Abstract
A geological map is a graphical presentation of geological observations and interpretations on a horizontal plane. A geological section is identical in nature to a map except that data are recorded and interpreted on a vertical rather than a horizontal surface. Maps and sections are essential tools in visualizing spatial, three-dimensional, geological relationships.
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Notes
- 1.
The ground surface is, of course, not always horizontal and, although this can usually be ignored in small-scale maps, it can have profound effects on the outcrop patterns of large-scale maps.
- 2.
By convention, large-scale refers to maps with a small scale ratio (that is, a large fraction) – e.g. 1:1,000 scale or 1:2,500 scale. Small-scale refers to large scale ratios (a small fraction) such as 1:100,000 or 1:250,000. Generally, anything over 1:5,000 should be considered small-scale, but the terms are relative.
- 3.
“Truth” and “fact” are slippery concepts that are often employed to claim authority and stifle debate. They are best not used in scientific contexts.
- 4.
The concept of multiple working hypotheses, now widely acknowledged as a basic part of the scientific method, was first enunciated by geologist Thomas Chrowder Chamberlin (1897).
- 5.
Today we recognize that geological processes are essentially chaotic (i.e. non-linear). Such systems typically exhibit what is called “scale-inv ariance”, meaning there is a repetition of characteristic patterns at different scales – the example often quoted being the comparison in shape between a rock pool and the coastline of which it is an element. Pu mpelly’s Rule is an early recognition of this type of relationship (see Pumpelly et al., 1894).
- 6.
Actually, as pointed out by the philosopher of science Karl Popper (1934), an experiment either falsifies a hypothesis or expands the range of conditions under which it can be said to hold good: it can never prove it.
- 7.
All theories in science, and that includes ideas on geology, must be formulated in such a way that they are capable of being falsified. For example, for field mapping purposes it is not very useful to postulate that “these outcrops constitute a metamorphic core complex” because there is unlikely to be a simple observation which can falsify that statement. Rather postulate “this outcrop is felsic gneiss, that outcrop is sandstone, this contact is a mylonite” – if these turn out to be false then the hypothesis may need revision.
- 8.
In our society from the earliest training we are unfortunately conditioned to think indoors, and to enjoy less cerebral pursuits outdoors. It is a syndrome that the field geologist must learn to break.
- 9.
Hip-ChainTM is a reel of disposable, biodegradable cotton thread. As it reels from its spool, a meter records the length wound off, and hence the distance travelled. The thread is then simply broken and left on the ground. Other brand names for similar measuring instruments are Fieldranger™, Chainman™ and T opofil™.
- 10.
GPS is operated by the US Department of Defence and is available free to all civilian users. At the time of writing (2010) it is currently the only commercially-available available GPS system. From 2013, on current estimates, the European Galileo satellites will provide an alternate coverage.
- 11.
The most commonly used grid is Universal Transverse Mercator metric grid ( UTM). A description of coordinate systems will be found in Sect. 10.5.
- 12.
However, in forested areas, GPS is a boon for airplane or helicopter operations. The geologist dropped off in a clearing in the rain forest to collect a stream sediment sample need never again fear that the helicopter pilot will not be able to find that particular hole in the canopy again.
- 13.
Highly skilled and experienced geologists are available who specialize in the field of air photo and satellite image interpretation. Their use is indicated for training purposes; where they have particular knowledge of the geology or landforms in the area to be mapped; or where there is little possibility of any substantial field access to the region.
- 14.
Photography for mountainous areas, where flying predetermined flight lines may be difficult, needs a wider side lap of 25% or more.
- 15.
The frame is made from wood and aluminium angle. It is articulated using standard wheeled drawer runners.
- 16.
Overlays are available in pre-cut sheets of clear to part-translucent drafting film. Clear sheets do not obscure the photo below, but are difficult to write on without special writing materials. Matt surface films readily take pencil marks but may have to be flicked out of the way when detail of the photo has to be observed.
- 17.
It is generally impossible to exactly align all the overlap area of the photos due to edge distortions.
- 18.
Air photos enlarged up to 20 times have been successfully used as mapping bases.
- 19.
An orthophoto is a distortion-free photographic image produced from standard air photos by computer scanning. Once in digital format the image is corrected for radial distortion and a Digital Elevation Model ( DEM) is used to correct for altitude differences across the scene. The process is known as ortho r ectification. An orthophoto map is an orthophoto to which metric grid coordinates and (sometimes) annotated line work identifying topographic/cultural features has been added – a process called georeferencing. For more on orthorectification and georeferencing see Chap. 10.
- 20.
Establishing a cross line at right angles to a base line can also be done using an opt ical square – a hand-held sighting instrument which enables two pegs to be placed so as to form a right angle with the observer.
- 21.
In very dense scrub or forest, the cleared grid line often provides the only practicable traverse route. Even here, however, every effort should be made to pick up significant outcrops between the lines and to map cross-cutting access lines such as any tracks or creek sections.
- 22.
This used to be called map-makers’ (or ge ologists’) wobble and is one of the things that can distinguish a great geological map from a merely pedestrian one. This is not artistic licence: C haos Theory describes what the geologist is doing. For each lithology, the map outline of the outcrops has a characteristic fra ctal dimension – a fraction somewhere between 1 and 2. The fractal number is lowest for a “smooth” outline such as the granite, and highest for a “rough” outline such as the quartzite.
- 23.
Strikes are the trace of planes on maps just as dips are their trace on sections.
References
Chamberlin TC (1897) Studies for students: the method of multiple working hypotheses. J Geol 5(8):837–848
Popper K (1934) The logic of scientific discovery. Basic Books, New York, NY
Proffett JM (2004) Geologic mapping and its use in mineral exploration. In: Muhling J, Goldfarb N, Vielreicher N, Bierlin E, Stumpfl E, Groves DI, Kenworthy S (eds) Predictive mineral discovery under cover. SEG 2004 extended abstracts, vol 33. University of Western Australia, Centre for Global Metallogeny, Nedlands, WA, 153–157
Pumpelly R, Wolff JE, Dale TN (1894) Geology of the green mountains. USGS Memoir 23:157p
Vearncombe J, Vearncombe S (1998) Structural data from drill core. In: Davis B, Ho SE (eds) More meaningful sampling in the mining industry, vol 22. Bulletin/Australian Institute of Geoscientists, Perth, WA, 67–82
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Marjoribanks, R. (2010). Geological Mapping in Exploration. In: Geological Methods in Mineral Exploration and Mining. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-74375-0_2
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