The best evidence for a very old earth are angular unconformities. Angular unconformities require several distinct steps each requiring significant lengths of time.
1. Deposition
2. Lithification (
Lithification - Wikipedia)
3. Sometimes metamorphosis
4. Tilting
5. Erosion
6. Deposition
7. Lithification
8. Tilting again on occasions.
Unconformity - Wikipediahttp://perlworks.com/..._unconformity/triassic_sandstone.jpghttp://perlworks.com/...conformity/syncline_unconformity.jpg
The nice thing about unconformities is that they are common and visible in road cuts everywhere. They do not require any assumptions about decay rates, the speed of light, absorption rates, etc.
What are the young earth explanations for angular unconformities?