Presenza di Esker in Val Menaggio (Lombardia, Lago di Como)

Abstract

The Val Menaggio connects the Valle del Lago di Como with the Valle del Lago di Lugano. The Val Menaggio was filled several times by an ice tongue of the Lago di Como glacier. The morphology of the valley is very peculiar. The Croce saddle separates the Val Menaggio from the Valle del Lago di Como; this saddle is built up of rock rises sensibly higher than the valley bottom. Moreover, in the middle of the valley the morphology is complex because of the occurence of two juxtaposed fans. The surface of the lower one is characterised by very elongated ridges with a radial arrangement. They approximately follow the maximum slope incline. These deposits are made of cleaned gravels with flow structures; the more elongated ridges are winding and forking. All these features give evidence on a fluvial deposition. Terraces are often associated with these ridges. These deposits rise above the underlying fan surface; this can only be explained with a subglacial deposition, so these deposits are eskers. This is the first finding of such structures in Lombardia, whose formation and preservation were possible thanks to particular conditions. The morphology of the saddle allowed the separation of a dead-ice tongue during the last glacial event. This ice melted "in situ" without any other movement and it produced only few melt-water. This fact prevented the erosion of the sediments deposited by subglacial streams. Moreover, since eskers are located in a relative higher position with respect to the valley plain, they are preserved from following erosion.