54 GEOLOGICAL MAPS AND THEIR INTERPRETATION the most arid parts of the land surface, the covering of soil, etc., means that the surveyor has had to judge the areal extent of the underlying Earth materials, from scattered exposures, in cliffs, river banks, road cuttings and the like, and other, indirect, evidence Geological maps, therefore, are continually being refined as additional evidence becomes available The Interpretation of Geological Maps Many of the uses of geological maps involve dealing with geological information in three dimensions New IT techniques are becoming available to facilitate this but much work still relies on interpreting the third-dimension from the flat map – the sheet of paper or computer screen Sedimentary rocks, being arranged in approximately tabular strata, lend themselves to this kind of treatment The strata comprise a three-dimensional configuration, originally a horizontal succession but in the case of marine deposits that have been uplifted to form land – the most widespread situation – now in some combination of horizontal, tilted, folded, and faulted layers The present-day land surface is an eroded slice, roughly horizontal, through this array Properly, therefore, a geological map is a plot of the intersections or outcrops of the strata with the present land surface, projected onto the horizontal plane of the map Note incidentally, in this context, the geological terms exposure and outcrop differ in meaning An outcrop of a formation, i.e., where it intersects with the land surface, and hence what is depicted on a geological map, may or may not actually be exposed, i.e., lacking modern cover and be directly visible From the patterns made by the outcrops on the map, a trained geologist can infer the three-dimensional configuration of the formations, that is, how they were arranged above the land surface before their erosion and, more importantly, how they are disposed below the ground, in the ‘subsurface’ The geologist can this in a quantitative or a non-quantitative way, the latter by simply assessing the geological patterns on a map and mentally visualizing – perhaps while sketching rough cross-sections – the spatial arrangements Some examples follow Visual Assessment A sequence of map formations passing progressively from older to younger implies: (i) that the succession has been tilted (if they were still horizontal only the uppermost division would outcrop), and (ii) the direction of dip (Figure 1) Even with such a preliminary interpretation it becomes possible to gauge what lies below the land surface in various places, apart from those sites where the lowest, oldest known formation is outcropping This simple principle, that Figure Block diagram and map to show how formations dip towards the direction in which they become younger (ignoring, as may be reasonable on a small scale map, topographic relief) In this example, the geological map shows that a journey west wards across the State of Mississippi would encounter progres sively younger formations The simplest and much the most likely interpretation is that the formations are dipping westwards, as evident from the front panel of the block diagram uK1, uK2, uK3, and uK4 are progressively younger Cretaceous formations, Tx, Te1, and Te2 are progressively younger Tertiary formations, and Qh is the youngest formation, of Quaternary age Air photographs and satellite images may be helpful; geological maps of the sea floor commonly involve seismic surveying outcropping formations pass from older to younger in the direction towards which they are dipping, needs care with larger-scale (usually contoured) maps of rugged country, where the topographic relief may interfere with and complicate the outcrop patterns However, further simple principles can be applied in such cases A dissected landscape developed in horizontal formations can give what at first glance is a bewildering geological map, but the outcropping formations, discerned by their boundary traces, will be exactly parallel to the topographic contours (because they, too, of necessity are horizontal; Figure 2) The outcrop patterns progressively deviate from this parallelism as the dip angle increases until, at the other extreme, vertical formations outcrop without any influence of topography Their outcrops cross topographic contour lines to give straight traces The more general case of dipping formations is best interpreted by assessing the direction and shapes of the V-shape outcrops made where they intersect with topographic valleys (Figures and 4) Note that these are geometric principles arising from the intersections