Explanation: In order to form coal, two processes might be undergone and they are as follows-
Peatification- It is the process by which the remains of trees and plants undergoes partial decomposition in a swampy type of environment, conserving organic matter. It is the first stage of coal formation.
Coalification- It is the process of complete transformation of the dead remains of plants and trees into coal.
usually when a river, ocean or lake...etc becomes deeper is because of the weather the more hydrogen and oxygen atoms the more water :D and also gravitys pull thats pulling it deeper is another reason.