The Case of the Missing Crater Rim

In this observation from NASA's Mars Reconnaissance Orbiter, these two craters perched at the edge of an outflow channel, appear to have lost a portion of their crater rims during a flood event.

Alternatively, it is also possible that the craters impacted the edge of the outflow channel after the flood occurred and we are seeing the difference in the strength of the material impacted.

https://photojournal.jpl.nasa.gov/catalog/PIA21881