Vallis (plural valles) is the Latin word for valley. It is used in planetary geology for the naming of landform features on other planets.
Vallis was used for old river valleys that were discovered on Mars, when probes were first sent to Mars. The Viking Orbiters caused a revolution in our ideas about water on Mars; huge river valleys were found in many areas. Space craft cameras showed that floods of water broke through dams, carved deep valleys, eroded grooves into bedrock, and traveled thousands of kilometers.
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Her Desher Valles, as seen by HiRISE.
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Her Desher Valles close-up, as seen by HiRISE.
Read more about this topic: Coprates Quadrangle