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: