Critical evidence for a great infra-Cambrian glaciation

The hypothesis of a widespread glaciation (Varangian) just before the Cambrian is familiar because in nearly every continent there are several deposits long interpreted as tillites and generally correlated together. The evidence on which this hypothesis is based is examined critically under three headings.The evidence for ice action in relation to alternative explanations of tillite-like rocks. Some criteria are inconclusive but others, or combinations of criteria, are decisive.The evidence for the distribution of tillites in space, in relation to:present distribution in the continents;late Pre-Cambrian distribution of continents;palaeo-latitudes determined directly or indirectly (i) by palaeomagnetism (ii) from the distribution of palaeoclimatic indicators;possible distribution of land-ice and sea-ice.The evidence for the distribution of late Pre-Cambrian tillites in time, in relation to:the stratigraphical correlation of tillites;the sequence and pattern of ice ages necessary to account for them;their relation to basal Cambrian transgressions and to Metazoan life in the Lower Cambrian. The evidence for ice action in relation to alternative explanations of tillite-like rocks. Some criteria are inconclusive but others, or combinations of criteria, are decisive. The evidence for the distribution of tillites in space, in relation to:present distribution in the continents;late Pre-Cambrian distribution of continents;palaeo-latitudes determined directly or indirectly (i) by palaeomagnetism (ii) from the distribution of palaeoclimatic indicators;possible distribution of land-ice and sea-ice. present distribution in the continents; late Pre-Cambrian distribution of continents; palaeo-latitudes determined directly or indirectly (i) by palaeomagnetism (ii) from the distribution of palaeoclimatic indicators; possible distribution of land-ice and sea-ice. The evidence for the distribution of late Pre-Cambrian tillites in time, in relation to:the stratigraphical correlation of tillites;the sequence and pattern of ice ages necessary to account for them;their relation to basal Cambrian transgressions and to Metazoan life in the Lower Cambrian. the stratigraphical correlation of tillites; the sequence and pattern of ice ages necessary to account for them; their relation to basal Cambrian transgressions and to Metazoan life in the Lower Cambrian. A limited amount of good evidence is necessary to test the hypothesis of an infra-Cambrian ice age. It is concluded that an ice age was sufficiently extreme to form marine tillites in the tropics. Earlier ice ages, possibly also in the "Eo-Cambrian", may be recognized.