Problem
Absolute ages (in numbers of years) can normally only be determined for igneous and some metamorphic rocks, while sedimentary rocks can normally only be given relative ages (like Cretaceous or Cambrian) based on the fossil life they contain, and by application of the Laws of Stratigraphy. However, to fully understand many parts of Earth's history, it is vital to be able to connect the relative ages of biological events (like mass extinctions) with the absolute ages of physical events (like major volcanic eruptions or episodes of mountain building).
1. Explain in general terms how researchers estimate absolute ages for sedimentary rocks that contain identifiable fossil communities (and therefore have specific relative ages in the geologic time scale).
2. Also, discuss at least two sources of uncertainty in the resulting age estimates.