The geography of the ancient past. Paleogeographers study the changing positions of the continents and the ancient extent of land, mountains, and shallow-sea and deep-ocean basins. The Earth's geography changes because its surface is in constant motion due to plate tectonics. The continents move at rates of 2–10 cm/yr (0.75–4 in./yr). Though this may seem slow, over millions of years continents can travel across the globe. As the continents move, new ocean basins form, mountains rise and erode, and sea level rises and falls. The best way to illustrate these changes is through a series of paleogeographic maps.