This large outflow channel is situated north of the Martian dichotomy boundary between the southern highland and the northern lowland.
Video: "Flight over Mawrth Vallis" from 2008 [4:01 min]
Copyright: ESA/DLR/FU Berlin (G. Neukum)
Most likely, the more than 600 km long and up to 2 km deep valley was formed by liquid water. The analysis of data collected by the OMEGA spectrometer onboard the ESA spacecraft Mars Express revealed the existence of clay minerals rich in iron and aluminum. They can be recognized in the movie as white and blue deposits. Comparable minerals on Earth usually result from the weathering of volcanic rocks and form in hydrothermal systems (e.g. by the interaction of a thermal source and water).
Link to release in press archive: Mawrth Vallis Animation (Jan 20, 2012)