Complex sentence: Today, hardly anything remains of the anti-tank ditches, but at the time they must have been the most conspicuous of all the fortifications; a few remain, much humbled, as field drains or field boundaries whereas others can be seen only as crop marks.
Split into simpler sentences: Today, hardly anything remains of the anti-tank ditches, but at the time they must have been the most conspicuous of all the fortifications. A few remain, much humbled, as field drains or field boundaries, whereas others can be seen only as crop marks.