Strabo Geography 3.2.4

[4] Turdetania, on the other hand, is marvellously fertile, and abounds in every species of produce. The value of its productions is doubled by means of exportation, the surplus products finding a ready sale amongst the numerous ship-owners. This results from its rivers and estuaries, which, as we have said, resemble rivers, and by which you may sail from the sea to the inland towns, not only in small, but even in large-sized skiffs. For the whole country above the coast, and situated between the Sacred Promontory ¹ and the Pillars, consists of an extended plain. Here in many places are hollows running inland from the sea, which resemble moderately-sized ravines or the beds of rivers, and extend for many stadia. These are filled by the approach of the sea at high tide, and may be navigated as easily, or even more so than rivers. They are navigated much the same as rivers the sea, meeting with no obstacle, enters like the flow of a river at flood-tide. The sea comes in here with greater force than in the other places; for being forced from the wide ocean into the narrow strait, ² formed by the coast of Maurusia and Iberia, it experiences recoils, and thus is borne full into the retiring parts of the land. Some of these shallows are left dry as the tide ebbs, while others are never destitute of water; others again contain islands, of this kind are the estuaries between the Sacred Promontory ³ and the Pillars, where the tide comes in with more violence than at other places. Such a tide is of considerable advantage to sailors, since it makes the estuaries both fuller and more spacious, frequently swelling them to a breadth of eight ⁴ stadia, so that the whole land, so to speak, is rendered navigable, thus giving wonderful facility both for the export and import of merchandise. Nevertheless there is some inconvenience. For in the navigation of the rivers, the sailors run considerable danger both in ascending and descending, owing to the violence with which the flood-tide encounters the current of the stream as it flows down. The ebb-tides are likewise the cause of much damage in these estuaries, for resulting as they do from the same cause as the flood-tides, they are frequently so rapid as to leave the vessel on dry land; and herds in passing over to the islands that are in these estuaries are sometimes drowned [in the passage] and sometimes surprised in the islands, and endeavouring to cross back again to the continent, are unable, and perish in the attempt. They say that certain of the cattle, having narrowly observed what takes place, wait till the sea has retired, and then cross over to the main-land.