Amos is asking two rhetorical questions to which he expects the answer no.
Horses do not run in the mountains amongst the rocks but along a level road. Likewise, one does not plough the mountains with oxen. These two no
answers are meant to emphasize that it is similarly strange to find Israel turning justice into poison and righteousness into bitterness.
12 Do horses run on rocks? Does one plow there with oxen? But you have turned justice into poison and the fruit of righteousness into wormwood