Complex sentence: In front of the line lay an otherwise unremarkable chalk outcrop, the Butte de Walencourt which gave some observation over the rear of the British lines in that area, which was to be the object of the next attack.
Split into simpler sentences: In front of the line lay an otherwise unremarkable chalk outcrop, the Butte de Walencourt, which gave some observation over the rear of the British lines in that area. This was to be the object of the next attack.