Judging from what is written in early Sailing Directions and navigation manuals it would appear that seamen of North-west Europe laid great store in their knowledge of the nature of the bottom in the seas in which they traded. And indeed, even at the present time, mariners are provided (although they may seldom use it) with fairly detailed information about the sea-bed, designedly to assist them in pilotage. A navigator may read, for example, that:

‘The bottom in the Western Approaches appears to consist, mainly of fine or coarse sand, a great deal of broken shell, occasional patches of pebble, gravel, small stones, and, now and then, mud. The sand is mostly white, but in numerous places yellow sand and also black specks are met with…. The greater proportion of yellow sand is found to lie southwards of 49°30′ N and the far larger portion of black specks northwards of that line. This peculiarity is so marked, especially between 9°40′ and 7°30′ W that the mariner running eastwards, and supposing his vessel to be to the southwards of this parallel, should certainly suspect his having been set northwards of his reckoning if the soundings disclose this mixture of black specks to any great extent and with any frequency.’