steep·en /ˈstipən, pṃ/
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.
v 1: become steeper; "The mountain side has steepened"
2: make steeper; "The landslides have steepened the mountain