Alcuni dati sulla modalita di progradazione della piana dell’Entella (Liguria orientale) in epoca storica

Some data on the mode progradation on the Entella Plain (Eastern Liguria) in historic time

Abstract

The plain of the Entella River, in eastern Liguria, is a typical example of deposition coast, whose formation in Holocenic time and for the most part in historic time, is directly tied to river deposits. On the basis of existing historical documents the progradation of mouth of the Entella is reconstructed: it appears that the seaward advance of the shoreline has come about since the Roman age up to 1810 A: D., when peak expansion was reached. Since 1810 A.D. onwards, however, the trend has been reversed to such an extent that, at present, the whole coastal area is being eroded. On the basis of historical documents are shown on fig. 1 the in Roman age, in the 15th century and in 1810 A.D., which visualize the progradation process. The finding of a stratigraphis sequence, shown in fig. 2 and schematically reproduced in fig. 3, located approximately at the reconstructed l5th century shoreline (fig. 1), makes it possible to verify the sedimentological characters of such deposition. The sequence made up of eight levels (fig. 3) including a deposit thickness which from the topographic surface (+2.40 m a.s.l.) reaches down to some tens of cm under sea level, shows a basal sequence (levels 6,7 and 8) upon which stand the storm wave deposits of levels 3,4 and 5, which are covered by a river deposit (level 2) polished of its fine particles by storm waves. These morphosedimentological findings indicate the existence, in the sequence described above, of upper shore deposits which, consequently recall the local presence of a former shoreline. Since the progradation of the plain, according to historical data (fig. 1), reached the area in the 15^th century, the age of the sedimentary sequence described above is regarded as being not older than the 15^th century.