Geochronology is a toolkit of geological dating methods that allow us to assign absolute ages to rocks. Shaun Eaves , CC BY-SA Cosmic rays were first discovered in the early 1900s, but it took almost a century until sufficiently sensitive particle accelerators became available to accurately count the small number of rare atoms produced when they hit Earth. The presence of cosmogenic nuclides in rocks and sediments at the Earth’s surface is an indicator of atmospheric exposure