It is formed by cores of polimetamorphic and polideformed of pre-alpine continental crust and/or ortogneiss derived from Variscan granitoids (270-315 Ma). These litholitical bodies forms the nappes cores, separated by strips of carbonatic rocks of the Mesozoic coverings of the original basement.
In the nappes building are included also strips of Mesozoic oceanic crust, divided into two greater ophiolitic units (Zermatt-Saas ZS and Antrona AS) and involving (dubitatively)other minor strips of anphibolitic and serpentinitic rocks of different structural position. The last recognized metamorphic event is of alpine age (38 Ma) and in anphibolitic fancies (>550 °C).

From south-east to north-west there are lithologies belongings to the Austroalpine and upper Pennidic and lower Pennidic multinappes system.

The structural asset of the Austroalpine and upper Pennidic units is verticalized (Loro-Vogogna/Domodossola) so the single units outcrops with low power.
The Austroalpine in the Ossola region is represented by the "Sesia-Lanzo -SL" area, formed by paragneiss, ortogneiss and metabasitics. Some kinds of beolas (the Vogogna beolas) belong to this unit lithologies.

Between the Austroalpine and the Pennidic nappes there is the Piedmont Area, formed by calcescists and green stones, which in the Ossola Valley outcrops in a weak way while it outcrops better in the Aosta Valley.

Upper Pennidic nappes are formed by ortogneiss with subordinated paragneiss, represented by the Mount Rosa nappe, by the Antrona ophiolits and by the Zona Camughera (Monte Rosa -Mr) and by the Orselina-Moncucco-Isorno area (OR- San Bernardo).
Beura and Pallanzeno beolas come from the Mount Rosa nappe while the Croppo beolas (Domodossola) come from the Orselina-Moncucco-Isorno area.
Lower Pennidic nappes, mainly formed by granitic ortogneiss, are visible to the north of Domodossola and from south to north of the Monte Leone (ML) - Pioda di Crana (PdC), Lebendun (LE) and Antigorio (AN) and from the Verampio Ortogneiss (VE).
From Domodossola to the north, the structural asset becomes subhorizontal and then in the north of the upper Val Formazza.
The Antigorio nappe, thanks to its subhorizontal asset, is a very powerful strip and so it is highly profitable: the different kinds of serizzo are a noticeable economic resource of the Ossola area.