It appears that MER-A only observed igneous rocks while on the floor of Gusev crater and began seeing sedimentary rocks as it climbed Columbia hills. I'm confused. Shouldn't sedimentary rocks be common on the crater floor? Why do they only appear in the Columbia hills? Did a lava flow fill the bottom of Gusev crater? If so, where did the lava come from? Also why are these sedimentary rocks so concentrated with salts? I read somewhere that the concentrations are as high as 40%. I'm an engineer and not a geologist. To my limited intuition, such high concentrations should be on the bottom of a dry lake bed. However we're seeing this in the vast level plains of Meridiani and in the Columbia hills. What's the mechanism for concentrating so much salt in places that are not obvious lake bottoms?

Gary