The problem I have with superposition is, if God created earth (rocks, land, etc) then the rocks or earth would be the same age. Or the age of Creation.
Forget about age for a second. All the LoS is saying is what came first with respect to what. If God created the Coconino sandstone in situ, then he probably had to have done so at least at the same time if not after the Hermit Shale. The point I am trying to get across is that the LoS should not be very controversial to anyone.
So then why is there a necessity to prove otherwise? I understand that some will say that it's non bias science and only the results matter in a given experiment. How this basic principle can apply to earth and rock(s) amazes me.
It is simply the best explanation for the evidence. We see sand piling up in basis today, it piles up on top of other "stuff". Why shouldn't we expect something like the Coconino sandstone to have been formed the same way especially when it has the same exact internal structure as sand dunes in nearby deserts?
Where does the material come from to form the layers?
Layers, is far too simplistic of a term. There are all kinds of rocks, some that are in layers, some that are not. Rocks that ARE in layers are (originally) built up from the eroded material from rocks up in mountains that are not in layers.
Batholith - Wikipedia
Sometimes, we can even trace the source of a particular "sand" to the mountain it came from by its particularly weird mineral components.
If the earth were billions of years old?
That is kind of the point. It takes a LONG time for granite to be butchered into sand by water and wind. Even in the most catastrophic flood anyone can dream up, it just doesn't happen. It was little things like this that lead the first creationist geologists to question a young age of the earth.
Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)