A Móra Ferenc Múzeum Évkönyve, 1970. 1. (Szeged, 1970)
LATEST PLEISTOCENE AND HOLOCENE HISTORY OF THE REGION By the end of the Pleistocene, three landscape units of different geological setting and character had been formed in the southern Great Hungarian Plain. 1. The Danube-Tisza Interfluve Ridge is a territory with a heterogeneous surface elevated 30 m high above the Danube Valley and almost 40 m high above the Tisza alluvium. Geologically, it is constituted by eolian sediments (loess and windblown sand) locally more than 100 m thick, deposited in alternation for a considerable part of the Pleistocene. (B. Molnár 1961, /. Miháltz 1965). The Uppermost Pleistocene wind-blown sands were redeposited during the dry phase of the Holocene hazel-nut stage, and the present-day near-surface pattern of the Danube-Tisza Interfluve has been developed. 2. During most of the Pleistocene the southern Trans-Tisza Region was sinking more rapidly than the aforementioned territory. Therefore it was continually flooded by river waters, and several hundred meters of fluviatile sediment have been accumulated in it. At the end of the Pleistocene, however, the rate of subsidence was slowed down, so that the loesses of the last glaciation (W 3 ) were deposited for the most part, on a wet surface, still covering a considerable part of the southern Trans-Tisza Region (/. Miháltz 1966, 1968). In the more humid period of the Latest Pleistocene and Earliest Holocene the rivers of the southern Trans-Tisza Region were incised into the Latest Pleistocene relief and, in many places, into the humid loess mantle, and their channels were carved out in it. It was due to the specific mechanism of accumulation by the rivers of the southern Trans-Tisza Region that the rivers often changed their channels, giving rise to abandoned basin stretches, ox-bows, etc. 3. Between the wind-blown sand and loessridge of the Danube-Tisza Interfluve and the fluviatile and Uppermost Pleistocene infusion loess area of the TransTisza Region lies the Tisza Valley —the depression carved out and filled up by the river at the end of the Pleistocene and the beginning of the Holocene (/. Miháltz, 1965, 1966). The Tisza Valley grows wider on both sides of the river south of Csongrád, attaining a maximum of 30 km in width. Its surface is covered by clayey infusion loess and Holocene fluviatile, flood-deposited and lacustrine sediments. ORIGIN OF NATRON LAKES Natron lakes occur in all three regions (landscape units) of the southern Great Hungarian Plain. The lakes of each region are in a close genetic connection with the geological history and structure of the region. NATRON LAKES OF THE DANUBE —TISZA INTERFLUVE RIDGE Out of the natron lakes of the Danube-Tisza Interfluve Ridge, Lake Kunfehér (/. Miháltz and M. Mucsi 1964), Lake Szarvas (M. Mucsi 1968) and Lake Petőfi at Soltvadkert (Fig. 1) {M. Mucsi 1965, 1966, M. M.-Faragó 1966) are geologically most explored. The lakes of the Bugac area are being investigated at present (B. Molnár and M. Szónoky 1969). At the end of the Pleistocene and in the dry phase of the Holocene hazel-nut stage, in the drainless depressions between the wind-controlled NW-SE-trending 66
