Many of these areas are spectacular when viewed from space. Most of these regions have been geologically stable for a long time with the last major tectonic or metamorphic event having occurred before the beginning of the Paleozoic.
Crystalline rocks comprise the bulk of the rocks exposed in shields.
Most of these rocks have experienced repeated cycles of deformation, intrusion, and metamorphism in which these processes acted while the rocks were buried at considerable depth. This implies that flow folding and other low-viscosity types of deformation dominated during much of a cycle. Cratonic shields have served as nuclei for continuing continental accretion as other shields, island arcs, fold belts, etc. are driven against their margins. Most shield rocks record structural over-printing from several tectonic cycles. Later uplift and erosion superimpose tensile fracture patterns over other patterns present.