Changes in geological climates over the last 570 million years have fluctuated considerably with quasi-periods of about 150 million years (warm-cool mode cycle) and about 300 million years (hothouse-icehouse cycle) (Figure 4.7). Mean global temperatures display periods of relative stability characterized by gradual and minor changes, and occasional bouts of more rapid change. The thickness of storm-bed deposits - tempestites - registers the 300-million cycle: thick tempestites, suggestive of more intense storms, are associated with icehouse phases, while thin tempestites, suggestive of less intense storms, are associated with hothouse phases (Brandt and Elias 1989). There is an interesting similarity between rates of warm-mode-cool-mode switching and rates of glacial-interglacial switching. As c/
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