Complex sentence: North of the seam is an early section of the wall built of large ashlar blocks with drafted margins around a prominent boss and laid out in homogenous header and stretcher courses one above the other.
Split into simpler sentences: North of the seam is an early section of the wall built of large ashlar blocks. These blocks have faces with drafted margins around a prominent boss and are laid in homogenous header and stretcher courses, one above the other.