Geomorfologia delle Langhe: il bacino della Bormida di Millesimo

Geomorphology of the Langhe district: the Bormida di Millesimo basin

Abstract

The head of this very elongated SN basin, i.e. about 1/3 of its length, is cut out of mostly crystalline pre-Tertiary rocks, whereas the remainder lies in sedimentary formations of the Bacino Terziario Piemontese. Its main and secondary watersheds consist of long ridges of uniform height cut out from a series of saddles lying in several SSE-NNW directions. At the height of the saddles, there is a first order of terraces now denuded of their alluvial cover. The correlation between the watersheds substantiates the eastward continuation of the Villafranchian glacis of the Langhe. The terraces and saddles mark the former consequent direction of the flow of the waters, roughly parallel to old bed of the Belbo. On the bedding plane slopes, very active dynamics can be observed on the form of extensive landslips. On the support slopes, there is a 2nd order of terraces that appear to join the talweg in the Pre-Tertiary Tertiary contact area. Lower down, there is a third order of terraces in the middle-distal sector of Val Bormida only. Longitudinal profile of the river reveals 3 large convexities. The two most upstream coincide with the areas where the 2nd and 3rd order of terraces disappear. Examination of the stream ranking shows that the areas of maximum disorganization coincide with the basin secturs corresponding to these two convexities. It may be deduced that the basin has undergone four erosion stages: A) lst stage: erosion of the glacis caused by a SSENNW stream net. B) 2nd stage: following twisting and seesawing of the Langhe monocline, the runoff shifted to the N. The new, lower base level wore away the valley bottom tu the 2nd order of terraces. C) 3rd stage: following the subsidence of the Alessandria area, fresh erosion wore away the sole tu the 3rd order of terraces. This stage is holocenic. Remounting erosion may still be active at the head of the basin. D) 4th stage: now active in the middle and lower part of the basin, this erosion has again worn down the talweg to expose the 3rd tier of terraces.