Why "The animals chased them right down to the bottom of the field"?
I am reading "Animal Farm" by George Orwell. In the chapter VIII I see the sentence "The animals chased them right down to the bottom of the field". Why the writer used "DOWN" and "BOTTOM" in the sentence? I think the animals run on the flat surface but not from the top of a mountain

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They saw that they were in danger of being surrounded. Frederick shouted to his men to get out while the going was good, and the next moment the cowardly enemy was running for dear life. The animals chased them right down to the bottom of the field, and got in some last kicks at them as they forced their way through the thorn hedge.
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Question

Why "The animals chased them right down to the bottom of the field"?
I am reading "Animal Farm" by George Orwell. In the chapter VIII I see the sentence "The animals chased them right down to the bottom of the field". Why the writer used "DOWN" and "BOTTOM" in the sentence? I think the animals run on the flat surface but not from the top of a mountain

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They saw that they were in danger of being surrounded. Frederick shouted to his men to get out while the going was good, and the next moment the cowardly enemy was running for dear life. The animals chased them right down to the bottom of the field, and got in some last kicks at them as they forced their way through the thorn hedge.
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Answer

Yes, Ivan, this is one of those weird things about English, but it is in fact regarded as very good English to say it just like this :-) .  Chasing the animals to the bottom of a field  (or running down to the bottom of the garden) simply mean that the animals were chased to that part of the farm furthest away from the farmhouse (or the part of the garden furthest away from the house), so right to the very end of the farm, as it were. 
One can even talk about going down to the very bottom of a country, which would mean its southern-most point.
Enjoy the book. It's a fantastic read!