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Foothills or piedmont are geographically defined as gradual increases in elevation at the base of a mountain range, higher hill range or an upland area. They are a transition zone between plains and low relief hills and the adjacent topographically higher mountains, hills, and uplands.  Frequently foothills consist of alluvial fans, coalesced alluvial fans and dissected plateaus.
Foothills primarily border mountains, especially those which are reached through low ridges that increase in size closer and closer to the mountain,  but can also border uplands and higher hills. 
Areas where foothills exist, or areas commonly referred to as the foothills, include:
- The Sierra Nevada foothills of California, USA
- The Foothills of the San Gabriel Valley in Los Angeles County, California, USA
- The Front Range along the Rocky Mountains in Colorado, USA
- The Wasatch Front along the Wasatch Mountains in Utah, USA
- The Rocky Mountain Foothills in British Columbia and Alberta, Canada
- The Alpine foothills around the European Alps
- The Silesian Foothills in Silesia
- The Siwalik Hills along the Himalayas in the Indian subcontinent
- The Catalina Foothills in Tucson, Arizona, USA
- The foothills in Western North Carolina and Northwestern South Carolina, USA
- The Margalla hills near the Himalayas in Pakistan
- The Duars, Chos and Terai on the foothills of Himalayas (India)
- The foothills around Boise in Idaho, USA
- 'The foothills' of the Dandenong Ranges in Melbourne, Australia. Generally the area from Ferntree Gully/ Boronia/ The Basin through to Belgrave.
- 'The foothills' of the Blue Mountains in Sydney, Australia.
- The foothills between Aeolis Palus and Aeolis Mons, Mars.
Another word for a foothill region is "piedmont", derived from "foot of the mount" in Romance languages.  The Piedmont region of Italy lies in the foothills of the Alps, and several other foothills in other parts of the world are called "piedmont", and include:
- The piedmont of the United States which consists of the eastern foothills of the Appalachian Mountains.
Ecosystems of piedmonts (foothills) are often known as submontane zones, relating to the higher montane ecosystems.
- Easterbrook, Don J. (1999). Surface Processes and Landforms (second ed.). Upper Saddle River, New Jersey: Prentice Hall. p. 530. ISBN 978-0-13-860958-0.
- Juanico, Meliton B.; Agno, Lydia N. Physical Geography. Goodwill Trading Co., Inc. p. 113. ISBN 978-971-12-0113-5.
- "foothill". Merriam-Webster. Retrieved 9 April 2016.
- "piedmont". Oxford English Dictionary (3rd ed.). Oxford University Press. September 2005. (Subscription or UK public library membership required.)
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