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Geology reconstructs the composition, structure and history of the solid Earth and other rocky bodies. Its observations include rock textures, fossils, layer relationships, deformation and numerical ages. A cross-cutting intrusion, for example, must be younger than the rock it cuts, even before either has a numerical date.
Physical models connect those relationships to processes such as plate tectonics, sedimentation, metamorphism and erosion. Distinguishing a directly observed structure from the process proposed to explain it is essential: similar present-day rocks can have different formation and alteration histories.