A valley.
A row or chain of mountains ranged in a line is called a mountain range. A narrow gap through a mountain range is a mountain pass.
Mountain ranges and belts are built through a process called orogenesis
A narrow gap through a mountain range is called a mountain pass. These passes are also sometimes known as gaps, saddles, and cols.
Plateaus?
it was a passage through mountain terrain.
A valley.
Pass
Plasmodesmata.
bone marrow.
osmosis
it could be a valley or a gorge
The name of the mountain range that runs through the mountains states are called the Rocky Mountains.
The Northwest Passage Apex;;;
A Crevasse
A passage through a hill ; underground, is called a TUNNEL. However, a passage between two hills (surface) is a VALLEY. A passage between two hills is dependent on its width. If the hills are low and far apart, then it is a VALE. However, if the hills are high and close together then it is a GLEN or RAVINE, or GORGE.
Smoothing an object's shape is called streamlining. It is used for improving passage through air as well as water.