The Upper Jurassic period appears to have been warmer than the Lias. Insects of a large size and corals again attained a very wide distribution, but there is enough difference in the marine faunas of different regions to indicate a greater development of climatic zones than in the extremely oceanic periods such as the late Triassic. Schuchert points out that the plants of Louis Philippe Land in 63° S. are the same, even to species, as those of Yorkshire. In the Cretaceous period the climate was at first similar to that of the Jurassic, and trees grew in Alaska, Greenland and Spitzbergen. These trees, however, show marked annual rings, indicating a considerable differentiation of seasons, while trees of this age found in Egypt are devoid of rings. Towards the close of the Cretaceous there were many crustal movements and great volcanic outbursts, accompanied by a considerable reduction of temperature, which led to the extinction of many forms of life and the rapid evolution of others. There is no evidence of glacial action during the Cretaceous, however, though at the beginning of the Eocene there was a local glaciation of the San Juan Mountains of Colorado. According to W. W. Atwood this glaciation was double, the first stage being of the Alpine mountain glacier type, separated by an interglacial from the second stage, which was of the Piedmont type (mountain glaciers spreading out on the plain at the foot of the mountain). This Eocene glaciation has been found nowhere else, however, and the climate of the Tertiary, which is discussed more fully in the next chapter, was in general warm and oceanic, becoming rigorous towards its close. Summing up, we find that in the geological history of the earth two main types of climate seem to have alternated. Following on periods of great crustal movement, and the formation of large land areas, the general climate was cool, with a marked zonal distribution of temperature, culminating during at least four periods in the development of great sheets of inland ice. It is in such a period, though, fortunately, not at its worst, that we are living at present. During quiescent periods, on the other hand, when these continents largely disappeared beneath the sea, climate became mild and equable, and approached uniformity over a great part of the world. At these times, as soon as the surface water of the sea in high latitudes began to cool, it sank to the bottom, and its place was taken by warmer water from lower latitudes. The oceanic circulation was very complete, but there were practically no cold surface currents. Instead, there was probably a general drift of the surface waters from low to high latitudes (with an easterly trend owing to rotation of the earth), and a return drift of cooled water along the floor of the ocean. The formation of sea-ice near the poles became impossible, while the widespread distribution of marine life was facilitated.