Have they always been here since the river was formed? Are some newer than others?
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1$\begingroup$ Where do all stones come from is far too broad a question to ask. I suggest you edit your question to ask about something more specific like only riverbed stones. $\endgroup$– bonJan 28, 2018 at 20:59
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1$\begingroup$ A vaguely related question: earthscience.stackexchange.com/questions/1048/… The answers (being somewhat different from each other) demonstrates the need for specifics. $\endgroup$– userLTKJan 28, 2018 at 22:16
1 Answer
Riverbed 'stones' - I assume you mean things like pebbles, boulders, etc. are pieces of rock that have weathered out and been deposited in the river. Some come from rock that is very close to where they are located and some have been transported from very far away. In general (and it is a very broad generalization) the rounder the stone, the longer it has been in the river and the more likely it is to have come from far away. Of course that depends on the hardness of the rock, and other factors, too.
Some rocks are newer than others. Some have been formed quite recently and some are billions of years old.