The northeastern two-thirds of illinois, in the midwestern united states, was last covered by glaciers only 14,000 years ago. the rest of the state was last covered by glaciers over 100,000 years ago. until the advent of modern agriculture, the recently glaciated area was a broad, grassy swamp, cut by very few stream channels. in contrast, the area that was glaciated over 100,000 years ago is not swampy and has been cut by numerous stream valleys. why? geology question