During an orogeny tectonic phases lead to mountain building, which causes deformation and metamorphism of rocks.
When new sedimentary layers are deposited on top of tilted ones, they will have an angle with the older ones, a structure which is called an angular unconformity.
The Alleghenian orogeny in North America (during the Carboniferous period) for example can be found as an angular unconformity between rock layers in large parts of that continent.
By analysing the sequence in which the minerals and structures were formed, more than one phase can be found in most metamorphic rocks (this is called polymetamorphism).
By radiometric dating the absolute age of the different phases of mineral growth can be determined, showing when the rock was under certain pressure-temperature conditions.