La dinamica dei versanti della Valle dei Templi di Agrigento

Abstract

In the course of time the ancient Agrigento - a superb Greek city - and later urban settlements have been expanding unti1 these days over a vast area - the one described in this study. This is un area of extreme archeological interest which is characterized by the presence of a regressive type of Pliopleistocenic succession, essentially consisting of clay, sand and calcarenite. From below upwards, one can distinguish a basement from the Miocene and Pliocene epoch followed, in transgressionm by the Trubi Formation and, further up, by the Mt. Narbone and Agrigento formations. Closing the lithostratigraphic succession, the terraced marine deposits of the Sicilian and Thyrrenian, and current debris, alluvial and coastal deposits can be observed. In the San Leone river valley, a series of morphostructural evidences suggest that here geomorphologic dynamics is very likely controlled by ~Canberry and Valley building~ type phenomena due to complete erosion of the calcarenite plaque. Among the most outstanding evidences are: a marked westward and eastward rotation of the structural axes, on the right and left side of the torrent respectively, the presence of remoulded clay stripes and levels, also at considerable depth, and sub-horizontal discontinuities. The calcarenites are marked by a thick mesh of cracks made more pronounced by erosion and dissolution processes. Eventually, this kind of dynamics results into the separation and i subsequent detachment and collapse of calcarenite blocks from the top of the huge plaque upon which the Greek city's famous Temples were built. Some remarkable mass movements that occurred during this last century should be viewed in the light of the complex dynamics just described. Especially important are three landslides: two of them inside the area of today's Agrigento, in 1944 and 1966 respectively, and one that occurred in 1976 along the eastem side of the Hill of Temples. The 1976 slide was caused by a remobilization that followed heavy rainfall: this conclusion was reached at the end of un empirical hydrological-statistica1 study based upon un origina1 method [15]. Daily cumulated rains (1 to. 180 days) were studied to cover the whole century, and this approach enables the recurrence interval of each rainfall event associated to the three investigated landslides to be determined. The results of the hydrological-statistica1 analysis were fully confirmed in the case of the 1976 slide, where it was found that rains are the main causative agent: these results were obtained by means of the physical and hydrological characterization of the soils in the landslide body and some simple calculations concerning the flow of infiltration water.