Near where I grew up there is a place called The Pannonian Plain. It's a remnant of a dried up sea.
So maybe we just don't call those geological formations valleys? https://en.m.wikipedia.org/wiki/Pannonian_Basin
Re:Giant 'Great Valley' Found on Mercury
by ubernostrum on Tuesday November 22, 2016 @12:19AM
(#13011834)
I believe Earth's oceans are generally thought of as the baseline, with the continents being relatively higher "plateaus" above them (rather than the continents being the baseline with ocean floors being "valleys" in between).
Although they were formed by a different process, this does sounds kind of like Earth's oceans. Do they not qualify as a geological formation?