 Through time the Rupert of the tectonic plates governs the distribution of the continents and also the sizes and the distribution of the ocean basins. So if you think about 230 million years ago it was Pangea and so we've got like a giant kind of almost like Pac-Man looking super continent and an ocean all around it. And as Pangea broke up you get the development of new ocean basins and that influences stuff like sea level. So as you get new ocean basins forming you've got new ocean ridges, they're normally a bit shallower than old oceanic floor that kind of is subsiding and sinking into, it's abducting. And so sea level changes through time because of this on very long geologic time scale. And so you also get just the physical distribution of the continents varies and so if you think about 50 million years ago South America and Antarctica were connected. Antarctica was still around the South Pole but it was quite warm in Antarctica and as the South America and Antarctica separated you've got seafloor spreading that started forming between there. That allowed for ocean to start going all the way around Antarctica and so this is in the area of the Drake Passage and so there's a lot of research that's being done on this topic and it's quite controversial. But one of the ideas is that as South America and Antarctica separated and Drake Passage formed then ocean current could go all the way around Antarctica without hitting any land and that made Antarctica cold. And then after that Antarctica became glaciated around 34 million years ago and we've been in that kind of state ever since. So before 34 million years ago there was no ice on Antarctica.