The part you refer to occurs in part 3 of the book. It is when Montag is drifting down the river after having fled his house where he'd kille Beatty. He had stopped at Faber's house and Faber gave him old clothes, a suitcase, and told him to head to the river. The "great, black creature" is the land he sees from the river at night. It goes on and on because he's not in the city any longer. The description adds to the tone of alienation and loneliness. Montag feels he is completely alone in the world because everything he has ever known, he has left behind. He has no idea of the direction in which he is headed. Even as this feeling is flowing over him like the comforting water of the river is flowing over him, he begins to transition to a peaceful and happier mindset. He realizes that the Hound, Millie, and the closed society he left is gone and he's happy to be free from them. In his fantasy, which stems from a childhood memory, Montag sees a barn with a hayloft and farm animals. He sleeps in the soft hay of the hayloft and in the morning, at the foot of the stairs leading to the loft, is some fresh fruit and a cold glass of milk. This represents a desire to return to simple, natural things in life. It is at this point that Montag stops his fantasy and steps from the river onto the land. He is also stepping into a new life.