Alan Dean Foster 
Carnivores of Light and Darkness 

It advanced not in the manner of a living creature but in the manner of sand. It had no arms and then a hundred, no feet but one as wide as the base of the advancing dune itself. Everywhere and all of it was dark red, like all the rust that had ever afflicted the metals of the world squeezed into a swiftly shifting pyramid rage. The dune howled and moaned and bellowed like some sky-scraping banshee unwillingly fastened to the Earth. And in the midst of all that geologic fury, two thirds up the face of the oncoming mountain were two eyes…