In the early history of our nation, the Appalachian Mountains acted for more than 150 years as a major barrier or dam to westward expansion. In only half that time, once that barrier was crossed in number, the pacific coast was populated, other physical "barriers" have created similar dams to human expansion in other world areas. Have students research the affects of other real or perceived barriers, e.g., the Ural Mountains, the Upper Amazon Basin, the interior of Africa, the interior of Australia or the desert areas of Central Asia. How did each of these, and others they may discover, serve to slow down or otherwise limit human penetration of these areas? For how long?