XLI. A course of 500 stadia having been accomplished, their next anchorage was in an estuary, which swarmed with fish, called Kataderbis, at the entrance of which lay an island called Margastana.[109], They weighed at daybreak, the ships sailing out in single file through shoals. The direction of the shoal was indicated by stakes fixed both on the right and the left side, just as posts are erected as signals of danger in the passage between the island of Leukadia and Akarnania to prevent vessels grounding on the shoals. The shoals of Leukadia, however, are of firm sand, and it is thus easy to float off vessels should they happen to strand, but in this passage there is a deep mud on both sides of such tenacity that if vessels once touched the bottom, they could not by any appliances be got off; for, if they thrust poles into the mud to propel the vessels, these found no resistance or support, and the people who got overboard to ease them off into navigable water found no footing, but sunk in the mud higher than the waist. The fleet proceeded 600 stadia, having such difficulties of navigation to contend with, and then came to an anchor, each crew remaining in their own vessel, and taking their repast on board. From this anchorage they weighed in the night, sailing on in deep water till about the close of the ensuing day, when, after completing a course of 900 stadia, they dropped anchor at the mouth of the Euphrates near a town in Babylonia called Diridôtis—the emporium of the sea-borne trade in frankincense and all the other fragrant productions of Arabia.[110] The distance from the mouth of the Euphrates up stream to Babylon is, according to Nearkhos, 3,300 stadia.