A gap is a low area between two higher-elevation landmasses, such as mountains. Gaps are similar to passes, but more rugged and difficult to navigate. The most rugged gaps are often called "notches." Notches are rarely crossed, and usually marked by steep cliffs on either side. Another name for a gap is a "saddleback," because wide gaps often have the shape of a saddle. There are peaks on almost every continent called Saddleback Mountain: Saddleback Mountain, Maine; Saddleback Mountain, Arizona; and Saddleback Mountain, Australia, are just a few. Gaps often help indicate a peak's prominence. Prominence is an expression of a peak's independence, or how isolated it is from other elevations. Prominence is the vertical distance between a summit and the lowest contour line (point of equal elevation with another peak). A U-shaped gap can indicate this contour line in a mountain ridge. Outside the U.S., gaps are often called cols. A key col, in fact, is a mountain's highest gap and an effective measure of that peak's prominence. The South Col, for example, is the gap between Mount Everest and Lhotse, the highest and fourth-highest mountain peaks in the world. The South Col bridges Nepal and the Chinese region of Tibet, and is the most popular site for the final camp of mountaineers climbing Everest. The South Col is also an entry into Everest's infamous "death zone," where altitude sickness can impair judgement and most climbers need supplemental oxygen. The South Col and most other geologic features in the Himalayas were created through the ongoing process of crustal collision. Crustal collision is a form of tectonic activity, where massive continental plates crash into each other. In this case, the Indian plate is crashing into the Eurasian plate. Material above the crashing plates continues to be uplifted, creating monumental, jagged peaks and rugged gaps. Water Gaps Other gaps are created through tectonic activity and the movement of flowing water. These gaps, created by rivers and glaciers, are called water gaps. Water gaps usually indicate the stream is older than the elevated area around it. Over millions of years, the process of tectonic uplift elevated the streambed, while the stream itself weathered the rock surrounding it. The majestic gaps of the Grand Canyon in the U.S. state of Arizona formed through the twin forces of the Colorado River and tectonic uplift of the Colorado Plateau. Water gaps have played an important role in regional development. The Chicago Portage is a water gap that helped shape American history. The Chicago Portage connects the watersheds of the Mississippi River and the Great Lakes. The Chicago Portage was formed as the massive Laurentide Ice Sheet retreated from the region during the end of the last ice age. As the ice sheet melted, it created the Great Lakes. Ancient Lake Chicago (which eventually became Lake Michigan) overflowed its banks, creating a gap in the Valparaiso Moraine. (Moraine is the hilly debris left by a glacier as it carves its way through the earth.) The Chicago Portage, which eventually included the Chicago and Des Plaines Rivers, is the single most important reason for the industrial development of the city of Chicago, Illinois. It facilitated trade between the Great Lakes and the Mississippi Valley. As shipping became more sophisticated, goods imported from the Atlantic Ocean (via the St. Lawrence Seaway) and the Gulf of Mexico (via the Mississippi River) could also be traded around the Chicago Portage. Just as the Chicago Portage facilitated communications in the Midwest, the Heavitree Gap allowed for trade in the isolated Outback of Central Australia. The Heavitree Gap was formed by the Todd River, an ephemeral stream, cutting through the MacDonnell Ranges as the region underwent tectonic uplift about 300 million years ago. Today, Heavitree Gap provides the main access point to the city of Alice Springs, Australia. Alice Springs was the site of a major gold rush in the 1880s, and thousands of prospectors flowed through the gap in search of a new life. Today, Alice Springs is the gateway to Australia's most famous natural landmark, Uluru (or Ayers Rock). Wind Gaps Wind gaps are former water gaps—narrow valleys through which a waterway no longer flows. Like water gaps, wind gaps can have an enormous impact on regional history. The Cumberland Gap, for instance, is a wind gap in the southern Appalachian Mountains of present-day Kentucky, Virginia, and Tennessee. The Cumberland Gap was created as an ancient creek cut through the Appalachians, once uplifted higher than the Himalayas. The Cumberland Gap has had strategic value for thousands of years. Native Americans used the gap as a key point for trade and seasonal migration. It became a major part of the Wilderness Road, the route European Americans took to settle in the "wilderness" of Kentucky and Tennessee from the original 13 colonies. Today, Cumberland Gap is part of the national park system.