When you try to "catch" yeast from the air: is it normal for the flour and water to split I'm trying to "catch" wild yeast for the first time. 36 hours ago I mixed a cup of flour and a cup of water and left it out on a shelf with a paper towel over the top. It's been between 45 and 70 degrees here over that time. 

Since then, the water and the flour has separated so that the water is sitting on top of the flour. 

Is that supposed to happen? Should I just keep waiting? Should I give up and try pineapple juice? 
What is the best way to catch wild yeast for sourdough?
Yes.  It seems like you should be stirring occasionally (when you feed your starter).

This site seems like a reasonable reference and, in the section called "Making your starter" it says:


  In between stirrings it's normal for the mixture to separate and for a small layer of water to be on the top.