Zur Talgeschichte zwischen Domodossola und Locarno : Valle d'Ossola, Val Vigezzo (Prov. di Novara) - Centovalli (Kt. Tessin) : 1. Teil

Abstract. In the Miocene, the drainage of the upperToce river with the Antrona, Ogliana di Pozzolo and Vigezzo Valleys as southernmost source branches, took place northwards to the Gries and Grimsel passes. As a Miocene Aar river, the upperToce deposited in the Swiss Lowland – as gravels testify – the Molasse fan of the Napf mountain area. The continental divide was located between Monte Rosa and Gotthard farther south, in the Ossola valley between Domodossola and the Anza river mouth, in the Vigezzo tributaries near Malesco.

In the younger Miocene the forming of the Helvetic nappes at the Pennine front, their advance and sliding northwards with uplif ting of the Aar massif interrupted this river course: the drainage was reversed. At the former divide S of Domodossola a lake was dammed. Along joints its outlet, eut the rock bar. From the rising Monte Rosa it took up the Anza river and followed its lower course.

During cool phases, first at the Miocene/Pliocene boundary, the Toce glacier entered the Vigezzo Valley. From LocarnoTicino ice entered the lowermost Maggia valley and directed Maggia and Onsernone ice into the Centovalli. Therefore the Vigezzo ice was pushed to the southern slope. It could only reach the Ticino glacier as a small ice flow with reduced erosion power.

BesideWürmian moraines and erratic boulders, the maximum extension of the north-alpine glaciers gives indications of older maximum stages also in the south-alpine areas. Uppermost carved rocks and transfluences in the Vigezzo valley show the highest ice level at 1900 m in Rissian, Mindelian or even older glacial times, at 1700 m in Würmian. From Malesco Vigezzo ice flowed over the Finero pass to the SE and reached the Ticino glacier. From the Lago Maggiore basinTicino ice entered the lowermost Cannobina valley, dammed the overflowed Vigezzo ice and caused the formation of roches moutonnées.

Rissian tili, cemented debris flows, interglacial lake deposits with a thermophilous flora, overlying Würmian gravel and tili represent the Pleistocene sequence in the Vigezzo valley filling. The youngest deepening of the Melezzo river ends in Rissian tili. Still in the early late-Würmian,Toce ice entered the Vigezzo valley as far as Malesco and took up the Melezzo and Loana glaciers. From the Gridone mountain hanging glacier tongues descended into the Centovalli; Onsernone, Maggia and Ticino glaciers had became independent.