#ravine  a deep narrow steep-sided valley (especially one formed by running water)
  supertype:  #valley__vale  a long depression in the surface of the land that usually contains a river
  subtype:  #canyon__canon  (North America) a ravine formed by a river in an area with little rainfall
  subtype:  #gorge  a deep ravine (usually with a river running through it)
     subtype:  #gulch__flume  a narrow gorge with a stream running through it
     instance:  #Grand_Canyon  #Olduvai_Gorge

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