It is uncertain how they were able to accomplish this feat. The late 4th century writer on Roman military affairs Vegetius mentions soldiers using reed rafts, drawn by leather leads, to transport equipment across rivers. But the sources suggest the Batavi were able to swim across rivers actually wearing full armour and weapons. This would only have been possible by the use of some kind of buoyancy device: Ammianus Marcellinus mentions that the "Cornuti" regiment swam across a river floating on their shields "as on a canoe" (357). Since the shields were wooden, they may have provided sufficient buoyancy