The planet over 6000 years old?

I think the geologists started to believe that the world was older than 6000 years because of the layering in the rocks. They could tell approximately how old the rock layers were because of their size and amount of layers. They could also discover that the different layers would hold different fossils of the life forms that died in the period. Some of these animals might have been around lots longer than anything within 6000 years.