Definition of "undulating"

The Borinage coalfield around Mons is another attractive area for the railway enthusiast; it is rather like South Lancashire, with its gently undulating landscape studded with slag heaps and pithead gear and criss-crossed by railway lines and tramways.

1960 February, J. N. Faulkner, “The Belgian Railways today”, in Trains Illustrated, page 86

In good poetry every word and phrase, as Professor McKail says, reverberates like the sound of a lyre, and leaves after it numberless undulatings. The verse exhales sweet sound, and light-like thought, as perfumes do; but we cannot explain just why.

1930, John Thomas Ingram Bryan, The Philosophy of English Literature, page 27