Combinations of metamorphic rocks and intrusions of plutonic igneous rock from in regions of crust that were once abnormally heated and thickened. Such thickening results in the uplift of mountain ranges at Earth's surface.
But as mountains rise, the relentless processes of weathering and sediment dispersion go to work, planing down the highlands and exposing once deeply buried rock. In this way nature opens a window for us into the plutonic realm.
On all of Earth's continents there are large areas of such flattened-off metamorphic and plutonic rocks, called shields. These regions make up the ancient cores of the continental blocks. They are rigid and buoyant enough to keep all of us air breathers' heads above water.
Or almost above water. From time to time the sea rises to cover the shields, spreading layers of sediment over the crystalline pavement of the shields. We can see an example of this right, here, at Nun's Corner near Santa Fe, where layers of limestone and shale rest on ancient weathered gneiss which itself once made land as an extension of the Canadian Shield:
Here in New Mexico, a renewal of crustal unrest has broken this arrangement of rocks into great blocks, some standing high, creating a new generation of mountains we call the Rockies. The great peaks and ridges of the Southern Rockies are made of exposed crystalline rock, resistant to erosion, projecting above their sedimentary cover. All of those mountains in the background of the photograph above are underlain by these ancient rocks.
As one of my students said, this is why we have to go up in the attic to see the basement.