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In images of Jezero crater on Mars, it looks like the delta fan is higher than the surrounding crater floor.

https://www.nasa.gov/image-feature/traversing-mars-jezero-crater

This indeed seems to be the case with closer-up images from the Perseverance rover taken from the edge of the delta.

https://www.jpl.nasa.gov/news/nasas-perseverance-sheds-more-light-on-jezero-craters-watery-past

Intuitively, I would guess that a river flowing would dig a kind of trench in the ground where it flows, so the ancient river bed would be lower than its surroundings. So how did the delta end up higher than its surroundings?

My hypothesis is that sediments were deposited, and erosion wore off the rocks around the delta, but somehow, the sediments from the delta were harder and didn't wear off as fast. Is this indeed the case, or is some other mechanism at play?

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    $\begingroup$ Bear in mind that, as a delta, it was emplaced under water, the crater being filled by a lake at the time. $\endgroup$ Oct 21 at 19:55
  • $\begingroup$ Relevant paper: doi.org/10.1016/j.pss.2012.02.003 $\endgroup$ Oct 22 at 10:19
  • $\begingroup$ Even if not an actual delta (that is, deposited under water), alluvial fans in fairly dry regions have a similar geometry. That is, most of the material is transported by fairly infrequent "flash flood" events, rather than by continual erosion & transport. $\endgroup$
    – jamesqf
    Oct 24 at 2:57

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