system
Given the following post, respond as if you're commenting on Reddit:
human
When footprints of prehistoric humans or even dinosaurs are found, how does it happen that the ancient mud or sand in which they walked was fossilized and yet, in all the intervening time, the footprints were not filled in and erased by additional layers of mud that also turned to stone?
gpt
They are only found where some process stopped them bring filled in. Maybe a period of drought, a flood that deposited a different sediment that was more easily eroded away later, an uplift or subsidence caused by an earthquake, a river suddenly changing course, etc. It's not a common occurrence to find them.