ИСТИНА

Our study presents of the comprehensive scientific research conducted at the ancient town of Bolghar (Tatarstan republic, Russia). Bolgar was intermittently the capital of Volga Bulgaria from the 10th to the 13th centuries. The UNESCO World Heritage Committee inscribed Bolgar Historical and Archaeological Complex to the World Heritage List in 2014 Our study is shown that the development and functioning of the town had not stable character: the periods of habitation turns by short stages of neglet. We also discusses the genesis of the so-called “sand” layer, a characteristic feature of the occupational layer in the central part of Bolgar. In previous researchers, this layer was associated with the consequences of the Bolgar defeat by the Mongol army in 1236, since a coal (fire) streak had always been found under it. Based on a complex of physicochemical and microbiomorphic methods our study shows that in all cases this layer is distinguished from the rest in terms of the content of gross phosphorus and phytoliths. The “sand” proper constitute only a small proportion of it, moreover, traces of thermal effects were not observed. Therefore, the genesis of this layer is not mineral, but mainly biogenic in nature. The authors propose several hypotheses about the origin of this layer as accumulations of manure of herbivores or remains of adobe structures transformed by natural processes. Thanks to their research, it was possible not only to reconstruct the natural environment during the existence of the city in the 10–14th centuries,