56 GEOLOGICAL MAPS AND THEIR INTERPRETATION into the structure known as a syncline; younger formations flanking older indicate an upwarp or anticline (Figure 5) In addition, the shape of the symmetrical pattern contains information on the three-dimensional arrangement of the fold Outcrop sequences that repeat in a translation pattern rather than a mirror-image reflection (Figure 5) imply the presence of a fault The effects of faults on outcrop patterns are diverse, depending on the nature of the fault and its orientation with respect to the displaced strata On published maps, faults are normally marked by a particular symbol – commonly a heavier line – indicated in the map key The actual nature of the fault displacement is commonly impossible to interpret from a map, because the same outcrop patterns can be generated by different kinds of fault motion Additional, often field-based, evidence is normally needed Unconformities are recognised on maps by the absence of strata representing a significant length of geological time, and also by discordant outcrop patterns in the case of an angular unconformity (see Unconformities) The latter is important in threedimensional map interpretations because the geometric arrangements will differ above and below the surface of unconformity Quantitative Treatments Figure Block diagram and map to show outcrop patterns typical of some structures The thrust fault is gently dipping and hence its trace is closely influenced by topography; steeply dipping faults crop out with relatively straight traces The trans latory repetition of outcrops is a typical effect of faults The mirror image repetition of outcrops is characteristic of folds In practical applications it is commonly not enough just to visualize or sketch the subsurface arrangements; the actual subsurface depths of particular formations and their thicknesses are often required Quantitative answers to such questions are usually arrived at through some combination of three methods First, a scaled geological cross-section can be constructed through the site of interest and the required data measured directly from the diagram Larger-scale maps commonly provide symbols indicating the dip angles measured at exposure by the surveyor, and extrapolations of surfaces to depth are based on these values However, with cross-sections there can be construction inaccuracies or even mistakes – irrespective of whether the work is done manually or by computer – though this pictorial approach has the advantage of making errors more readily apparent A second approach utilises trigonometric calculations (Figure 6) Both these approaches rely on assumptions, such as the dip angle as measured at exposure being maintained at depth, and they Figure Schematic block diagram and map to illustrate how the thickness and subsurface depth of dipping formations can be calculate trigonometrically from map information