Hi Loafers. I've been lurking and learning from the Fresh Loaf contributors for some months but this is my first post. Hopefully someone can suggest a cure for my sourdough blues. I've been home bread-baking for 15 odd years with commercial yeast leavening, but about a year ago decided to experiment with sourdough. My starter was based on 1/3 wholewheat flour to 2/3 white. It came to life with gusto. I live in Africa and I guess our wild yeast are really wild! 1kg of dough rises well and almost as fast as with commercial yeast, but the baked loaves (which look magnificent) often have huge cavities below the top crust running the full length of the loaf. I think it's known as "floating crust". Slices fall apart and are virtually useless. I've never had this problem with conventional leavening. What causes it? And how do I prevent it?