This chapter introduces basic geology and discuss the effect of rock structures in relation to rock properties. Rock formation is a complex process that occurs in the lithosphere, and it is accompanied by tectonic activities such as plate movements. Tectonic movements lead to the formation of geologic structures such as faults, folds and shear. Geological faults are planar rock fractures that show evidence of relative movement. The degree of competence determines the rock mass conditions in the vicinity of faults; that is, if the rock is incompetent, the fault may be barely visible, while in competent strata, the rock may be completely crushed. Bedding planes divide rocks into beds or strata; they represent interruption in the sedimentary process or repeated sedimentation cycles. A shear zone, which varies from a few centimeters to several meters, is commonly associated with structural instabilities such as landslides. Over geologic time, rocks are exposed to weathering, a process that transforms hard rocks into soft soil.