Although the actual duration of the Ice Age is by no means accurately known, we can be quite sure that the time represented is far longer than that of postglacial time. That it must have lasted fully 500,000 years seems certain when due consideration is given to amount of time necessary to bring about the repeated changes of climate between the glacial and interglacial stages; the amount of plant accumulation during the interglacial stages; the amount of weathering and erosion of the various layers of glacial deposits. Some estimates run as high as 1,500,000 years for the duration of the Ice Age, and an average is about 1,000,000 years, which probably indicates, at least roughly, the order of magnitude of the time involved. When it is considered not only that the fact of the great Ice Age was not even thought of until 1837, but also that many factors enter into the general problem of the climate of geologic time, it is not surprising that the cause (or causes) of the glacial climate is still not definitely known. A few of the various hypotheses which have been advocated to account for the glacial climate will now be very briefly referred to. One is that the increased cold (not more than 10 to 15 degrees for the yearly average) was brought about by the notably increased altitudes of late Tertiary and early Quaternary times in northern North America and Europe. In this connection it is interesting to note that the four times of real glaciation during geologic time (mid-Proterozoic, early Paleozoic, late Paleozoic, and early Cenozoic) did occur directly after great crustal disturbances and notable uplifts of land. According to this hypothesis the interglacial stages would have to be explained by a rather unreasonable assumption of repeated rising and sinking of the glaciated lands. Another hypothesis, long held in favor, is based upon certain astronomical considerations. Thus we now have winter in the northern hemisphere when the earth is nearest the sun, but in about 10,500 years, due to wobbling of the earth on its axis, our winter will occur when the earth is farthest from the sun, thus making the winters longer and colder, and the summers shorter and hotter. After a much longer period of time the earth will be millions of miles farther from the sun in winter than in summer and this would still further accentuate the length and coldness of the winters. The interglacial stages represent the 10,500 year periods when the earth in winter (northern hemisphere) is nearest the sun. A difficulty in the way of accepting this hypothesis is that it is inconceivable that each glacial and interglacial stage lasted only 10,500 years. Another objection to the hypothesis as an explanation of Ice Ages is that it is directly opposed by the fact of widespread glaciation at low latitudes either side of the equator during the late Paleozoic Ice Age.