvia a manmade ditchby the waters of another watercourse known as the Wieldządz Stream (Pol. Struga Wieldzącka) flowing to the east of the town (i.a. Mrózek, 1969). Downstream of the kaszownik millpond the Toruń Stream was channelled (Fedorowicz, 1993).
One key element of the primary hydrology in the study area is the depth of the water table and its associated subsurface waters. These are influenced by shortand long-term fluctuations caused by the extent of rainfall and changes resulting from: deforestation of the medieval town and its surroundings; agricultural drainage and draining of wetlands; regulation of the Vistula; land drainage for Prussian fort construction excavations; urbanisation, including both an increase in non-porous surfaces which hamper the infiltration of rainwater into the substrate, and the limiting of contact between the waters of the Vistula and the waters of the water table; and exploitation of the water table.
It is not possible to precisely determine the depth of the water table in the analysed area for the thirteenth century. Reconstructions of thirteenth-century rainfall in Poland (Przybylak, 2008) may suggest that the water table was then lower than average. Its relative increase may have been brought about by deforestation  (Biskup, 1998). associated with the town's construction. All the remaining factors listed above undoubtedly contributed to its lowering, mainly in the second half of the nineteenth and in the twentieth century. Due to the uneven elevation of the low-porosity substrate and the strictly local influence of certain factors (e.g. exploitation of water), the extent of lowering of the water table since the thirteenth century has not been uniform and may have ranged from 0.5 to 3 m (Fedorowicz, 1993).
The reconstruction of the depth of the water table, which differs from that of today, was attained by reference to the digital elevation model of the study area's primary, reconstructed surface. Assuming that the water table has undergone a general lowering, raising it by 1 m allows for a hypothetical determination of the extent of wetlands. These are areas on the map where the water table is no deeper than 2 m. Areas marked in this way as wetlands mainly occur in the northern part of the analysed area, adjacent to watercoursesand in the vicinity of the Vistula floodplain. Moreover, these supposed wetlands in the shape of belts, perhaps associated with the course of a waterway, divide the area of the medieval town. In the existence of wetlands, we may also find the reasons for the eastern part of medieval Toruń being set back from the Vistula.
The map also shows drill points where biogenic deposits (mainly peats) were discovered. For legibility, each drill site has been surrounded by a 50-m buffer representing the hypothetical extent of these deposits. Their occurrence is associated with areas of supposed wetlands. These deposits appeared in drilling profiles, often below layers of embankments or sand dunes. A lot of drillings in the northern part of the described area found gyttja (an organic or organic-mineral sediment occurring on lake beds), providing evidence of the existence here of what were probably significantly earlier (Late Weichselian and Early Holocene) shallow water reservoirs (i.a. Noryśkiewicz, 2011). Silts lying to the north of the Old Town in Toruń are of anthropogenic origin and are in fact associated with the moat becoming overgrown and disappearing in front of the earthen bastions. The reconstruction is marked with the maximum extent of flood lands, based on the maximum historical reach of Vistula floodwaters recorded in the sixteenth century.

Conclusion
The reconstructed characteristics of certain abiotic environmental components of the area where Toruń was founded in the thirteenth century, and where its suburbs began to develop, are significantly different from those of the present day. Over the eight centuries of the town's eventful history, the primary landscape of this area has practically been transformed into a cultural landscape marked by intensive human activity.
It combines the features of a natural and a cultural environment. The primary relief, surface geology, and, above all, hydrology, all underwent a fundamental transformation.
The presented reconstruction of the aforementioned elements of the natural environmental of Toruń's Old Town and close suburbs is approximate in nature. Despite their many limitations, the procedures employed represent the first attempt at their quantitative reconstruction. The results, presented on maps, may form the foundation of a reconstruction of past biotic environmental components, including the natural soil and flora coverings. Moreover, it may form the premise on which to verify hypotheses regarding the conditions of Toruń's founding and spatial development.
Software ESRI ArcGIS 9.3 (with Spatial Analyst and Geostatistical Analyst extensions) and Global Mapper 17 were used for the manipulation and mapping of all spatial data sets. The Poland CS2000 zone 6 (ETRS89) coordinate system was used.