A semi-habitable Mars and the differences between the plains and the craters

I'm developing a world based on a terraformed Mars, which was cut off from interstellar society and regressed to a pre-industrial level of technology. Mars has an ocean, but most of the settled people live in craters, since this is where almost all the arable farmland exists. My explanation for this is that the Martian regolith is sandy and therefore porous, and the only place this is not true is craters and places where rivers and lakes previously laid down impermeable beds of clay. In addition, the crater floors are closer to the water table. In between, the land is a 'wet desert', where mosses and other simple things can grow but otherwise the rain drains too quickly to allow for trees etc. In these plains, nomadic herders scratch a living from animals specially adapted to eat the sparse moss and / or lichen.

My question is: how realistic is this? Could porous regolith prevent plant growth, but allow for lichens and / or mosses? How quickly would soils form in highland regions from whatever plant life there was? Is there any other way I can justify arable farmland being limited to isolated craters and floodplains? Also, how much and where would impermeable lava flows also allow for a raised water table? What might coastlines and rivers look like - also arable?

Edit: I realise mosses aren't necessarily what would would grow in place of trees, I just mean some small, sparse plant-life that would grow in land that wasn't arable, but would potentially feed some sort of nomadic herd.

• Have you had a look through Space Exploration SE? you may find resources to answer your questions there & if what you want is real world info on Mars regolith its makeup & properties etc that may be a better place to ask : here's some links to some Mars related questions there ~ Cyanobacteria & Lichens on Mars ~ How far down can we dig. Apr 5, 2020 at 22:42
• Welcome to worldbuilding . Please take the tour and when you have some free time read-up in the help center about how we work, whilst you're at it, you will see the that we deal with one question at a time here. Please edit yours down to a single question. We encourage you to post further questiions in seperate threads. Apr 5, 2020 at 23:05
• Mars has an ocean then it was some long time after Mars was terraformed, it already has a non-trivial atmosphere and so on. Which makes it plausible for the regolith to no longer be a fine dust by coalesced into bedrock - got rid of its perchlorates, with the chlorine in the calcium salts washed as more soluble salts (sodium) and replaced by calcium carbonate resulted from the thawed carbon dioxide in the soil. I don't think your 'wet deserts' are realistic at all. Apr 5, 2020 at 23:08