Complex sentence: The Fourth Army had crossed Bazentin Ridge, which exposed the German rear-slope defences beyond, to ground observation and on 18 September, the Quadrilateral, where the British advance had been frustrated on the right flank, was captured.
Split into simpler sentences: The Fourth Army crossed Bazentin Ridge, which exposed the German rear-slope defences beyond to ground observation. On 18 September the Quadrilateral, where the British advance had been frustrated on the right flank, was captured.