As the nineteenth century turned to the twentieth, the overwhelming majority of geologists thought that Earth’s great geographic variety was primarily the consequence of bodies of rock moving up and down. Put simply, mountains were places that recently moved up and oceans were places that had recently moved down. Sometimes it was suggested that regions had gone from being high to being low in several episodes. The proposed driving mechanism for these changes were cooling of the Earth and gravitational instabilities. Cooling was used to explain contraction, compression, and formation of mountain belts.