The geological history of Greece has produced a generally highly indented coastline, with many islands lying in the neighbouring seas; some coastal areas, however, feature long stretches of high cliffs, or of sandy shallows. Marine erosion has further developed coastal indentation, and has produced many typical erosion features. Sea currents around Greece, which are governed by the inflows of water into the Mediterranean at Gibraltar and at the Hellespont, are regular, owing to the weakness of Mediterranean tides, and are strongest in straits and off major headlands. The Greek climate is also regular, falling into two distinct seasons; summer features strong, regular, northerly winds, clear skies, and great heat and drought, while winter is typified by eastwards-moving depressions which bring strong, stormy, and unpredictable winds, as well as cloud and rain, especially in coastal areas, where winds blowing off the sea meet land barriers (ch.1). Such topographical, oceanographic, and meteorological conditions meant that headlands and straits were areas of particular navigational difficulty. Yet these same conditions entailed a ready availability of shelter, in natural harbours, in river mouths, and behind promontories and islands. Areas composed of unbroken cliffs, or of flat, featureless shallows, on the other hand, afforded no such shelter (ch. 2). The physical environment thus also strongly influenced the routes taken by mariners, especially with regard to the degree to which ships 'hugged the coast'. Headlands and islands were the principal points at which major changes of direction were made, and, along with some other prominent topographical features, took on the role of milestones and landmarks, for which they were well suited. Incidental structures such as towers, tombs, and temples, took on similar roles, and were particularly important in inshore navigation. Where, due to distance or darkness, terrestrial landmarks were not visible, sailors' knowledge of other aspects of the physical environment was of paramount importance for safe and successful navigation (ch. 3).