Geologists and paleontologists based the geologic table on the relative positions of different strata and fossils. They estimated the time scales based on studying rates of various processes. They estimated weathering, erosion, sedimentation, and how long it took to turn sediment into hard rock. The discovery of radioactivity in 1896 and its geological applications by radiometric dating happened in the first half of the 20th century. It made possible the absolute dating of rocks, and the discovery of the age of the Earth.