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This question assumes that each of these layers is annual, and this is obviously not the case. Numerous experiments have been done on the formation. You can take a section of Green River formation and grind it to powder, drop it into moving water and it will resort itself into many fine layers. It has been shown that the layers are not annual at all. There are places in this formation where over 1500 layers in some areas and only 1000 in others, all between the same to ash layers called "event horizons." See Creation Magazine June-Aug. 1997. This subject is dealt with in great detail at the Institute for Creation Research (619) 448-0900, or you can contact their web page at www.icr.org .