geological maps and map-making

geological maps and map-making A geological map shows the rocks that would be seen at the Earth's surface if all the soils were removed. The various rocks are distinguished either by colours or by black and white patterns (Fig. 1a). The reliability of such maps depends on the extent to which the rocks crop out, on the ability of geologists to interpret what happens beneath the soil between one rock outcrop and another, and on the geologists' knowledge of the rocks they are dealing with; different types of rock were formed differently and occur in different ways.

The attitudes and structures of rocks are indicated on a map by symbols. Using the information provided by the map, cross-sections can then be drawn to show how the rocks continue in depth (Fig. 1b). Cross-sections, are however, merely projections, and their reliability depends on the reliability of the surface mapping. In some instances, subsurface information from drill holes, mine workings, and geophysical surveys provide supporting factual information. There is, however, a limit to how far geology can be extended downwards with any certainty: at best, perhaps no more than 4 or 5 km, usually much less. Below that is conjecture, but at least it is educated conjecture, based on a knowledge of how rocks behave.

There are many reasons for making geological maps. The most obvious is to aid the search for water, oil, and minerals, for they are all associated with specific types of rock and rock structures. Planning and engineering projects also require the information provided by geological maps: the construction of dams, tunnels, and high-rise buildings are just a few examples. Geological maps are in fact essential to economic development and also for the protection of the environment, such as in limiting the spread of industrial pollutants. It is to be regretted that this is not always appreciated by the powers that be. There are academic reasons too—we would like to know how our planet was formed, its history, and, possibly, even its future. We might even eventually learn to predict earthquakes and volcanic eruptions in time to warn the people who may be affected.

Because there are so many uses, there are many different types of geological map, produced at many different scales. Reconnaissance maps of geologically poorly known regions—and they still exist—may first be made at 1 : 25 000 or at even smaller scales. Maps for more general use are more likely to be surveyed at 1:50 000. In well-developed areas, geological surveying may be 1:10 000 or even larger scales, whereas maps for mining and engineering purposes are commonly made at 1 : 500, or larger still. Of course, a map made at one scale in the field may be reproduced at a smaller scale for publication: thus the 1:50 000 geological map series of Great Britain was complied from field information gathered at 1 : 10 000.

Geological map-making

During the past 50 years or so, geological map-making has changed radically. Before then, geologists took topographic maps into the field and covered the ground on foot. In some underdeveloped regions they even had to make their own topographic maps, often by plane-tabling, because of the total lack of any reliable base maps. In the field they systematically identified and recorded the rocks they encountered, measured their attitudes, traced the contacts between different rock types, and took rock samples for laboratory examination. If the rocks being investigated were fossiliferous, they would identify the fossils they found and take specimens for further examination and comparison with fossils from elsewhere; rocks can be dated by fossil evidence and correlated with rocks in other regions, often very far afield.

In the later 1940s aerial photographs began to become more available. By the early 1950s excellent topographic maps were being produced from these photographs, thus solving some of the geologists' problems. In addition, methods of photogeological interpretation that had been developed by oil companies were now being adopted more generally. These were methods by which ‘stereo’ pairs of aerial photographs were viewed through a stereoscope to give a three-dimensional image of the ground below, which was then systematically examined. Many geological features—often invisible on the ground—could be seen, even under soil cover. Changes in tones and textures indicated different rock types, fine erosional patterns distinguished softer rocks from harder, and changes in vegetation showed that plant life has preferences for soils covering one type of rock rather than another. Contacts between rock groups, fault lines, and major joints in rocks could also be traced on photographs, often far more easily than on the ground. Geologists still had to work on the ground to confirm what they saw on the photographs, and of course they could not measure the dip of strata, identify a rock properly, or collect fossils, except in the field; but photogeology proved to be of enormous benefit in geological interpretation. In addition, aerial photographs made locating oneself on a map a great deal easier.

Other aids to geological mapping were also developing at much the same time. Hand-held magnetometers could distinguish magnetite-bearing rocks from those that were not magnetic. Thus the Karroo dolerites, important aquifers in southern Africa, could be easily traced despite soil cover. Subsurface mapping was aided by geophysical methods of many types. The systematic measurement of gravity could outline the shape of large buried masses of less dense material, such as salt domes, while seismic surveys could penetrate to considerable depths by recording the reflection and refraction from deep geological boundaries of waves produced by small surface explosions.

Airborne geophysical methods were also developed. Recording magnetometers flown over large areas could distinguish rapidly between magnetic and non-magnetic rock groups. A scintillometer could make a radiometric survey at the same time which would distinguish potassic from sodic granites. Such methods were particularly useful in covering large areas quickly, especially in mineral surveys. The term ‘remote sensing’ was coined for such methods.

Since the 1970s satellites have also played their part in geological map-making. Satellite images (resembling colour photographs, some taken in ‘false colour’ extending beyond the visible spectrum) can often show major structures in astonishing detail. Satellites can also help a field geologist who has a hand-held GPS (Global Positioning System) instrument. GPS is in theory capable of giving a position to within 10 m or better, but the Pentagon restricts civilians to a ‘coarse acquisition’, accurate only to between 50 and 100 m. This can still be useful in the field.

J. W. Barnes

Bibliography

Barnes, J. W. (1995) Basic geological mapping (3rd edn). John Wiley and Sons, Chichester.
Maltman, A. (1990) Geological maps: an introduction. Open University Press, Milton Keynes.