Late glacial river-bed changes on the Little Hungarian Plain, based on preliminary chronological, geological and paleontological data

Abstract

Comprehensive chronological, geological and paleontological investigations were conducted as part of archaeological excavations in 2011 and 2012, prior to the construction of the M85 motorway between Gyor and Csorna, Hungary. These studies clearly show that the alluvial fan that underlies much of the Little Hungarian Plain was built up by streams flowing in a southeasterly to northwesterly direction from the nearby Bakony Hills, and continued to form until the end of the last glacial period. The northern part of the fan, now named the Csorna Plain, became inactive (i.e. became a fossil river-bed system) at about 25-15 ka, when the Rába and Marcal rivers changed theirflowdirection fromsouth-north towest-east.As a result of this change in flow direction, the Rába and Marcal rivers became incised, capturing the Bakony stream beds, stopping sediment deposition on the northern side of the alluvial fan (essentially the left bank of the Rába-Marcal river system), although the southern part of the fan continue to form as before. On the northern side of the fan, the sediment surface dried out due to falling groundwater levels, and aeolian sand-drifts began to form. Eventually, accumulation of the sand-drift sediments ceased due to the deposition of loess-type sediments, which fixed the surface, conserved the sand-drift shapes, and contributed to the straightening and eventual canalization of the fluvial channels. Geoarchaeological examinations indicate that the development of present fluvial features were strongly affected by the settlement and tillage activity of human communities on the Csorna Plain.