The answer to the question 'In a series of undisturbed rock layers where shale lies between sandstone below and limestone above, the __." would be limestone is youngest. Out of all the rock layers, the limestone is dated to be the youngest of all.
Age of sedimentary rocks refers to the time that took for the material making the rock layer to be deposited.The determination of relative ages of sedimentary layers is based on the order in which the geologic events took place.
Therefore, if a layer of sediment overlies another layer , it means that the overlying layer must be younger.
Same case if a layer of of shale lies between sandstone below and limestone above, it means that the limestone is the youngest among the three rock layers.
