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Analysis of the distribution of pollen grains of various species contained in surface layer deposits, especially peat bogs and lake sediments, from which a record of past climate may be inferred. Because the lake sediments accumulate over time, a core of the mud will show that the mud at the bottom will be the oldest and the mud at the top will be the newest. By separating the samples of the core, we can get a record of how the vegetation around that site has changed. This has shown us that the area around the Great Lakes was tundra 11,000 years ago. For more details, please see the Palynology entry.
