National Geographic : 1958 Aug
which about 8,000 years earlier had crunched almost as far south as Cincinnati, then re treated. Estimates had placed the destruction of the Two Creeks forest at about 23,000 B. C. But radiocarbon tests of the spruce wood and peat show that this date is much too high; the forest was overwhelmed at approximately 9000 B. C. Lake Huron and Lake Michigan were still half choked with glacier ice a thousand years later, and the ice did not completely leave the Lakes area until about 5000 B. C. Pollen Tells the Story of Climate A long list of carbon dates for other loca tions gives us a picture of the alternate dwin dling and readvancing of the Wisconsin ice sheet over a period of 30,000 years. This timetable has been closely tied by radiocarbon to the movements of the Scandinavian ice sheet in Europe. Three additional techniques of chronology assist radiocarbon in surveying glacier move ments. One is the counting of varves, the 248 David S. Boyer, National Geographic Staff thin annual layers of sediment left in glacial lakes by the melting ice. Another is pollen analysis. Pollen, one of the most indestructible of organic things, sur vives for thousands of years in bogs and peat beds, silt deposits, and lake sediments.* When these geological formations are excavated, pol len samples can be taken from each layer, identified, and counted. The predominating pollen of each stratum tells what kind of vege tation thrived when the layer was deposited, and from this information specialists can de duce the climate of that period. Spruce and fir spell cold weather; these trees grew right up to the edge of the glacier. But the appearance of pine pollen indicates retreating ice, and large amounts of oak and hickory pollen mean warmer weather. A third piece of supporting evidence for the glacial chronology comes-surprisingly enough-from tiny marine animals known as * See "Lifelike Man Preserved 2,000 Years in Peat," by P. V. Glob, NATIONAL GEOGRAPHIC MAGAZINE, March, 1954.