Principles of geological mapping and map reading pdf

The line of outcrop in Fig. The difference height of topography minus height the present topography but has since been eroded of geological surface equals the depth of burial at any point away, and on the map. Using again the data from Fig. Isobaths, lines linking all points of equal depth of burial, surface relative to the topography.

In the case of valleys, can then be drawn dashed lines in Fig. As soon as we tilt the beds away from this critical position they will start to exhibit a V-shape. If we visualize the bed to be rotated slightly upstream it will 2. The A dipping surface that crops out in a valley or on a ridge bed can be tilted still further upstream until it becomes will give rise to a V-shaped outcrop Fig.

The way the horizontal. Downstream-pointing vees are produced when the beds dip downstream more steeply than the valley gradient Fig. Finally, vertical beds have straight outcrop courses and do not vee Fig. This type of problem is frequently encountered by geological mappers. On published geological maps all contacts are shown. However, rocks are not every- where exposed. Whilst mapping, a few outcrops are found at which contacts are visible and where dips can be measured, but the rest of the map is based on interpretation.

The following technique can be used to interpret the map. Using the known dip, construct structure contours for the thin bed. These will run parallel to the measured strike and, for a contour interval of 10 metres, will have a spacing given by this equation see Section 2. Others are drawn parallel at the calculated spacing. The crossing points of the topographic contours with the structure contours of the same height, yield points which lie on the outcrop of the thin limestone bed.

The completed outcrop of the thin limestone bed is shown in Fig. These join lines are structure contours for that tion 2. Draw as many structure contours as possible to test the assumption of constant dip planarity of surface. This confirms that the limestone bed What assumptions are involved? However formations of rock, differs from the true thickness TT , because from Fig. This equation can be used to calculate the true thickness if 2.

The horizontal thickness HT is a distance measured at right angles to the strike The true or stratigraphic thickness of a unit is the distance between a point on the base of the unit and a point of the between its bounding surfaces measured in a direction same height on the top of the unit. It can be found by taking perpendicular to these surfaces TT in Fig. The vertical thickness is the altitude. Uniformly Dipping Beds 15 Fig. The horizontal thickness is given by the horizontal separation of any pair of structure contours of the same altitude one for base, one for top.

The horizontal thickness in this example is m. Contour lines and isobaths are examples of lines drawn on a map which join points where some physical quantity has equal value. Isochores are lines of equal vertical thickness From Fig.

On thickness unless the ground surface is horizontal. In cross- , scale maps for instance, the presence of valleys sections care must be taken when interpreting thicknesses. On the other hand the run the true thickness will only be visible in cross-sections of geological boundaries on, say, the , geological parallel to the dip direction of the beds.

Only where dips are gentle and relief is high e. Give a three-figure compass direction. The photograph shows dipping beds of Carboniferous b What is the approximate angle of dip?

The north c Write down the attitude of the bedding as a single direction is shown by an arrow in the sand. Interpret the run of the geological boundaries and a angle of dip complete the map. At which cutting will railway passengers observe the steepest dip of strata? NB: apparent dips are observed in the cuttings.

What is the strike of the contact? Explain why the angle of dip seen in the drawn section What is the dip direction of the contact? What is the angle of dip of the contact? Use a formula to calculate the dip observed in the section and to check the accuracy of the cross-section. Examine the relationship between geo- formations exposed in this area.

SE corners of the map. Draw structure contours for the line X to Y. Borehole Elevation Does the seam crop out within the area of the map? Margam Park No. Construct the line of outcrop of the Margam Park No.

In each map the bed has a different dip. On map a the the dip is 3. On this express the way the outcrop pattern of beds exposed on map show the general form of topographic contours a ridge varies depending on the dip of the strata. Find the dip and dip direction of the beds. Calculate the stratigraphic and vertical thickness of the Calculate the 30, 60, 90 and m isobaths for the Llynfi Beds.

The line capable of A geological surface which is curved is said to be folded. To demonstrate shape, as viewed on serial sections cross-section planes this place both hands on a tablecloth and draw them which are parallel like those made by a ham slicer , remains together; the shortening of the tablecloth results in a number constant Fig. This is true whatever the attitude of the of folds. Serial sectioning of a non-cylindrical fold The structure known as folding is not everywhere produces two-dimensional fold shapes which vary from one developed equally.

For example the Upper Carboniferous section to another Fig. The crustal deformation 3. Zones of concentrated The single curved surface in Fig. The deformation and folding are called fold belts or mountain lines which separate adjacent folds are the inflection lines. For example, the present-day mountain chain convex to concave or vice-versa. Between adjacent inflec- of the Andes is a fold belt produced by the shortening of the tion lines the surface is not uniformly convex or concave but rocks of South America since the end of the Cretaceous there are places where the curvature is more pronounced.

A corrugated iron roofing sheet or a row of greenhouse roofs has the form of a set of cylindrical folds. Folds which cannot be generated by translating a straight line are called non-cylindrical. An example of this type of shape is an egg-tray. Figures 3. B: Non-cylindrical fold. This is called the hinge zone.

The hinge line is the line of The axial surface of a fold is the surface which contains maximum curvature. Like the inflection lines, the hinge line the hinge lines of successive harmonically folded surfaces need not be straight except when the folding is cylindrical.

For obvious reasons this surface is sometimes Hinge lines divide folds into separate limbs. The axial surface need not The terms introduced so far can be used for a single be planar but is often curved.

Folding usually affects a layered sequence so that a number of surfaces are folded together. Harmonic folding is where the 3. Where this matching As a result of folding, the overall length of a bed as of folds does not exist the style of folding is called measured in a direction perpendicular to the axial surface is disharmonic Fig. B: Disharmonic folding. A: Upright folding with hinge lines Fig. The orientation of the axial surface is described by this is indicative of the direction in which the strata have means of its dip direction and angle of dip.

When the axial been most shortened. Folds are inclined when the axial plane dips and are the two most important directional characteristics of a the term upright is reserved for folds with steeply dipping fold. Figure 3. As explained example of upright plunging folds. Because the fold plunge. Either Fig.

Such Which of the folds in Fig. The answer folds are called reclined Fig. Another directional feature of the fold is the direction in which the limbs of the fold converge or close. This direction Where folding affects a sequence of beds, and where their of closure is a direction within the axial surface at right relative ages are known, the facing of the folds can be angles to the fold hinge Fig.

The direction of facing is the direction in the On the basis of the direction of closure, three fold types axial surface at right angles to the fold hinge line pointing are distinguished: towards the younger beds Fig. Folds may face antiforms: close upwards upwards, downwards or sideways.

Folding 33 Synclines face towards their inner arcs or cores. Anticlines can be either antiformal, synformal or neutral. The same is true of synclines. The tightness of a fold is measured by the size of the angle between the fold limbs. The interlimb angle is defined as the angle between the planes tangential to the folded surface at the inflection lines Fig.

When one limb of a fold is the mirror image of the other, and the axial surface is a plane of symmetry, the fold is said to be symmetrical Fig. There exists a common The size of the interlimb angle allows the fold to be misconception that the limbs of a symmetrical fold must classified in the following scheme: have equal dips in opposite directions. This need not be the case, but the lengths of the limbs must be equal Fig.

Such folds are called overturned folds. More commonly occurring they possess the same interlimb angle. Nevertheless the non-cylindrical folds possess a well-defined but curved shapes of the fold differ significantly with respect to their hinge line. Four types having the shapes of whalebacks, curvature. This rounded shape contrasts with angular fold in periclinal folds. They are doubly-plunging with points on Fig. Chevron, accordion, or concertina folds are their hinge line called plunge culminations and depressions where the direction of plunge reverses.

Periclinal folding gives rise to closed elliptical patterns on the map or outcrop surface Fig. B: Periclinal folds exposed on a horizontal outcrop surface, Precambrian gneisses, Nordland, Norway. The closed oval arrangements of layers also characterizes the map pattern of larger-scale periclinal folds. The layering limbs, though the bed thickness is constant if measured in a around the limbs also undergoes distortions or strains which direction parallel to the axial surface Figs.

The latter are called similar folds because the upper in the fold relative to others. The careful measurement of and lower surfaces of the bed have identical shapes. One important class of folds in this scheme has 3. A B Fig. Compare the variation of stratigraphic thickness of the layers in each example. Tilt the card so that the fold plunges. With chalk, sketch in the run of structure contours on the folded card and draw, on a map, how these contours will appear as seen from above Fig.

Repeat this for other angles of plunge to investigate how the plunge affects both the shape of the contours and their spacing. Tilt the fold enough to make one limb overturned, i. Draw structure contours for non-cylindrical folds. Note that the contour lines for the one limb cross with those of the other limb Fig. Since the different heights at the same point on the map.

The shape of a set of structure contour lines depicts the shape of horizontal serial sections through the folded surface. Since cylindrical folds give identical cross-sections on parallel sections see Section 3.

Non-cylindrical folds give rise to more complex structure contour patterns Fig. Concentric circular contours indicate domes or basins. The various types of periclinal folds have characteristic contour arrangements Fig. These lines are called the trough lines and crest lines respectively. The recognition of Fig. If the structure contours are given, the crest or trough line can be drawn in. For a cylindrical fold this line is parallel to the hinge line, so that its plunge is measured straight from the map.

The direction of plunge can be shown on maps by an arrow Fig. The angle of plunge is calculated by solving the triangle in Fig. Note that the contour spacing mentioned is the separation of the Fig. Folding 37 Fig. Determine the plunge of the fold axis. As the limbs are not parallel they will intersect Fig. Let X be a point on the line of intersection of height h metres. The structure contours of elevation h metres for each of the limbs must intersect at X.

These can be drawn parallel to the strike of each limb Fig. Adopting a convenient scale, use the known angles of dip and the equation in Section 2. These intersect at a point Y, which like X is a point on the line of intersection. The height difference between X and Y is 10 m and their horizontal separation is 20 m. The triangle in Fig. The fold axis thus Fig. It can be readily shown by means of a figure similar to Fig.

Also, if the dip on any part 3. Therefore the above method to calculate the fold The angle of plunge is obtained by looking for a dip plunge can be applied to calculate the plunge of the line of direction which is, as near as possible, parallel to the fold intersection of any two surfaces Fig. If both surfaces plunge direction. The angle of dip there will be approx- are planar dip uniformly their line of intersection will be imately equal to the angle of plunge. No part of a folded straight plunge uniformly.

It is important to remember that these methods apply only to cylindrical folds. In other words our usual view of folds tends to be two-dimensional, similar to folds viewed in cross-sections. For example, Fig. With Fig. On allows the true shape of a fold structure to be seen.

This exercise demonstrates the importance of the orientation of the slice through the fold for governing the fold shape one observes. A Map Fig. The fold profile or Fig. It corresponds to our view of the fold as we look down the fold hinge line. In order to be able to interpret the observed geometry in terms of the three- dimensional shape of the fold, the technique of down- Fig.

This entails oblique viewing of the different directions of closure. The view so obtained corresponds to a true profile of the fold structure. Use down-plunge viewing thickness. Providing the direction of plunge is known, the apparent 2 How does the fold classify in the tightness direction of closure on a horizontal surface indicates classification Section 3. The last example serves to emphasize how oblique parallel Section 3. The synform plunges towards the sea. The fold is a gently plunging, upright synform.

The hand points down the hinge line of a synformally folded quartz-rich layer in the gneisses. Note the way the fold appears to close on the horizontal and steep parts of the outcrop surface. On maps of more or less flat terrain this effect is most marked when plunges are low. Oblique sectioning of folds on non-profile planes produces a distortion of the shape which can be most easily visualized by reference to a plunging circular cylinder, say a bar of Fig.

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Download Now Download Download to read offline. Geological mapping May. Geological mapping. Field studies must thus go far beyond mere mapping and collecting of individual rocks or structures. In this section we shall discuss the planning stage of the project. The other two stages will be discussed elsewhere in this course unit. Some of the most important recommendations that are required when one is planning for a field project include: i Determine if other geologists are working or have worked in the area or near it.

Establish the known problems in this area. Consider whether preparation of other topographical maps is required besides base maps. Establish the most efficient methods of surveying. From the compilation of the field data that is obtained when a geologist studies rock outcrops. The rock outcrops in the field are usually studied by following: i River courses from downstream to upstream ii Making traverses across the strike in the survey area iii Following roads and paths across the strike in the survey area 2.

From aerial photograph interpretation and any other available satellite imageries. In this process, geologists interpret the geology from aerial photographs using stereoscopes and plot the map from these interpretations. Thereafter the geologist goes to the field area very briefly to observe and label the rock units that he has interpreted. You will learn more about aerial photographs and their interpretation in Lectures 6 and 7 of this unit.

It consists essentially of the following: - i Hammers- A geological hammer with a pick or chisel point at one end Fig 1. Hammers have occasionally been known to shatter when used heavily on rock, and as a precaution against this and the very much greater chance of flying rock fragments striking you in your eyes, you should wear some sort of shatter-proof goggles when hitting hard rock.

Your eyes are far too precious to run the risk of damage! The actual weight of hammer will depend upon the kind of rock that you will be attacking. If it is very hard you must have a heavier hammer. For areas composed of hard rocks, a 2lb hammer is necessary. This heavy hammer is used for collecting hard rock specimens such as gneisses, lavas, and hornfelses. Click here to sign up. Download Free PDF. Structural Geology and Map Interpretation Aniket Mitra.

A short summary of this paper. Download Download PDF. Translate PDF. Eos, Vol. Clearly, the book is not all without expressing my deepest and sincerest "Nevertheless, was a milestone in my my own original ideas but builds on things I thanks to my family, who have always been life, for it was then that I published my mono- have learned from many others.

If credit is to tremendously supportive. I especially want graph, which Simon referred to as the "big be given, I want to share it with all of these to mention my two sons, Jeff and Will, for blue book". I suppose this is my own interpre- people.