Superposition gives an order, not a numerical age. A widespread, recognizable species that existed briefly can help correlate layers at different sites. Disturbance and reworking must be considered.

The model has a domain of validity. Explain its assumptions before using it in a changed context, then compare your prediction with evidence.

• Relative dating orders events as older or younger. In undisturbed sedimentary layers, lower layers formed before higher ones.
• Folding or overturning can invalidate a simple lower-equals-older reading.