Webster’s Revised Unabridged Dictionary · 1913
Steepen
Steep′en (stēp′'n), 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.