The best description of uniformitarianism is the present is the key to the past. People who are proponents of this theory believe that everything, same laws and processes that are happening today, have already happened in the same way, meaning that in the future, the same thing will also happen again. These laws and processes have always operated in the past, are operating now, and will operate in the future. So, we only need to look at the present events to understand what used to happen in the past.
