Complex sentence: But the Ulstermen advanced during the bombardment by pushing forward small trenches the depth of a man, then cutting the barbed wire which was 30 inches in depth and height in places (before bombardment).
Split into simpler sentences: But the Ulstermen advanced during the bombardment by pushing forward small trenches the depth of a man. Then cutting the barbed wire which was 30' in depth and height in places (before bombardment).