Water goes downhill, straight up through the roots when it can. The ridge just lets the soil saturate, so the roots pull it up by capillarity, then drop it where it can. If the soil is dry it’ll stay in the cracks, if wet it’ll seep through the root mats like a sponge. You can test it by sticking a stick in the soil and watching for condensation on the bark. That's how the oak moves its water.