Encyclopedic Entry

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.

Vocabulary

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.

Credits

Media Credits

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Editor

Jeannie Evers

Producer

Caryl-Sue, National Geographic Society

Sources

Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

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