The barbarians thought that Romans would not be able to cross it without a bridge, and consequently bivouacked in rather careless fashion on the opposite bank; but he sent across a detachment of Germanic tribesmen, who were accustomed to swim easily in full armour across the most turbulent streams. [...] Thence the Britons retired to the river Thames at a point near where it empties into the ocean and at flood-tide forms a lake. This they easily crossed because they knew where the firm ground and the easy passages in this region were to be found; but the Romans in attempting to follow them were not so successful. However, the Germans swam across again and some others got over by a bridge a little way up-stream, after which they assailed the barbarians from several sides at once and cut down many of them. (Cassius Dio, Roman History, Book 60:20)