A narrow valley is a V-shaped valley, or river valley. A narrow valley has steep sloped sides that look like the letter V. They are formed by a strong stream.
Ravines and chasms.
canyons
There are more than one term for a deep narrow valley. Glens, canyons, ravines, fjords are terms you could use.
what do people build to cross deep rivers or valleys?
Has hills and mountains separated by deep, narrow valleys and the Yalu and Tumen rivers separate North Korea from China.
The Marianas Trench, and many other deep undersea valleys, can be found on the ocean floor.
A deep valley with steep sides would be called a gorge or canyon.
The term used for narrow deep valleys is "ravine."
a glen
a deep, steep-sided, narrow river valley
Those deep valleys are mainly in Wales, in the southwest of Britain.
cracks
Ravines.
Ravine
deep narrow valleys and mountains
Ravine, gorge, canyon.
There are more than one term for a deep narrow valley. Glens, canyons, ravines, fjords are terms you could use.
Deep V-shaped valleys are commonly known as river valleys and are formed by the erosive action of flowing water over time. These valleys typically have steep sides and a narrow bottom, shaped by the continuous erosion of the river cutting through the land.
You may be thinking of a fjord (or fiord), a narrow, deep inlet (generally in a coastal landscape) formed by glacial activity.