Complex sentence: The mountain range extends for 320 Km in an east to west direction, forming the southern edge of the Khorat Plateau, from the Mekong River until they merge with the highlands of the Sankambeng, the southern arm of the Dong Phaya Yen Mountains in Thailand.
Split into simpler sentences: The mountain range extends in an east to west direction, forming the southern edge of the Khorat Plateau. They stretch for 320 Km from the Mekong River until they merge with the highlands of the Sankambeng, the southern arm of the Dong Phaya Yen Mountains in Thailand.