Geological field mapping techniques

Geological field mapping techniques are very important for the geological study of the area. It provides all the detailed information about the structure, lithology, geomorphology, hydrology, mineral resources, etc. The geological maps are used in various sectors like geotechnical engineering, plant set-up, etc. A map is a two-dimensional representation of the three-dimensional features. A map either can be scaled or cannot be. In this post, we will learn how to draw a geological map for structural features of the area.

Geological Field Mapping Techniques
Three-dimensional geological view of the area.

Before drawing a geological map, let us know about some basics.

What is a Map? 

A map is a two-dimensional representation of the all physical elements which are seeing over the ground surface such as man-made features like buildings, dams, bridges, etc, natural features like ridges, stream and rivers, hills, etc. A map is established at a scale like 1:50,000, 1:10,0000, 1:250,000, etc. All the elements on the map are represented by symbols, colours, shape, etc.

What is a Geological Map?

A geological map is just the representation of the all-natural features on the plane-surface, it can be seen in the form of topographic maps, aerial photographs, or satellite images. The aims of the geological map are to show the surface distribution of the rock units, the locations of the interface of contact between adjacent rock units, the locations of structural features like faults, folds, joints, fractures, shear zones, foliations, cleavage, unconformity, etc. In the geological map, the standard geologic symbols are used (Geological Maps of Indian States).

Always remember that the topographic and geological map is projected onto the horizontal plane, therefore, the distances measured on the horizontal surface are the horizontal distances; it is not the actual ground distance.

What is Contour Lines?

Contour lines are the imaginary lines which pass through the equal height of elevation above or below the sea level. It is a significant feature of a topographic map. Basically, the contour lines show the elevation and the shape of the physical features in the area. For example, a circular contour represents the hillock, an elliptical contour for ridges, V-shaped contour for River valley, U-shaped contour for Glaciar Valley, etc.

When all the contour lines are parallel to the structural traces of a bed, it indicates about the horizontal structure. On the other hand, the intersection of contour lines and structural traces represents the inclined structures like folded, tilted, faulted, etc.

Rules of Contour Lines

  1. On the contour lines, every point has the same elevation.
  2. In the Case of Convex outcrop, the contour spacings decrease from the top to bottom. On the other hand, the contour spacing increases from the top to bottom.
  3. The contour lines never touch or cross to each other except cliff.
  4. Contour lines are closer together in steeper terrain and farther apart in the flat terrain.
  5. V-shaped contour pattern represents the river valley. The tip of V points to the upstream or uphill direction. 
  6. U-shaped contour pattern represents the glacier valley. 
  7. Contour lines on the opposite sides of the valley or ridge occur in pairs.

How to draw a Topographic Map Cross-section?

Before constructing a topographic map cross-section, needs to draw a profile plane of the area. A topographic profile plane tells us about the physical features of the area. It also gives you a side view of the sub-surface conditions like tilted, folded, faulted beds, intrusions, unconformity, etc. A profile plane is constructed by the joining of the contour lines along with a given cross-section line. (fig1a, 1b). 

For a geological purpose, we should know an idea about the sub-surface, because all the commercial activities like geotechnical structures, mineral resources, hydrological conditions, etc. can be understood with the help of a topographic map.

After the construction of a profile plane, look at the map carefully. Look at the contour patter, structural traces (e.g. any repetition, overlapping, overstep, missing of a bed). Then draw a strike line over the map. A strike line is drawn through the intersection pints where a contour line intersects to the structural trace (upper trace or lower trace) at least two points. The Second strike line is drawn parallel to the first strike line which also passes through another contour line (fig1a).

Since we know that every strike lines has own elevation, thus, we can obtain the dip-direction of the bed. The dip of the bed is carried out along the given cross-section line (also called a traverse line). The dip is measured by the inverse tangent of the contour interval or height and the horizontal spacing or horizontal equivalent. If the cross-section line is perpendicular to the strike of the bed, the true dip is always obtained (fig1a).

Now mark the upper and lower traces over the profile plane and draw beds with measured dips (fig2a, 2b). 

Always remember that the folded structures are identified by the repetition of beds, the fault is by missing of beds, and an unconformity is by the erosional surface where the older sequence is overlain by the younger sequence (fig3a, 3b, 3c).

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