During the last 1.8 million years, glaciers repeatedly advanced and retreated across the region, leaving behind a thick, complex blanket of intermixed layers of mud, clay, silt, sand, and gravel.
(glaciers; retreated; across the region; T:During the last 1.8 million years; T:repeatedly)
glaciers
retreated
across the region T:During the last 1.8 million years T:repeatedly