How do geologist know what they are looking at really is what they say it is?

Take a look at this picture: In this is see lots of layered slabs and on one slab in the upper right area what looks a lot like a dried mud surface, but there is also lots of indications of weathering, the lower bald spots that look like loose soil or dirt.What can be said about how the stuff in this picture was created?

It is mars but that is partly why I am asking the question.The topic is "How do geologist know what they are looking at really is what they say it is?" and here is such an example.What I see was described above but we have real geologists here that I hoped could perhaps expand on what is seen in the image.