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River Rock such as that pictured below can actually be any of the three rock types: igneous (such as andesite, basalt or rhyolite), metamorphic (such as slate), or sedimentary (such as chert or limestone). Since river rocks are tumbled, shaped and rounded in mountain streams and and river beds over very long periods of time, the appearance can be very similar among all three types. Photo by Michael Marcotte Return to List Back to Michael's Hobbies page Back to Michael Marcotte's HomePage