Rimmed depressions of the central part of the

Rimmed depressions of the central part of the East-European Plain (Moscow region) Alexander N. Makkaveev, Vadim V. Bronguleev (Institute of Geography RAS, Moscow, Russia) geomorph@rinet. ru bronguleev@gmail. com Distribution of rimmed depressions over the region (geomorphic types are generalized) Deposits in the vicinity Lake Krasnoye Lake Svetloye About 30 isometric lake or bog basins, surrounded by ramparts, were revealed in the Moscow region. They have a resemblance to the remnants of degraded pingo (though they are mainly of a larger size) or to meteoritic craters. One of them (the Smerdyachie lake) was described earlier as an astrobleme, although this has not been definitively established. Rims of the two other lakes (Svetlopye and Krasnoye) were investigated by the authors. Oblique bedding in the upper layers of the rim 2 -d terrace of Moscow r. Test pits Rimmed depressions are mostly located on the low fluvioglacial plains, glaciolacustrine lowlands, and river terraces at the marginal zone of Moscow glaciation (Varthian stage). P h o t o Ribbony beds Moraine boulders in sandy deposits of the rim The scheme of rim formation (depth) (astrobleme supposedly) The depth-diameter dependence (oval) does not fit the Baldwin curve, but it can not disprove their impact origin, because of the infillment of the depressions with the sediments. The rim of the lake Svetloye consists of sand small amount of loamy sand loam. The character of bedding indicates fluvial origin of sediments and alternating conditions of accumulation. The only outer boundaries for rim formation on the surrounding flat plain might be the ice walls of under- or intraglacial lake. The preglaciation karst lake was overlaid by the thin margin of the Moscow ice shield, the decay of which over the lake (due to pressure of karst waters or to sagging of the broken ice into water) has led to intraglacial lake formation. After the ice melting (120 -100 ka) the sediments, accumulated near the ice walls and leaned to them, formed the rim. Ibyuk pingo Could such form exist here during cold Ice Ages? This structure reminds ejecta of meteor crater. At the diameter of 400500 m the initial depth of the crater should have been 80 m. It is larger than the depth of carboniferous rocks in the region but no traces of them on the surface are known. The other possibility of its formation – degradation of the pingo, grown here in the periglacial conditions, which slopes produced accumulation of the deluvium at their foot. The depression and the rim have close volumes: 390 and 450 x 103 m 3 correspondingly. Conclusions: About 30 rimmed lake and bog depressions were first described in the Moscow region. Some of them may be an impact craters, but their location indicates the action of glacial and cryogenic processes. The structure of the two rims analyzed has proved the diversity of their origin. One of these rims probably was formed by fluvial accumulation in the intraglacial lake, the other one – by slope processes on the ancient pingo.