Yes, regarding the heat sink rock. Think about your scenario about standing on the rock versus the soil or wood. If you were to try that in the morning, the rock would feel cool a lot longer than the other materials. Later in the evening, after the sun goes down, it remains warmer much longer, because it has absorbed so much more energy during the day. Now if you don't supply enough energy (as per the 60W bulb example) to increase the rock's temperature to the same as the rest of the enclosure, then it will constantly be a heat sink; because energy goes from warm to cold.