Outflow channels are probably relics of catastrophic flooding on Mars long ago. They appear only in equatorial regions and generally do not form extensive interconnected networks. Instead, they are probably the paths taken by huge volumes of water draining from the southern highlands into the northern plains. The onrushing water arising from these flash floods likely also formed the odd teardrop-shaped “islands” (resembling the miniature versions seen in the wet sand of our beaches at low tide) that have been found on the plains close to the ends of the outflow channels. Judging from the width and depth of the channels, the flow rates must have been truly enormous—perhaps as much as a hundred times greater than the 105 tons per second carried by the great Amazon river. Flooding shaped the outflow channels approximately 3 billion years ago, about the same times as the northern volcanic plains formed.
According to paragraph 2, all of the following are true of the outflow channels on Mars EXCEPT:
A. They formed at around the same time that volcanic activity was occurring on the northern plains.
B. They are found only on certain parts of the Martian surface.
C. They sometimes empty onto what appear to have once been the wet sands of tidal beaches.
D. They are thought to have carried water northward from the equatorial regions.