The dura consists of an outer periosteal layer, which is tightly adherent to the skull, and an inner meningeal layer. In most regions the two layers appear fused into a single one, and some textbooks describe it that way.
Deep to the dura lies the arachnoid, a delicate membrane that is easily separated from the dura in most places, but pressed tightly against it by the pressure of the CSF lying in contact with it's inner surface. The third meningeal layer is the pia, a fine cellular layer which is adherent to the surface of the brain. As shown here , the pia dips into the complex sulci and fissures that characterize the surface of the hemisphere, whereas the arachnoid does not. Since these two membranes form the borders of the subarachnoid space, small pools of CSF accumulate in these regions.