I would really appreciate it if you could help me with my question
what is the role of 'since '? I thought, part of sentence after 'since' is IC, Independent clause.,but I think in this case, it doesn't make sense.

1.In lowland country almost any spot on the ground may overlie what was once the bed of a river that has since become buried by soil.

Question

I would really appreciate it if you could help me with my question
what is the role of 'since '?  I thought, part of sentence after 'since' is  IC, Independent clause.,but I think in this case, it doesn't make sense.

1.In lowland country almost any spot on the ground may overlie what was once the bed of a river that has since become buried by soil.

Answer

Another way to say after.  Very good catch as it is not really needed to improve or clarify that sentence.  Another writer making English even harder to understand.