A glen is a deep, narrow valley, especially in the mountains of Scotland or Ireland.

How can Valley be deep?

Question

A glen is a deep, narrow valley, especially in the mountains of Scotland or Ireland.

How can Valley be deep?

Answer

A valley is deep if the land to either side slopes sharply.

Answer

Se você está no topo da montanha, faz sentido chamar o vale de "profundo / deep".
Eu entendo porque você perguntaria isso.