GEOLOGICAL MAPS AND THEIR INTERPRETATION 53 GEOLOGICAL MAPS AND THEIR INTERPRETATION A Maltman, University of Wales, Aberystwyth, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction Geological maps at their simplest are portrayals of the distribution of geological materials at the Earth’s surface (see Geological Field Mapping) Any covering, say of agricultural soil or concrete, is ignored Such a picture of areal distributions is itself highly useful, but the real power of geological maps arises from the further interpretations that can be made from them – into the third-dimension below the ground and back into geological time Because of this breadth of possibilities, geological maps are a fundamental tool of both academic and applied geologists, and the governments of most developed countries underwrite a national body responsible for the geological surveying of the nation Indeed, geological maps have been referred to as ‘the visual language of geologists’ Today, yet new possibilities are blossoming as GIS (see Remote Sensing: GIS), spatial databases, and digital imagery are applied to geological map methods How best to harness all this tremendous potential is the subject of much current dialogue Fundamental map principles, however, are unchanging, and it is these that are summarized here The Nature of Geological Maps The distributions of the different kinds of rocks are plotted onto some kind of topographic base image, traditionally a topographic map Scales vary from very small, such as 1:1 000 000 or less (which may summarize the geology of entire regions), through medium scales such as 1:100 000 to 1: 25 000 (showing particular areas in reasonable detail), to largescale maps at scales of 1:500 or even greater (typically used for specific sites of some commercial activity) Small-scale maps commonly have to neglect topographic relief, but at larger scales topographic contours can be shown, and the interaction between the physiography of the land surface and the underlying geology can be important in three-dimensional interpretations The distribution plots are increasingly being linked into Digital Elevation Models, to aid visualisation of the interplay between geology and topographic relief The areal extents of the various materials are shown by different ornaments in the case of black and white maps, or by different colours in the case of most published maps, sometimes with additional letter or number symbols to aid distinction The geological ornaments/colours and symbols are explained in an accompanying key or legend While in some cases the Earth materials in a given area fall naturally into groups that are convenient for depiction at a particular map scale, in many cases the surveyor has to judge how best to make appropriate sub-divisions Small-scale maps commonly portray divisions based on the geological period in which the material formed; some more specialized geological maps may show divisions according to the fossils contained in deposits, or some other particular characteristic The majority of geological maps, however, are litho-stratigraphic: the divisions are based on the type of Earth material and their stratigraphic position The various divisions of material types are, conventionally, arranged in the map key in order of their genesis, upwards from oldest to youngest The actual geological ages may or may not be added On medium- and larger-scale maps, each division is commonly referred to as a map ‘unit’ or ‘formation’ It is important to understand that although a particular formation may be coloured boldly on the map, and given some imposing name, it has been defined subjectively by the surveyor It may be quite indistinct on the ground; another worker may have divided the units differently It is also the convention to make a major distinction between solid bedrock and any overlying, unlithified, geological materials Some geological maps ignore the latter, except where they are especially significant, and are thus referred to as bedrock or – in the UK – solid maps On the other hand, maps that emphasize them are variously referred to as surficial, superficial, Quaternary or – again in some UK usage – drift maps Many published geological maps, which by definition depict things in the horizontal plane of the Earth’s surface, are accompanied by geological crosssections These portray the arrangement of the material in the vertical It is this complementarity of maps and sections that underscores the three-dimensional aspects inherent in geological maps, a topic expanded in the following section Mention was made above of the subjectivity of the units into which the Earth materials are divided for the purpose of a map Also intrinsic in a geological map is the interpretive nature of the distributions shown Unlike almost all other kinds of maps, much of the information shown is interpretive Except in