# Formation of petrified trees

In my book, I came across this question:

Give reason why formation of petrified tress is considered as a degradation and a sedimentation action of the underground water.

My book explains that underground water replaces the original plant material in trees with silica and other inorganic materials, and states this as a sedimentation action of underground water. But I don't understand the part of "degradation action" in the question, I think underground water only replaces the plant material in trees so why we consider this to be a degradation action? Or the degradation action mentioned in the question is something else? And if yes, could you explain it for me? Thanks in advance!