Ammianus Marcellinus Res Gestae 15.10.11

[11] At the same time, having an eye to the common welfare, he advised his brother, Gnaeus Scipio, to proceed to Spain and hold off Hasdrubal, who was planning to burst forth in like manner from that quarter. But Hannibal learned of this from deserters, and being of a nimble and crafty wit, came, under the guidance of natives from among the Taurini, through the Tricasini and the extreme edge of the Vocontii to the passes of the Tricorii. Starting out from there, he made another road, where it hitherto had been impassable; he hewed out a cliff which rose to a vast height by burning it with flames of immense power and crumbling it by pouring on vinegar; ¹ then he marched along the river Druentia, dangerous with its shifting eddies, and seized upon the district of Etruria. So much about the Alps; let us now turn to the rest of the country.