As the Roman baggage train came into view, the Belgic force suddenly rushed out of the trees and surprised their opponents, overwhelming the Roman cavalry. They crossed the shallow river at full speed and charged up the hill against the legions setting up camp, giving them no time to get into battle formation. It seemed to Caesar that the Nervii came on with incredible speed, all at once pouring out of the trees, charging across the river and overrunning his legionaries.