"Do horses run on the rocky crags? Does one plow the sea with oxen? But you have turned justice into poison and the fruit of righteousness into bitterness - you who rejoice in the conquest of Lo Debar and say, 'Did we not take Karnaim by our own strength?'" NIV translation
Amos gives two examples of ridiculous questions where the answer is obviously no. A horse does not run over rocky terrain because it would stumble and break a leg. Likewise, you do not use an ox or anything else to plow the sea to prepare it for planting. Amos shows that it is just as ridiculous to think that they were able to gain military victory on their own.
For us, it is just as ridiculous to live life without Jesus Christ. Why would anyone want to reject the Lord and go their own way? That, too, is just as ridiculous as trying to plow the ocean.
"For the Lord God Almighty declares, 'I will stir up a nation against you, Israel, that will oppress you all the way from Lebo Hamath to the valley of the Arabah.'" NIV translation
Once again, Amos shares the judgment of God.
Lebo Hamath would be northern Lebanon and the valley of the Arabah would be the Dead Sea area.
This shows that the judgment will be on all of Israel. It also hints at the fact that Assyria would be the tool used for the coming judgment.