Strabo Geography 7.1.5

[5] The Hercynian Forest is not only rather dense, but also has large trees, and comprises a large circuit within regions that are fortified by nature; in the center of it, however, lies a country (of which I have already spoken ¹ ) that is capable of affording an excellent livelihood. And near it are the sources of both the Ister and the Rhenus, as also the lake ² between the two sources, and the marshes ³ into which the Rhenus spreads. ⁴ The perimeter of the lake is more than three hundred stadia, while the passage across it is nearly two hundred. ⁵ There is also an island in it which Tiberius used as a base of operations in his naval battle with the Vindelici. This lake is south of the sources of the Ister, as is also the Hercynian Forest, so that necessarily, in going from Celtica to the Hercynian Forest, one first crosses the lake and then the Ister, and from there on advances through more passable regions—plateaus—to the forest. Tiberius had proceeded only a day’s journey from the lake when he saw the sources of the Ister. The country of the Rhaeti adjoins the lake for only a short distance, whereas that of the Helvetii and the Vindelici, and also the desert of the Boii, adjoin the greater part of it. All the peoples as far as the Pannonii, but more especially the Helvetii and the Vindelici, inhabit plateaus. But the countries of the Rhaeti and the Norici extend as far as the passes over the Alps and verge toward Italy, a part thereof bordering on the country of the Insubri and a part on that of the Carni and the legions about Aquileia. And there is also another large forest, Gabreta; ⁶ it is on this side of the territory of the Suevi, whereas the Hercynian Forest, which is also held by them, is on the far side.