Below it, the broad lowland marked by a dry river donga lay in shadow, while the early sky gathered
What does "marked by" mean in the sentence above?
Does it simply mean that there is a dry river donga in the lowland, or does it mean that the lowland is surrounded by a dry river donga?
From a distance, if you trace the path of where the river once was (the dry river), you could easily see Donga. The path of the old river showed/marked/pointed to/made it easy to see.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.