Wife and i drove out to an area called the Painted Hills today. We hadn't been there yet. Small area but pretty interesting. I always thought these soft, layered formations were sedimentatious but they were igneous. Lava dust carried minerals like iron ores (reddish layers) and magnesium oxides (black blotches) and leached them into the soft clays that formed from the dust. What i found interesting is that nothing grows on these hills. All around them sagebrush, rabbitbush, grasses, cactus grow - but nothing on the actual hill. Couldn't find any reasons for that in the literature they provide.