Man vs. Nature means that a person is dealing with something in nature that could harm him, his family, or society. For example: a hurricane could be a force of nature that could harm you, your family, or society. Another example might be a volcano. Another one might be a storm at sea. The antagonist is nature, and man has to overcome it and survive through it.
When Montag escapes the Hound, he changes into Faber's clothes, douses himself with liquor, and jumps into the river. He finds the river very comforting.
"He floated on his back.....the river was very real; it held him comfortably and gave him the time at last, the leisure, to consider this month, this year, and a lifetime of years. He listened to his heart slow. His thoughts stopped rushing with his blood." (pg 140)
However, when he reaches the land, it is a different story. After he has floated a while, his heels scrap on the pebbles of shore. He sees the land as a menacing creature.
"He looked in at the great black creature without eyes or light, without shape, with only a size that went thousands of miles, without wanting to stop, with its grass hills and forests that were waiting for him." (pg 141)
When he steps from the river, he is overcome with fear. The land is causing the fear. It brings back memories of a fearful childhood incident.
"The land rushed at him, a tidal wave. He was crushed by darkness and the look of the country and the million odors on the wind that iced his body. He fell back under the breaking curve of darkness and sound and smell. He whirled......He wanted to plunge in the river again and let it idle him safely on down somewhere. This dark land rising was like that day in his childhood, swimming, when from nowhere the largest wave in the history of remembering slammed him down in salt mud and green darkness, water burning mouth and nose, retching his stomach, screaming! Too much water!
Too much land." (pg143)
He sees two eyes in the darkness, and he thought it was the Hound. He gave a last loud shout, and the eyes exploded and were gone. It was a deer. He then started walking and was filled with all the scents of the earth. Suddenly his fear diminished.
"He stood breathing, and the more he breathed the land in, the more he was filled up with all the details of the land. He was not empty. There was more than enough here to fill him. There would always be more than enough." (pg144)
However, his greatest comfort comes when he finds something manmade; railroad tracks.
"Here was the path to wherever he was going. Here was the single familiar thing, the magic charm he might need a little while, to touch, to feel beneath his feet as he moved on into the bramble bushes and the lake of smelling and feeling and touching, among the whispers and the blowing down of leaves." (pg 145)
It was the fear of the unknown, of what those forest and hills could offer him. He was unfamiliar with the land and he had no idea how to survive. He was terrified.