Lineamenti geomorfologici delle unità terrazzate del terzo ordine nel Bacino del Metauro (Marche settentrionali)

Geomorphologic features of the third-order fluvial terrace of the Metauro River Basin (Marche)

Abstract

The so called third-order fluvial terraced units morphologically constitute the main part of the wide valley flats of the Marchean rivers. A detailed geomorphological and stratigraphical survey of some sample areas and observations extended to the whole basin, led us to recognize and to differentiate two genetically distinct fondamental types of terraces, i.e. erosional and polygenic terraces. The terms «polygenic» groups some varieties of terrace subtypes, both mainly erosional and mainly depositional. In order to correlate the 3rd-order terraced unit throughout the whole basin and to fix some criteria useful also for contiguous river basins, it has been made an attempt to recognize the alluvium filltop, it is to say the remains of ancient flood-plain. This surface is seldom preserved because it has been extensively eroded during the subsequent re-incision stage, during which new miner terraced have been moulded. These minor terraces, often represent unpaired meander cut-off terraces and cannot be referred to some «sub-orders»; in other cases, they constitute paired terraced units indicating minor events superimposed to the main one.