Mercer, who's battery was well behind the crest of the ridge speaks, and I'm working just from memory here, of seeing the tops of the bearskins and plumes of the French cavalry when they were rallying, before renewing their charge, at the lower end of the forward slope. The ridges were not very pronounced and it could be that the French artillery were firing at Allied standards or groups of mounted officers or any other feature taller than an infantryman. Of course with nothing visible they may have fired blind, concentrating on skimming their shot just over the crest of the ridge and hoping they hit something. A possibly heretical view, I know, but I suspect that the French artillery would have felt it incumbent on them to do something even if lacking a visible target.