Titus Livius (Livy), The History of Rome, Book 21
D. Spillan, A.M., M.D., Cyrus Evans, Ed.

36. They then came to a rock much more narrow, and formed of such perpendicular ledges, that a light-armed soldier, carefully making the attempt, and clinging with his hands to the bushes and roots around, could with difficulty lower himself down. [2] The ground, even before very steep by nature, had been broken by a recent falling away of the earth [p. 735]into a precipice of nearly a thousand feet in depth. [3] Here when the cavalry had halted, as if at the end of their journey, it is announced to Hannibal, wondering what obstructed the march, that the rock was impassable. [4] Having then gone himself to view the place, it seemed clear to him that he must lead his army round it, by however great a circuit, through the pathless and untrodden regions around. But this route also proved impracticable; [5] for while the new snow of a moderate depth remained on the old, which had not been removed, their footsteps were planted with ease as they walked upon the new snow, which was soft and not too deep; [6] but when it was dissolved by the trampling of so many men and beasts of burden, they then walked on the bare ice below, and through the dirty fluid formed by the melting snow. [7] Here there was a wretched struggle, both on account of the slippery ice not affording any hold to the step, and giving way beneath the foot more readily by reason of the slope; and whether they assisted themselves in rising by their hands or their knees, their supports themselves giving way, they would tumble again; nor were there any stumps or roots near, by pressing against which, one might with hand or foot support himself; so that they only floundered on the smooth ice and amid the melted snow. [8] The beasts of burden sometimes also cut into this lower ice by merely treading upon it, at others they broke it completely through, by the violence with which they struck in their hoofs in their struggling, so that most of them, as if taken in a trap, stuck in the hardened and deeply frozen ice.