"Erosional features in the outflow channels often form around obstacles such as craters..."
"Tiu Vallis appears to have started from an area of collapsed terrain (a region known as Hydaspis Chaos), moved northward through a fairly narrow channel, and then spread out and eroded a large area to the north and west. A more detailed view of the source shows the chaotic terrain, the initial channels, and various eroded features:
Erosional features in the outflow channels often form around obstacles such as craters..."
http://www.ig.utexas.edu/research/projects/mars/education/picture_pages/hydaspis_chaos.htm