The river catchment area
The mountain catchment area of the Brenta river extends over a surface area of more than 1,500 sq.km of very varied territory in geological terms, where ligneous (or magmatic), sedimentary and metamorphic rocks emerge.
The first were formed by cooling of molten rock, effusive when this happened above the surface, and intrusive when this happened below. The second were formed on sea beds thanks to depositing of flood materials and organic remains (shells and skeletons…). The third are the result of fusion by compression or heating of the first two.
A horizontal fracture – called the Valsugana fault – divides the Brenta mountain basin in two: it was formed 5 million years ago and set off the erosive process that created the valley. The pebbles that we find on the river bed are a sort of geological “memory” of the Brenta since its waters have eroded them from every rocky stratum it crossed and then transported them downstream.
We can therefore identify their various mineral compositions, discovering their places of origin: granite (with its closely-mottled black and grey crystals) must come from the central Lagorai mountains; porphyry (with its reddy-brown or grey-green colouring) comes from its peripheral areas; heavily striped rocks (gneiss) come from zones where ligneous rock and sedimentary rock fused together; the uniformly whitish pebbles consist of limestone: dolomite rock comes from the nearby Dolomites via the Cismon river, while white, grey or pink limestone pebbles come down from the Asiago plateau or from the Grappa massif and the Brenta valley itself.