Complex sentence: For the past two million years, massive sheets of ice have repeatedly scoured the landscape; the last glacial period ended with the retreat of the Laurentide Ice Sheet from the Boundary Waters about 17,000 years ago.
Split into simpler sentences: For the past two million years, massive sheets of ice have repeatedly scoured the landscape. The last glacial period ended with the retreat of the Laurentide Ice Sheet from the Boundary Waters about 17,000 years ago.