The Collaborative International Dictionary of English v.0.48:

Steepen \Steep"en\ (st[=e]p"'n), v. i. [imp. & p. p. Steepened
   (st[=e]p"'nd); p. pr. & vb. n. Steepening.]
   To become steep or steeper.
   [1913 Webster]

         As the way steepened . . . I could detect in the hollow
         of the hill some traces of the old path. --H. Miller.
   [1913 Webster]

WordNet (r) 3.0 (2006):

steepen
    v 1: become steeper; "The mountain side has steepened"
    2: make steeper; "The landslides have steepened the mountain
       sides"