By means of pollen studies it has been possible at least to venture estimates of the ecological conditions which existed and developed during post-glacial periods. Climate is no doubt the most important factor in determining the extensive formations of vegetations and their replacements. Weaver and Clements state: "Climate may produce new areas for succession through the destruction of existing vegetation." If this be true, then the pollen analyst can estimate, on the basis of forest types as indicated by fossil pollen findings, what climatic conditions prevailed over wide geographic areas, since the forest types would be directly dependent on the climate, making the two synonymous within greater or lesser degree. While to date 22 Indiana bogs have been studied in detail, the overall picture of climatic and vegetational changes within the state is still somewhat incomplete. As new discoveries are being added to the present store, the picture of past vegetation evolves more clearly, and this in turn gives an estimate of the climatic conditions which prevailed since the retreat of the continental ice sheets which once covered much of Indiana.