a deep, steep-sided, narrow river valley
The term used for narrow deep valleys is "ravine."
a glen
There are more than one term for a deep narrow valley. Glens, canyons, ravines, fjords are terms you could use.
The different types of river valleys include V-shaped valleys, U-shaped valleys, and canyon valleys. V-shaped valleys are typically narrow with steep sides, carved by erosion from a river. U-shaped valleys are wider and have more gradual slopes, formed by glaciers. Canyon valleys are deep and narrow with steep sides, often formed by a combination of tectonic activity and erosion.
Those deep valleys are mainly in Wales, in the southwest of Britain.
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.
The deep narrow valleys on the Moon are called rilles. These are long, winding depressions that can stretch for many kilometers across the lunar surface. Rilles are thought to have been formed by a combination of ancient volcanic activity and tectonic forces.
Ravines.
Ravine
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.
A ditch with deep sides is commonly referred to as a "gorge" or "ravine." These terms describe narrow, steep-sided valleys that are typically formed by the erosion of rock and soil by a river or other water flow. In some contexts, a "trench" may also be used to describe a similarly deep and narrow excavation.
deep narrow valleys and mountains