Sedimentary rocks can be dated using radioactive carbon, but because carbon decays relatively quickly, this only works for rocks younger than about 50 thousand years. So in order to date most older fossils, scientists look for layers of igneous rock or volcanic ash above and below the fossil.

What types of rocks are radioactive?

Thorium abundanceIgneous RocksSedimentary RocksSyenites and phonolitesShales, clays, mudrocksPelagic clays and siliceous oozesGranites, rhyolites and intermediate igneous rocksSandstonesBasalts and other mafic rocksPhosphorites5 more rows•3 Dec 2019