Complex sentence: But the key terrain feature was the ridge running from Hannut through Crehen and Merdorp, forming a watershed: north of the ridge, the Petite Gette flowed north into the Escaut River, south of it, the Mehaigne flowed south into the Meuse.
Split into simpler sentences: But the key terrain feature was the ridge running from Hannut through Crehen and Merdorp. North of the ridge, the Petite Gette flowed north into the Escaut River, south of it, the Mehaigne flowed south into the Meuse.