Source : Webster's Revised Unabridged Dictionary (1913)

Glen \Glen\, n. [Of Celtic origin; cf. W. glyn a deep valley,
   Ir. & Gael. gleann valley, glen.]
   A secluded and narrow valley; a dale; a depression between
   hills.

         And wooes the widow's daughter of the glen. --Spenser.

Source : WordNet®

glen
     n : a narrow secluded valley (in the mountains)