I notice that when I let my dough proof in a bowl for more than 1-2 hours, the top of it becomes thick and dried out, while the bottom (the part touching the bowl) becomes particularly moist. I used a formula for French bread with roughly 70% water, 2% salt, 1% yeast (relative to flour content); I coat the dough in a bit of olive oil before proofing; and I cover the bowl with plastic, though it doesn't seem to help. Since the book I bought on bread-making (Bread Baking: An Artisan's Perspective) suggests proofs of around 4 hours, this is posing a big problem. I also tried making a poolish once and that ended up being a thick, dried out mess too. Could it be that the air in the bowl is causing the exposed part of the dough to dry out?