Tacitus, Agricola. Chapter 23. The Geography and Inhabitants of Britain.
23. The fourth summer he employed in securing what he had overrun. Had the valour of our armies and the renown of the Roman name permitted it, a limit to our conquests might have been found in Britain itself. Clota and Bodotria, estuaries which the tides of two opposite seas carry far back intot he country, are separated by but a narrow strip of land. This Agricola then began to defend with a line of forts, and, as all the country to the south was now occupied, the enemy were pushed into what might be called another island.
Tacitus, Agricola - translated by Alfred Lord Church and William Jackson Brodribb.