The climate of the Great Lakes Land was becoming milder and milder In the last glacial period, about twelve thousand years ago, the glaciers flowing from the mountains of the Scandinavian peninsula covered Europe along the curve which on a contemporary map would overlap the line connecting Vilnius, Warsaw, Berlin and Hamburg. 

The glacier, slowly moving ahead, destroyed everything that was unable to escape. 
The blocks of ice broke trees in the forests. 

Coniferous trees, once broken, excreted ample amounts of resin. Under high pressure of water, the resin was transformed into amber, while slivers of wood were changed into jet. 

Towards the end of this glacial period the temperature of air started to grow, which initiated melting of glaciers. 

Further increase of temperature made the blocks of ice melt away even more rapidly. Water getting from beneath the glacier grooved troughs which today can be observed as trough lakes. Mikolajskie and Beldan lakes were formed in this way. 
Post-glacial lakes, such as Sniardwy lake, appeared in the places where a block of glacier melted without any outflow. 

After the glaciers had melted down, end moraines, having been formed by the moving ice, created the hills, while ground moraines covered the soil with infertile rubbles. 

The climate of the Great Lakes Land was becoming milder and milder