The deep ocean currents are the cold ocean currents. They appear in the Arctic and Antarctic circles. As the warm ocean currents come from the low latitudes toward the high latitudes, they bring in warmer water on the surface. They start to cool off though as the near the high latitudes, so gradually they become cold ocean currents. As they become cold, the water is sinking below the less dense warmer water that is coming from behind, making a turn in the opposite direction and moving as deep ocean current toward the low latitudes. Once it reaches the low latitudes, this water will get warmer, and it will move toward the high latitudes, and the process constantly continues.
When examining a cross section of rock layers, the oldest layer is on the bottom. This is because over time, newer rock layers continued to form on top of the oldest one.
