When closer they exchanged their slings and discharged from them egg shaped pebbles which they had gathered from the bed of the Trebia.

When within still closer distance with the third slings they poured in volleys of much larger and heavier stones, with such tremendous force that it seemed as though they were sent from catapults.

Against such a storm of missiles the Roman skirmishers could make no stand, and were instantly driven back.
Their Cretan archers, after shooting away their arrows with but small effect, for the strings had been damped in crossing the river, also fled behind the heavy troops; and these in turn were exposed to the hail of stones.

Disorganized by this attack, the like of which they had never experienced before, their helmets crushed in, their breastplates and shields battered and dented, the front line of the Romans speedily fell into confusion.

Sempronius ordered up his war machines for casting stones and javelins, but these too had been injured in their passage across the river.
The hail of Carthaginian missiles continued until the Roman light infantry were forced to fall back; and the slingers were then recalled, and the heavy infantry of the two armies stood facing each other.