So the Tharsis Bulge was the most important factor in shaping the surface of Mars.  The other major factor was meteor fall.  In the Noachian Age, three to four billion years ago, meteors were falling on Mars at a tremendous rate, millions of them, and thousands of them planetesimals, rocks as big as Vega or Phobos.  One of the impacts left behind Hellas Basin, 1600 kilometers in diameter, the largest obvious crater in the solar systems; although Daedalia Planum appears to be the remains of an impact basin 4500 kilometers across.  Those are big; but then there are areologists who believe that the entire northern hemisphere of Mars is an ancient impact basin.

These big impacts created explosions so cataclysmic that it is hard to imagine them; ejecta from them ended up on the Earth and the Moon, and  as asteroids in Trojan orbits; some areologists think that the Tharsis bulge started because of the Hellas impact; others believe that Phobos and Deimos are ejecta.  And these were only the largest impacts.  Smaller stones fell every day, so that oldest surfaces on Mars are saturated with cratering, the landscape a palimpsest of newer rings obscuring older ones, with no patch of land untouched.  And each of these impacts released explosions of heat that melted rock; elements were broken out of their matrix and fired away in the form of hot gases, liquids, new minerals.  This and the outgassing from the core produced an atmosphere, and lots of water; there were clouds, storms, rain and snow, glaciers, streams, rivers, lakes, all scouring the land, all leaving unmistakable marks of their passage-flood channels, stream beds, shorelines, every kind of hydrologic hieroglyphic.