Map Interpretation for Structural Geologists

Narayan Bose, Soumyajit Mukherjee

• Publisher: Elsevier
• Year: 2017
• Print ISBN: 9780128096819
• eBook ISBN: 9780128096581
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

Map Interpretation for Structural Geologists covers various topics, from deciphering topography using contour patterns to interpreting folds, faults, unconformities and dykes. By interpreting several types of maps, this book gives readers the confidence to solve difficult geologic questions related to map interpretation in the classroom and in the field. Interpreting geological and structural maps is an inseparable part of learning structural geology in the undergraduate curriculum and postgraduate development.- Features approximately 30 full-color geological or structural maps and their solutions, from basic to the most complex- Includes content appropriate for undergraduate and graduate students and professional geoscientists alike- Presents a self-learning guide and teaching manual with minimum instruction required

Chapter 1

Topography

Abstract

This chapter discusses how topography is represented in topo-sheets along with contour lines. Map representations for various topographic features, e.g., valleys, spurs, domes, basins, etc., are discussed. Interpreting topographic maps and drawing their cross-sections are described in this chapter.

Keywords

Topography; cross-section; valley; spur

In geological maps, the undulation of the topography is represented by height contours. These contours represent lines along which all the points have the same altitude. Height contours are represented usually by broken lines and the numbers attached to them indicate the altitude of the corresponding contour. A vertical cross-section (herein referred to as “cross-section”) accompanies a geological map, in almost all cases. Cross-sections represent various topographic and sub-surface structural geological features. Concept 1.1A exemplifies of how a three-dimensional topographic feature is presented in a map (as height contours) and in a cross-section. Concepts 1.1B and 1.1C show how various topographic features can be understood from height contours.

Concept 1.1A Representation of a basin in various ways. Schematic diagrams. (A) Three-dimensional view. Height horizons are marked. (B) Note how the height contours look similar to those in map view. The height increases as one moves away from the centre (as indicated by the arrow “i”). (C) Vertical section along MN marked in (A).

Concept 1.1B Contours of topographic features: (A) Basin, where height increases (“i”) away from the centre. (B) Dome, where altitude decreases (“d”) away from the centre. (C) Hill. Here the slope of the topography along E–W is much gentler than that along N–S (note spacing between adjacent contours). (D) Plateau. Here an elevated flat surface is bound by “steep” slopes. (E) Water divide. Here elevated portions together function as a water divide, since two oppositely draining valleys generate from this part. One drains towards NE and the other towards SW.

Concept 1.1C Contours for valley and spur. (A) A northerly draining valley. Note height increases from the central part towards the flanks (“i” direction). (B) A south sloping spur. Note height falls towards the flanks (“d” direction). (C) A north sloping spur. (D) A south draining valley. (E) A combination of valleys and spurs, where the valleys drain towards south. (F) A combination of valleys and spurs. All the valleys drain towards the central part.

Map 1.1 has only height contours. The task is to describe different topographic features present in Map 1.1, as well as to draw a suitable cross-section.

Map 1.1 Part of a topographic map. Cropped from map 31 of Platt (1951).

MAP 1.1

Map 1.1 represents an undulating topography. The maximum height reaches 900 m or more, and the minimum height is 500 m or less. Six valleys are present (marked in Fig. 1.1A). The central elevated region works as a water divide (also see Concept 1.1B E) as two valleys (V5 and V6) slope in opposite directions. Fig. 1.1B: cross-section along the “MN” line in Fig. 1.1A.

Cross-section 1: Drawing the topography. For drawing a vertical cross-section, first select the line along which the section will be drawn (Fig. 1.1A). The line should be selected in such a way, that it passes through the maximum number of (if possible, all) litho-units and structural-/topographic features in the map. The next job is to mark all the points of intersection on this line and transfer them to the x/horizontal-axis of the graph (Fig. 1.1B). The y/vertical-axis represents elevation and here the scale should be the same as that of the map (in most of the cases/unless stated otherwise). Now from the points marked on the x-axis, the topographic elevation points are marked accordingly (see the black solid circles in Fig. 1.1B). Finally, these points are joined by freehand drawing/by a smooth curve. The following things should always be mentioned in the cross-section: scale, a caption stating the line along which the cross-section has been drawn (such as line MN in Fig. 1.1B), and proper marking of the line on the x-axis.

Points A–F: ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• Preface
• Acknowledgments
• Chapter 1. Topography
• Chapter 2. Horizontal and Inclined Beds
• Chapter 3. Unconformity
• Chapter 4. Folds
• Chapter 5. Faults
• Chapter 6. Summary maps
• Chapter 7. Miscellaneous maps
• References
• Index