There have always been continental glaciers on the Earth. By studying the oxygen isotope relations of the sediments in the seabed, it has been found out that glacial periods occur regularly. The climate has changed throughout the history periodically from cold to warm. For this there are several reasons, such as changes in Earths orbit and in the angle of Earths axis, continental drifts, increased volcanic activity, and changes in Suns thermal energy. Also glacial periods are recurring. During the cold periods, glaciers expand. Between glaciation periods there are warmer periods. If a glacier stops expanding but does not melt, it is called interstadial period. If the climate gets so warm that the glacier starts melting, it is called inter-glacial period. 

For the last 2,5 million years, Finland has repeatedly been covered by glaciers. Between the glaciation periods, there have been warmer inter-glacial periods. However, it cannot be said for certain how many times Finland has been covered by a continental glacier, because there is no complete layer formation from which the number of previous glaciation periods could be counted. During the past million years, there have apparently been ten glaciation periods, which lasted approximately 100 000 years. The inter-glacial periods lasted approximately 10 000 years. The latest glaciation period that occurred in Finland is called Veiksel period. 

Reasons for glacial periods