Why can't we date sedimentary rocks directly? How do we assign numerical ages to intervals on the geologic column, to produce a geologic time scale?

We cannot date sedimentary rocks directly because they are formed from accumulated and cemented particles, making their age different from the particles they contain. Radiometric dating, which works for igneous and metamorphic rocks, is not suitable for sedimentary rocks due to the lack of useful isotopes. Instead, we assign numerical ages to intervals on the geologic column by dating interbedded igneous rocks using radiometric dating methods. This provides a minimum or maximum age for the sedimentary layers, allowing us to estimate the numerical age of intervals on the geologic time scale. The geologic time scale is developed through the correlation of rock layers, fossils, and radiometric dating methods, providing a quantitative record of Earth's history and insights into the evolution of organisms, environments, and geologic events.