Shadows of Empire — Lester Del Rey

As the battalions of Earth withdraw from Mars, in the fading days of an interplanetary empire, puzzles remain. The V'nothi had disappeared before the Pyramids were put up, leaving only pictures of themselves in the ruins of Mars, looking like big, good-natured Vikings, complete to brawn and winged helmets. Archaeologists were still swearing every time they looked at those pictures and wondered what men on horseback were doing on Mars, and why no bones had ever been found.

But one person in the Battalion seems to know more than he should about the ancients of Mars...