A Móra Ferenc Múzeum Évkönyve, 1971. 1. (Szeged, 1971)

Molnár, Béla–Mucsi, Mihály–Magyar, László: Latest quaternary history of the southern stretch of the Tisza valley

longer exposed to the surface. For instance, in the line of boreholes 19 and 20 the loess layer has been entirely worn away by fluvial erosion. In this zone there is still a depression on the surface, which has not ceased attracting surface drainage (Fig. 1 and 3). 3. The loess is in many places overlain by 3 to 5 m of lenticular, though never peaty, sediments which often pinch out and vary in granulometric composition between clay and small sand (curves 1, 3, 6, 7, 10, 14, 15 of Fig. 4). The finest clay sediment ever observed in the territory also appears here and it always occurs near the surface or is totally exposed to it (curves 1, 3, 7, of Fig. 4). The heavy minerals composition of the loess-covering sands showed in all of the cases a mixture of sources (Danube-and-Tisza origin), whereas the underlying ones did always suggest an origin from the Tisza river. (B. Molnár, 1964, 1966, 1967). The Danube-derived fraction of the heavy minerals is always more rounded, because of the greater distance of transportation, as compared to the fraction deriving from the Tisza flood-plain. There are differences in the shape of the quartz grains, too. The loess-covering sands show the predominance of grains belonging to the 3rd type of attrition. The most essential differences in the precentage distribution and the grain shape of the minerals have been indicated by encircling the respective figures in Table 1. The loess-covering layers differ from the loess-underlying ones in both carbonate content (15—22%) and

—immediately under the surface

—pH (8,5—9,2) (Fig. 2 and 3). On account of the physical conditions of occurrence it seems to be evident that the loess is overlain by fine-grained alluvial and lacustrine sediments and windblown sands of mixed, Danube-and-Tisza, origin. The wind-blown sands at the junction of the Danube-Tisza Interfluve and the Tisza valley directly overly the loess, being exposed to the surface in many places (Fig. 2, Section I, Borehole 6—9). In other places, closer to the Tisza river (these cases being more frequent), they pinch out between alluvial and lacustrine deposits. On the geological map, scale 1:10 000, made on the basis of the results of investigations (Fig. 1), the coarsest sediments small-grained wind-blown sands) are shown to occur in the highest morphological position in the NW part of the territory. The wind-blown sands covering the surface end with SE-trending tongues whose direction coincides with the predominant wind direction of the Danube-Tisza Interfluve (R. Wagner 1931). To the SE of the contiguous wind-blown sand area there are isolated occurrences of fine wind-blown sands which either crop out from below alluvial or lacustrine sediments, of overlie the loess formation. These too form minor dunes rising some 40 to 50 cm above their background and coinciding in direction with the afore-mentioned wind-blown sand tongues. Heavily alkalized in most of its outcrops, the loess formation occurs in the NE and S parts of the territory. The depressions between loess and wind-blown sand are occupied by Lake Dongér and minor abandoned river channels blanketed by clay and fine silt. The age of the formations. An important evidence for the understanding of the geohistorical evolution of the territory is provided by the loess layer which is present throughout the territory and represents the last glaciation of the Pleistocene. In the beds underlying the loess Mrs. Miháltz found and identified Betula, Salix and Alnus pollen grains accompanying Pinus silvestris constituting about 50% of the pollen spectrum. Of the fossil remains of plants other than trees the pollen H