The direct clue for interpretation of rock type / lithologic unit comes from the tone (colour) of the image. For example, the acidic and arenaceous rocks appear in lighter tone as compared to the basic / argillaceous rocks. Similarly, coarse grained rocks having higher porosity and permeability appear brighter on the image as compared to fine-grained rocks having higher moisture retaining capacity. The highly resistant rock formations occur as different types of hills depending upon their texture and internal structure; whereas, the easily erodible rocks occur as different types of plains and valleys. While dendritic drainage indicates homogeneous rocks, the trellis, rectangular and parallel drainage patterns indicate structural and lithological controls. The coarse drainage texture indicates highly porous and permeable rock formations; whereas, fine drainage texture is more common in less pervious formations. The coarse textured and light coloured soils indicate the acidic / arenaceous rocks rich in quartz and felspars; whereas, the fine textured and dark coloured soils indicate basic / argillaceous rocks. Thus, by combining all these evidences, it is possible to interpret different rock groups / formations. Though, one or two recognition elements, mentioned above, may be diagnostic for the identification of a particular rock type, the convergence of evidences must be considered by studying all the recognition elements conjunctively (figure 8.1). However, limited field checks are a must to identify the rock types and to make necessary corrections in the interpreted map based on field evidences. Once, the rock types are identified, the contacts can be extended over large areas with minimum ground control. The identification, correlation and extrapolation of rock types are possible based on similar spectral and morphological characters.