Palynological analysis suggests that the so-called "Lower Tuff" is not a tuff, but rather a massive, poorly-bedded volcanic shale representing many centuries of sequential accumulation. Lower Tuff time was characterized by several proximal holocaustic fires and at least one major draw-down of lake level, marked most conspicuously by the widespread fossil stump horizon. The overlying "Lake Shale" unit comprises five thousand or more years of lake history. Lake Shale time was characterized by aperiodic cycles of alternate shallowing and deepening of the ancient lake. During shallow water intervals, water quality was very poor. Such conditions resulted in a low-diversity lacustrine fauna and facilitated preservation of insects and assorted plant materials such as leaves. Fluctuations in lake water level resulted from a dynamic equilibrium between watershed catchment and runoff, a warm climate that resulted in high rates of evaporation, and the integrity of the downstream containment structure(s).

Key words: Florissant, limnology, paleoecology, palynology