The glacial record yields a pattern of ice-ages that shows distinctive characters for different periods. However, ages of deposits generally provide constraints on the time sequence which because of uncertainties are still too wide to be used critically for or against any regular or cyclic history. It seems better to assume no periodicity until such can be argued. Models for cyclic mechanisms with a longer period than the Milankovitch cycle that have been proposed include major chancge in tilt of Earth’s spin axis, galactic year, and tectonic cycles. Evolutionary tectonic changes or changes in atmospheric CO2 are more likely to have been contributing factors; but the late Proterozoic (Vendian-Varangian) ice-ages with at least equatorial glaciation remain enigmatic.

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