This closeup view from NASA's Curiosity rover shows finely layered rocks, deposited by wind long ago as migrating sand dunes.
Credits: NASA/JPL-Caltech/MSSS
My guess is water.
Mars may have been completely inundated with water, in the beginning, pole to pole, and no dry land at all. It eventually either sublimated into space, or sank into the core, or both.
WHo knows, even Earth’s massive oceans may have come from MARS!..............