• gap
    The Cumberland Gap cuts through the southern Appalachian Mountains.

    Photograph by Bruce Dale, National Geographic

    Dreamtime
    Heavitree Gap is sacred to the Arrente people of Central Australia. Their legends say that the gap was created by the movement of giant caterpillars across the ancient Dreamtime landscape.
    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.
  • Term Part of Speech Definition Encyclopedic Entry
    altitude sickness Noun

    illness caused by reduced oxygen levels at high elevations.

    ancient Adjective

    very old.

    bank Noun

    a slope of land adjoining a body of water, or a large elevated area of the sea floor.

    carve Verb

    to cut or slice through.

    cliff Noun

    steep wall of rock, earth, or ice.

    Encyclopedic Entry: cliff
    communication Noun

    sharing of information and ideas.

    continent Noun

    one of the seven main land masses on Earth.

    Encyclopedic Entry: continent
    contour line Noun

    line joining points of equal elevation.

    debris Noun

    remains of something broken or destroyed; waste, or garbage.

    development Noun

    construction or preparation of land for housing, industry, or agriculture.

    elevation Noun

    height above or below sea level.

    Encyclopedic Entry: elevation
    enormous Adjective

    very large.

    ephemeral stream Noun

    body of water that flows only after a fall of precipitation.

    facilitate Verb

    to help or make easier.

    gap Noun

    steep-sided opening through a mountain ridge.

    Encyclopedic Entry: gap
    geologic Adjective

    having to do with the physical formations of the Earth.

    glacier Noun

    mass of ice that moves slowly over land.

    Encyclopedic Entry: glacier
    Great Lakes Noun

    largest freshwater bodies in the world, located in the United States and Canada. Lake Huron, Lake Ontario, Lake Michigan, Lake Erie, and Lake Superior make up the Great Lakes.

    hill Noun

    land that rises above its surroundings and has a rounded summit, usually less than 300 meters (1,000 feet).

    Encyclopedic Entry: hill
    ice age Noun

    long period of cold climate where glaciers cover large parts of the Earth. The last ice age peaked about 20,000 years ago. Also called glacial age.

    ice sheet Noun

    thick layer of glacial ice that covers a large area of land.

    Encyclopedic Entry: ice sheet
    indicate Verb

    to display or show.

    industrial Adjective

    having to do with factories or mechanical production.

    infamous Adjective

    having a very bad reputation.

    isolate Verb

    to set one thing or organism apart from others.

    key col Noun

    highest gap (col) of a mountain peak.

    landmark Noun

    a prominent feature that guides in navigation or marks a site.

    landmass Noun

    large area of land.

    majestic Adjective

    very impressive and formal.

    Midwest Noun

    area of the United States consisting of the following states: Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, and Wisconsin.

    migration Noun

    movement of a group of people or animals from one place to another.

    moraine Noun

    material, such as earth, sand, and gravel, transported by a glacier.

    Encyclopedic Entry: moraine
    mountain Noun

    landmass that forms as tectonic plates interact with each other.

    mountaineer Noun

    someone who climbs mountains.

    national park Noun

    geographic area protected by the national government of a country.

    navigate Verb

    to plan and direct the course of a journey.

    navigate Verb

    to plan and direct the course of a journey.

    Outback Noun

    remote, sparsely populated interior region of Australia.

    pass Noun

    gap or break in rugged terrain, such as a mountain ridge.

    Encyclopedic Entry: pass
    peak Noun

    the very top.

    prominence Noun measurement of the elevation of a mountain's summit by the vertical distance between it and the lowest contour line encircling it and no higher summit.
    prospector Noun

    person who searches or mines land for precious metals.

    retreat Verb

    to go back to a familiar or safe place.

    river Noun

    large stream of flowing fresh water.

    Encyclopedic Entry: river
    route Noun

    path or way.

    rugged Adjective

    having an irregular or jagged surface.

    shipping Noun

    transportation of goods, usually by large boat.

    sophisticated Adjective

    knowledgeable or complex.

    strategic Adjective

    important part of a place or plan.

    stream Noun

    body of flowing water.

    Encyclopedic Entry: stream
    summit Noun

    highest point of a mountain.

    supplemental Adjective

    additional.

    tectonic activity Noun

    movement of tectonic plates resulting in geologic activity such as volcanic eruptions and earthquakes.

    tectonic uplift Noun

    movement of plates beneath the Earth's surface that causes one part of the landscape to rise higher than the surrounding area.

    trade Noun

    buying, selling, or exchanging of goods and services.

    uplift Noun

    elevation of the Earth's surface due to tectonic or other natural activity.

    valley Noun

    depression in the Earth between hills.

    water gap Noun

    small opening where flowing water has carved through a mountain range.

    watershed Noun

    entire river system or an area drained by a river and its tributaries.

    Encyclopedic Entry: watershed
    weather Verb

    to change as a result of exposure to wind, rain, or other atmospheric conditions.

    wilderness Noun

    environment that has remained essentially undisturbed by human activity.

    Encyclopedic Entry: wilderness
    wind gap Noun

    steep-sided opening through a mountain ridge that does not contain a stream.

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