for the mammoth bobbing shape that overtopped the ridge was only a head - a mitred double head - and below it in terrible vastness loped the frightful swollen body that bore it ; the mountain-high monstrosity that walked in stealth and silence ; the hyaena-like distortion of a giant anthropoid shape that trotted blackly against the sky , its repulsive pair of cone-capped heads reaching half way to the zenith .