Which statement describes uniformitarianism? The continents at one time were all part of a large landmass, but then they separated and moved apart. In an undisturbed layer of rock, older rocks lie at the bottom and younger rocks lie at the top. The forces that act to change Earth's surface today are the same forces that acted upon Earth's surface in the past. Layers of sedimentary rock contain more fossils than layers of metamorphic or igneous rock.