ИСТИНА

Cultural layers of ancient cities, or urbosediments are formed from the wastes of human household activities that begun to accumulate a long time ago and from their modern analogues that constantly expand and cover large areas on the Earth's land surface. Urbosediments of ancient towns have a large exchange capacity, are enriched with technogenic elements, and contain features of past stages of urbopedogenesis; they also serve as parent materials for modern urban soils. Urbosediments in the north of European Russia are formed under conditions of humid climate and flat relief and are represented by deep (2–5 m) organic layers, in which the loss on ignition is about 50–80%, and which occur are in a permanently waterlogged state. For several centuries, there was a relatively rapid accumulation of the remains of wooden buildings and other organic matter, as well as the accumulation of P, S, Ca, Cu, Zn, Pb, As, etc. Nowadays, organic cultural layers are characterized by the decomposing organic matter and intensive carbon dioxide emission into the atmosphere. Urbosediments formation occurs under the following three main groups of processes: primary sedimentation, as well as the subsequent transformation of the accumulated sediment under the influence of soil (pedogenic) and diagenetic processes. Soil processes occur on the surface of accumulating anthropogenic sediments, which are particularly pronounced in periods of slow accumulation and formation of soil horizons. Soil processes are expressed in the formation of separate profiles of under-developed soil, corresponding period of slowing or stopping of sedimentation, as well as the appearance of soil characteristics, dispersed in the bulk of the habitation deposit. Soils and soil horizons occurring in the bulk of habitation deposits of urban and rural areas indicate periods of sedimentation, the formation of natural vegetation, in some cases they have signs of a temporary appearance of agricultural landscapes. Weakly-developed thin soils are characterized by homogenization, aggregation, transformation of the humus state. Habitation deposits are accumulated in steps (unevenly) in a regular income of various debris and waste. There have been interruptions from different backgrounds and of different duration. The first group of short breaks is related to the cutting and displacement of previously accumulated sediments, including as a result of leveling works (lining of the sites for construction of new buildings). They don’t show in habitations deposits morphologically, and marked only by a break (hiatus) in the stratigraphy and chronology of archaeological material. The second group of long-term interruptions associated with the cessation of construction in a particular area and the appearance of gardens, orchards and vacant lots. These breaks correspond to the poorly developed soil formed as a result of processing the material previously accumulated cultural deposits. The thickness of these horizons may vary from a few centimeters (a series of soils 17th–19th centuries, Yaroslavo Dvorische archaeological excavation, Velikii Novgorod) to half a meter (13th-14th centuries, Ilmensky archaeological exxcavation, Velikii Novgorod). Formation of these soils occurs in conditions of natural wastelands and in agrogenic activities (arable and vegetable gardens, orchards). Soils in the wet organic layers (peat-like mass with wooden chips) characterized by loss of organic carbon. Thus, soil horizons in cultural layers mark the periods of desertion and changing types of economic use of the territory, and also allow us to give a chronological assessment of the duration of these breaks (from tens to hundreds of years) in thickness and severity of the properties.