Complex sentence: The region north of the mountains was a production center for grain, cotton and heavy farm machinery while its two main oilfields, at Maykop, near the Black Sea, and Grozny, adjoining the Caspian, produced about 10 per cent of all Soviet oil.
Split into simpler sentences: The region north of the mountains produced grain, cotton and heavy farm machinery. Its two main oilfields, at Maykop, near the Black Sea, and Grozny, near the Caspian, produced about 10 per cent of all Soviet oil.