How old are fossils?

A concise survey of ways scientists estimate the ages of fossils and the rock layers that contain them, outlining relative approaches based on stratigraphy and observable processes and absolute approaches enabled by radioactive elements. It explains methods grounded in rates of erosion, sedimentation, and salt accumulation, highlights uncertainties from variable rates, breaks in the rock record, and limits of uniformitarian assumptions, and emphasizes the immense scales involved by citing vast sedimentary sequences exposed in regions such as the Grand Canyon. The account offers rough numerical estimates to convey the magnitude of geological time without claiming precise values.