Beside the African routes into Europe, along the south shore of the Mediterranean and over the Sicilian and Gibraltar land bridges, while they lasted, two others must be noticed. One of these extended through Asia Minor and across the land bridge at the Dardanelles, while the second led westward along the northern border of the Caspian and Black Seas and the Caucasus Mountains. The most southerly of these four routes through Africa were probably the first to be travelled, the most northerly last of all. We shall have to study these routes more closely in a later chapter. It was at some time during the Glacial period, the Great Ice Age, when a vast ice-cap covered northern Europe with glaciers extending far southward and advancing or retreating according to climatic conditions, that man arrived in Europe. During the first Glacial Epoch the advance of the ice covered the most northern part of Great Britain and the Rhine valley almost as far south as Cologne; Scandinavia was completely buried, like central Greenland to-day, and North Germany probably to the Harz Mountains. Eastward the southern edge of the ice sheet ran nearly along the line of 50° N. lat. across Russia. In Siberia the effects were less marked and the limits were much farther northward. Between the parallel of 50° and the northern edge of the Alpine glaciers a zone was left ice-free, but three-fifths of Germany was overwhelmed. Southern England and France, not yet separated by the English Channel, formed one great habitable province, and but a small part of France was glaciated. The climate was tempered by proximity to the sea.[17] The average yearly temperature of northern Europe was probably not more than 4°-6° Cent. (39°-43° Fahr.), which is colder than at present. But the formation of these enormous masses of ice demanded heavy snowfall and a moist or very damp climate. Hence the edge of the great ice sheet advanced or retreated according to climatic conditions. There were four periods of advance before the final retreat of the ice, not counting minor oscillations.[18] These are known as the Gunz, Mindel, Riss, and Wurm Glacial Epochs. Alternating with these were the interglacial epochs of ice retreat--the Gunz-Mindel, Mindel-Riss, and Riss-Wurm; while the final retreat is usually termed Post-glacial. During the first and second interglacial epochs the climate appears to have been warmer than at present. But at times dryness may have contributed to the retreat of the ice even more than warmth, and then the climate would have been continental, harsh, and extreme.