Source : Webster's Revised Unabridged Dictionary (1913)

Steepen \Steep"en\, v. i. [imp. & p. p. {Steepened}; p. pr. &
   vb. n. {Steepening}.]
   To become steep or steeper.

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

Source : WordNet®

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