Lithological features and morphological changes in the Mrożyca River valley floor in Brzeziny town and their affect on possibilities for urban development (from the Middle Ages to modern times)

Abstract

Research on the problem of urban floods and their importance for urban development has been widely discussed and analysed in contemporary literature (cf. Ciupa 2009; Kundzewicz 2012). However, the issue considered here concerns fossil traces of changes in the Mrożyca floodplain dating to the Middle Ages and the factors that favoured construction in such an area, as well as its abandonment and long- -term disuse. Brzeziny is located barely 2 km from the Mrożyca springs, and the valley bottom in the studied area of the city is morphologically poorly developed. The geomorphological processes that took place there in the Vistulian and Holocene, but also that are occurring today, are significantly influenced by slope processes. Another important distinguishing feature of the upper Mrożyca valley that agriculture and settlement were very late in becoming factors conditioning the pace and course of slope processes in the Holocene, and to a large extent also river processes, especially in small river valleys. More intense agriculture and settlement only appeared in the “inner” part of the Łódź Hills only in the early Middle Ages (e.g., Kamińska 1971; Dylik 1971; Chmielowska 1975; Sikora 2009).

The site is located in the source section of the valley, about 1 km below the river sources. Above the sources, there is a severalkilometre-long dry section of valley (Fig. 3). In-tensive slope processes that cause the displacement of large amounts of sediments occur during spring thaws, especially those that occur on still-frozen ground (Photos 1 and 2) and during summer downpours.

Among the geomorphological forms, attention should be paid to the dense network of denudation troughs and valleys, and to long denudation slopes (Fig. 3) that formed in the Vistulian. Their development continued in the Holocene and intensified significantly under anthropogenic pressure (Twardy 1995, 2000, 2003, 2008, 2017).

Within the analysed urban plot, the surface layer is a ~0.5–1.3-m layer of anthropogenic embankment comprising brick fragments, stones and municipal waste, mixed with mineral and organic formations. The embankment layer is underlain by a series of organic-mineral deposits laid down on mineral mud formations with a small admixture of organic matter (Photo 4). Beneath this series of organic-mineral formations, there are mineral silts with a small admixture of organic matter and river sands.

The research at the Brzeziny 8 site was rescue archaeology preceding the construction of a commercial facility on the site. During two seasons of archaeological research (2017– –2018), relics of a stone road of up to 4 m wide were recorded. The road was made of three rows of large stones that served as footpaths, with a fine cobblestone arranged between them (Photo 6). On the northern side of the stone road, the remains of two buildings set on stone foundations were explored. The lower beams of a timber-frame wall structure were also preserved. To the east of the buildings there was a utility section with a separate gutter (Photo 7). Three wooden wells were registered behind the buildings (Photo 8). On the southern side of the stone road, relics of another wooden house on stone foundations were recorded, as well as a fragment of carefully laid cobblestones (Photo 9). Ceramic and metal artefacts, as well as a large collection of coins (~300 items) and dendrochronological dates allow the discovered objects to be dated as having been in use in the first half of the 16th century.

The introduction of residential buildings and construction of road structures at the Brzeziny site clearly prove that, during the construction period, the examined part of the bottom of the Mrożyca valley allowed such development. The riverbed cut more deeply than today's, so the upper water table was lower than now. The stone road structure is overlain by mineral-organic silts, confirming that the study area flooded and the material was deposited in the conditions of a pond with weak flow. The impeded outflow of water from the slope to the riverbed caused flooding and swamps in this part of the valley bottom.

Archaeological and geoarchaeological research at the site shows rapid, heavy accumulation of fluvial and deluvial material and almost 1 m of aggradation of the Mrożyca valley floor over the last ~300 years. The factors that con-tributed to the significant accumulation and rapid uplift of the Mrożyca valley floor in Brzeziny are both natural; a) geology and lithology, b) topography and c) palaeoclimate; and anthropogenic: i) the nature of agriculture in the Brzeziny area, and ii) hydrotechnical and road infrastructure in the valley in and around the site. Factors from both groups acted concurrently, their effects intertwining and overlapping.

The presented results of research at the archaeological site in Brzeziny, as well as geomorphological and geological research, allow several phases of changes in the physiognomy and cover of the studied area to be presented. The situation preceding the entry of medieval settlement was designated as the first phase (Fig. 7). The second phase was the development of the floodplain area adjacent to the river, when the stone roads were built. This phase can be correlated with the 16th and 17th centuries. The third phase is when settlers left the floodplain area, perhaps due to flooding being more frequent and reaching the built-up area of the terrace. It can be correlated with the late 17th and early 18th centuries. The fourth phase is the development of a wetland within the floodplain. The material covering the remnants of the structure from the second phase has the character of sediment deposited in ponds, but containing a significant share of organic material, which indicates the functioning of wetlands with specific moisture-loving vegetation. The end of this phase in the site area was due to direct anthropogenic interference – the construction of an embankment – but some improvement in the accessibility of the valley bottom may be associated with the Mrożyca channel having been straightened and deepened in the 20th century.