The _geologic-climatic_ method of dating combines the principle of stratigraphy with an interpretation of the meaning of natural occurrences. When geologists demonstrated that the earth had recently gone through a period when enormous glaciers advanced and retreated across the northern and southern hemispheres, they began to speculate on the effects of such conditions on the landscape. The surface would be scraped down to bare rock by the advancing ice sheet. When the ice retreated, its load of rocks and cobbles would tend to be left behind, while the smaller particles of dirt would be flushed away in the rivers formed by the melting ice. If the archaeologist found tools among the boulders and cobbles, those tools should date to the period when the cobble and boulder stratum was formed, the period when the glacier was retreating. Here the geology gave clues to the climate, and if the age of the climatic event was known, artifacts associated with the geology could be dated. It is through the geologic-climatic method of dating that archaeologists discovered that human beings lived in southern Europe at the margin of the last great glacier, and therefore the date of their occupation was on the order of 20,000 years ago. It is not necessary to use the geologic aspect of geologic-climatic dating if one has other clues to the climate. Plant and animal fossils are clues to ancient climates, since living organisms have a tendency to live in climates to which they are best adapted. If we find the fossil bones of a giraffe or an ostrich in the Sahara Desert, we can conclude that at some time in the past the Sahara was not a desert but a veldt, since giraffes and ostriches live in the veldt today. If stratigraphy allows us to relate artifacts to those fossils, we can maintain that the makers of those artifacts lived at the time the veldt existed in what is now the Sahara area. Now if we can relate the fossils to a date when such a climate could have obtained in the area, we have a date for the artifacts. Animal fossils are relatively rare, and fossil remains of plants which can be seen with the naked eye are even rarer. But microscopic pollen grains are not particularly rare in the stratigraphic sequences of sediments, and are directly associated with artifacts in many kinds of sediment. The pollen of most plants is protected by a tough coat like that on many seeds. Millions of pollen grains are produced by local vegetation each year, and a percentage of them are buried in the yearly accumulation of sediment. Once they are buried, the tough outer covering is preserved. [Illustration: (uncaptioned)]