Mariner 4, 6, 7, were flyby missions that could only photograph small regions of the Martian surface. They saw portions of the surface that suggested Mars was drab and cratered like the Moon, and geologically dead. This was changed profoundly by the orbiter Mariner 9, which went into orbit around Mars in late 1971. When it arrived the entire Martian surface was engulfed in a dust storm that left almost no surface features visible.

When the dust storm finally subsided in early 1972, Mariner 9 discovered that we had been badly misled by the earlier flyby missions that had seen only small (in retrospect, unrepresentative) portions of the surface. Mariner 9 found evidence for a planet having many interesting geological features:

1. Meteor craters and volcanic plains (the largest crater is Hellas, which is 2000 km across).
   
2. Huge volcanic cones (the three round features at the left of the adjacent image are volcanic cones).
   
3. Gorges larger than the Grand Canyon here on Earth (the feature in the center of the adjacent image is a canyon system, Valles Marineris, that extends over a region the width of the United States).
   
4. Vast sedimentary deposits in the Polar regions.
   
5. Valleys that looked as if they could be water-formed (but these don't coincide with the "canals" that people erroneously thought they saw from Earth in earlier times).

Information supplied by: http://csep10.phys.utk.edu