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?
Thanks in advance, WH
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.