Igneous rocks are uncommon. As the mountain belt rises, great thicknesses of sandstone and conglomerate are deposited on its flanks and frequently bury much of the foreland. These rocks are typically shallow-water or terrestrial, often red in color, and are called molasse deposits. The stable interior of the continent will often be covered with thin layers of shallow-water or terrestrial sedimentary rocks. This thinly-mantled region is the platform The Shield Areas where ancient crystalline rocks are exposed over wide areas are called shields .
Every continent has at least one shield.
The shield and adjacent platform together make up the craton. Driving from Green Bay to Wausau takes you from a platform into a shield. Metamorphic Zones One of the best indicators of former subduction is the presence of paired metamorphic belts , a belt of typical Greenschist and Amphibolite metamorphism flanked by a belt of Blueschist metamorphism. The rising magma from the descending plate heats the crust, resulting in greenschist and amphibolite metamorphism in the igneous arc. This sort of metamorphism, called granulite metamorphism, occurs deep in the crust just about everywhere simply due to the normal geothermal gradient.