"The Athabasca Valles streamlined mesas on Mars rise 100 meters (330 feet) above the channel floors at their upslope ends and range from a few hundred to a few thousand meters long. Their flat upper surfaces "support an aqueous origin rather than a glacial one," the scientists conclude in their paper.
"The mesas are composed of fine, horizontal layers behind impact craters, adding to evidence that were formed by deposition during flow in the lee of an object. Other streamlined forms, also layered but not flat-topped like the streamlined mesas, more likely were formed by erosion during floods over pre-existing layered terrain, Burr said."

However, the fact that these features were formed by hydrovolcanic activity raises the possibility that some of the craters seen in this region are actually maars rather than impact craters.

See for example this flat-bottomed crater given to accompany the story:

Compare to these examples:

Espenberg Maars
Alaska Science Forum
http://www.gi.alaska.edu/ScienceForum/ASF2/245.html

and

HYDROVOLCANIC PROCESSES
http://www.geol.ucsb.edu/~fisher/hydro.htm

Bob Clark