Fran, 'basic noise' and it should look gritty, then you just lower the height of the terrain....this particular thing is very difficult to get right but you just have to keep going back to the terrain editor, add noise, lower, raise a little, add noise, etc until it looks the way you want it. Unfortunately, for this tut there's no, click this you get that, everything requires tweaking, tweaking tweaking, remember I had a month to do this challenge image, it didn't go together and look great first try. The tutorial itself is a starting place....=)

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