He knew a few things from talking to Vagabonds and from examining the Doctor's expansive maps: that the Ij broadened into an inland sea called Ijsselmeer, which was protected from the ocean by the island called Texel. That there was a good deep-water anchorage at Texel, but between that island and Ijsselmeer lay broad sand-banks that, like the ones at the mouth of the Thames, had mired many ships. Hence his astonishment at the size of the merchant fleet in the Ij: he knew that the great ships could not even reach this point.