How did Thomas and Minho escaped from the Grievers that were chasing them?

Thomas and Minho lured the Grievers to the edge of the horizon..... the cliff.

Just as the first Griever’s arm extended out to nip at them, Minho and Thomas dove in opposite directions, each toward one of the outer walls of the corridor. The tactic had worked for Thomas earlier, and judging by the horrible screeching sound that escaped the first Griever, it had worked again. The monster flew off the edge of the Cliff. Oddly, its battle cry cut off sharply instead of fading as it plummeted to the depths beyond.