Glacial periods were epochs in the history of the Earth when vast portions of the Earth's surface were covered by ice probably 5,000 ft. thick. The most extensive periods of glaciation occurred in the Pleistocene epoch, the Carboniferous period, the Permian period, and the Huronian period. It was thought that glaciation was caused by the cooling of the Earth's surface, but this hypothesis was disproved by the discovery of several periods of glaciation, one of which was in the very early history of the world. Other reasons, such as the shifting of continents, the shifting of the Earth's axis, elevation of regions through crustal movement into the upper colder air, changing of ocean currents, have been advanced; but none is conclusive proof of the reason for glacial periods.
