Expanding glaciers also cause sea levels to decrease. Scientists study these processes, as well as ice cores from glaciers and sediment cores from lakes and oceans, to learn about ice ages that predate the Pleistocene Epoch. Scientists use these findings to determine what factors may influence the occurrence of future ice ages. During an ice age several geologic changes occur. These alterations range from changes in the shape of the land to a decrease in sea level.
Water freezes and settles within the growing glaciers.
When this process occurs, shallow ocean waters that cover the continental shelves, or the edges of the continents, recede and uncover the submerged land. Advancing ice sheets can block water drainage pathways and create glacial lakes. Elsewhere, rivers are diverted from their original pathways to courses along the ice margin. The added weight of glacial ice sheets causes Earth's crust to lower by as much as several hundred meters. The ground in some areas becomes frozen throughout the year and forms permafrost, or permanently frozen ground.