The Collaborative International Dictionary of English v.0.48:

Subjacent \Sub*ja"cent\, a. [L. subjacens, p. pr. of subjacere
   to lie under; sub under + jacere to lie.]
   1. Lying under or below.
      [1913 Webster]

   2. Being in a lower situation, though not directly beneath;
      as, hills and subjacent valleys.
      [1913 Webster]

WordNet (r) 3.0 (2006):

subjacent
    adj 1: lying nearby but lower; "hills and subjacent valleys"
           [ant: superjacent]