Some Changes are Major and Some are Minor

Some changes have a major impact on the environment, while some changes have a minor impact.
"For millions of years, Illinois was covered by a shallow inland sea teeming with life. Then, about 320 million years ago, the water receded, and broad areas of dry land emerged. Land provided habitats for a whole new set of plants and animals."

Illinois climate has also changed through time. In addition to being underwater sometimes and being dry land at times, Illinois has had tropical climate and frozen climate. The climate has generally been warming since the last ice age 12,000 years ago.