| The oldest rocks, layers 1,2, and 3, were deposited in succession, and they contain fossils that establish their relative age as Late Cretaceous. The granite dike cutting through the shale (#1) and sandstone (#2) must be younger as it shows contact metamorphism with those rocks. Scientists verify this observation by using isotopic methods to determine the age of the dike in years (85.mya). Since the dike is younger than the shale and sandstone deposits, they must be older than 85 mya. The lava flow on top of layer 3 has been dated isotopically at 80 mya. Therefore, we can deduce that layer 3 and its fossils must have been deposited between 80 and 85 mya. Contact metamorphism occurred when the hot lava flowed onto layer 3, but there is none between the lava flow and the limestone (#4). Why? The lava (80 mya) had cooled and solidified before the limestone was deposited, and so layer 4 must be younger than 80 mya. (Modified from Edwards and Pojeta, 1994) |