Metamorphic rocks form when sedimentary or igneous rocks are buried deep within the Earth during mountain building. In response to the higher temperatures and pressures encountered there, new minerals grow and old features of the rock are destroyed. By these processes, shale changes to schist, limestone to marble, and platonic rock to gneiss. Metamorphic rocks are common in the Columbia, Omineca and Cariboo ranges of the eastern Cordillera, the Shuswap Highland and Coast Mountains. The presence of metamorphic rocks at the surface shows those parts of the Cordillera that were the most strongly uplifted and deeply eroded.
Geological time is measured in terms of both relative and absolute age.
By itself, this method of dating can only be applied locally. However, many sedimentary rocks contain fossils, the petrified remains of animals and plants, and older rocks contain different kinds of fossils than younger rocks. Because similar changes in fossil types occur all over the Earth, rocks in widely separated regions containing similar fossils are assumed to have formed at the same time. Absolute ages , shown in millions of years before the present on the map, are determined by measuring minute changes in amounts of radioactive elements found mainly in igneous rocks. The changes act like a geological stopwatch that starts as soon as the igneous rock cools down and the products of radioactive decay start to accumulate.