Ice

There are various forms of ice.ICE AGEWhen most people hear the words Ice Age, they think of glaciers covering much of North America and Eurasia, animals like mammoths and saber-toothed cats, and Cro-Magnon people painting cave walls. These things come to mind because the words "Ice Age" often refer to the last time that glaciers extended over a large portion of the Earth's surface. The ISM online exhibit The Midwestern U.S. 16,000 Years Ago provides more information on this glaciation in the Midwestern United States. The amount of ice on the Earth's surface has varied greatly through time. For example, the extent of ice in North America has changed dramatically since the height of the last glacial advance 20,000 years ago.During most of the last 1 billion years the earth had no permanent ice. However, sometimes large areas of the globe were covered with vast ice sheets. These times are known as ice ages. Illinois has experienced changes in rocks and fauna during these times. This Web module tells how some of these changes happened. Knowledge of the changes and their causes helps increase understanding about ancient rock formations and animals remains found in Illinois sites as evidenced in the Museum's specimen collections and site reports.For example, the Online Exhibit The Midwest 16,000 Years Ago presents evidence that snowshoe hares are found today in the northern part of the midwest, yet 16,000 year old remains were found in Kentucky and Missouri, much farther south. If a type of animal lives in a certain type of climate now, we may infer that its ancestors lived in that type of climate, too. Does that mean that Kentucky and Missouri were once as cold as Wisconsin is today? What other evidence can we study to find out?What are Ice Ages?Ice Ages are intervals of time when large areas of the surface of the earth are covered with ice sheets (large continental glaciers). The term is used to describe time intervals on two very different scales. It describes long, generally cool intervals of Earth history (tens to hundreds of millions of years) during which glaciers advanced and receded. The term also describes shorter time periods (tens of thousands of years) during which glaciers were near their maximum extent. These shorter intervals are also known as "glaciations."In addition, the term "Ice Age" is sometimes used to refer to the last major glaciation that occurred in North America and Eurasia. When used in this way, the first letters of both words are often capitalized. This is the way the term Ice Age is used in the Midwestern U.S. 16,000 Years Ago exhibit.When have Ice Ages occurred?Many glacial advances and retreats have occurred during the last billion years of Earth history. These glaciations are not randomly distributed in time.Instead, they are concentrated into four time intervals. Large, important glaciations occurred during the late Proterozoic (between about800 and 600 million years ago), during the Pennsylvanian and Permian (between about 350 and 250 million years ago), and the late Neogene toQuaternary (the last 4 million years). Somewhat less extensive glaciations occurred during portions of the Ordovician and Silurian (between about 460 and 430 million years ago).During each of these intervals, many glacial advances and retreatsoccurred. For example, over 60 glacial advances and retreats have occurred during the last 2 million years.If "ice age" is used to refer to long, generally cool, intervals during which glaciers advance and retreat, we are still in one today. Our modern climate represents a very short, warm period between glacial advances...