sentences:  In the third and most recent period starting about 1.8 million years ago, glaciation eroded previous rock formations and deposited deep layers of glacial till over most of the state, and created the beds and valleys of modern lakes and rivers.Minnesota's geologic resources have been the historical foundation of the state's economy.Precambrian bedrock has been mined for metallic minerals, including iron ore, on which the economy of Northeast Minnesota was built.