Sargasso of Lost Starships by Poul Anderson

It was a strange little army, thought Donovan, as strange as any the Galaxy had ever seen.

He looked back. The old ruined highway went down a narrow valley between sheer cliffs of eroded black stone reaching up toward the deep purplish heaven. The sun was wheeling westerly, a dull red ember throwing light like clotted blood on the dreariness of rock and ice and gaunt gray trees; a few snowflakes, borne on a thin chill wind, drifted across the path of march. A lonely bird, cruel-beaked and watchful, hovered on great black wings far overhead, waiting for them to die.