Nirgal Vallis that runs in two quadrangles has features looking like those caused by sapping. Picture taken with THEMIS.
|Naming||the word for "Mars" in Babylonian|
Nirgal Vallis is a long river channel bordering the Coprates quadrangle, Phoenicis Lacus quadrangle and Margaritifer Sinus quadrangle of Mars at 28.4° south latitude and 42° west longitude. It is 496 km long and is named after the word for "Mars" in Babylonian. The western half of Nirgal Valles is a branched system, but the eastern half is a tightly sinuous, deeply entrenched valley. Nirgal Valles ends at Uzboi Vallis. Tributaries are very short and end in steep-walled valley heads, often called "amphitheater-headed valleys." The shape of these valley heads is like cirques on the Earth.
Nirgal Vallis and sapping 
Nirgal Valles is one of the longest valley networks on Mars. It is so large that it is found on three quadrangles. Scientists are not sure about how all the ancient river valleys were formed. There is evidence that instead of rain or snow, the water that formed the valleys originated under ground. One mechanism that has been advanced is sapping. In this case, the ground just gives away as water comes out. Sapping is common in some desert areas in America's Southwest. It forms alcoves and stubby tributaries; these features are visible in the pictures below of Nirgal Vallis taken with Mars Odyssey's THEMIS.
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