The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

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

WordNet (r) 3.0 (2006):

glen
    n 1: a narrow secluded valley (in the mountains)