Chiefly three processes can be attributed to the formation of both outliers as well as inliers, which are as follows

(a) Outliers and inliers:

A limited area, where younger rocks are completely surrounded by older rocks, it is known as an outlier. In a similar way, an area in which the older rocks are completely surrounded by rocks of younger age, known as ‘inliers’.

Chiefly three processes can be attributed to the formation of both outliers as well as inliers, which are as follows:

1. Folding:

Folds which attains the form of anticlines and synclines, after undergoing a period of erosion at the surface pro­duces outliers and inliers.

Anticlinal folds produce inliers and synclinal folds produce outliers.

2. Faulting:

Because of the dislocation of rocks due to fault­ing, sometimes it so happens that older rocks are surrounded by younger ones or the vice-versa and accordingly produces outliers or inliers.

3. Erosion:

Erosion plays a major role in the formation of outliers and inliers only in cases where the dispositions of the beds are horizontal. Sometimes deep valleys are being cut by the agents of erosion and when the erosion exposes underlying rocks around the younger-remnant bed, the resulting structure is termed as outlier; this process is equally applicable for the formation of inliers.

(a) Nappes and windows:

Nappe is a large body of rock that has moved forward for more than one mile, from its original position either by over thrusting or by recumbent folding. Because of the translation of rocks in nappes, various types have been recognised:

(i) Autochthonous nappes, where the rock beds have not been translated.

(ii) Para-autochthonous nappe, in which the sheets of rocks have to undergone relatively small translation and which can be traced back origin or roots.

(iii) Allochthonous nappe, here the rock sheets have been translated for great distances.

Window:

Sometimes through erosion of overlying nappes t younger rock beneath it becomes exposed. Thus, the area where erosion has penetrated a nappe (i.e., both large recumbent folds well as thrust) exposing rock lying beneath is known as a Window or fenster.

(c) Klippe:

With continued erosion of a nappe, and window sometimes a remnant of the thrust sheet is left as a relict block which is called a ‘klippe’ (cliff).

Geological Surveying and Mapping-Use of Contour and Topographical Maps’

Important Points:

(1) Contour:

These are imaginary lines which joins points of equal elevation. The representation of relief features on a map is being made through drawing of contour lines.

The contour lines when are found circular, it represents a hit or mountain. But when they are circular and at the same time closely spaced, it indicates a vertical or nearly vertical peak. The contour taper in any direction to show a valley or river in that direction.

(2) Topographical maps:

These maps are to represent the sacrificial relief features of a particular area (both positive and negative) on a sheet of paper. It includes not only the cartographies representation of hills, mountains and rivers but also other feature like temples, location of villages etc.

With the use of clinometer compass, by the back-bearing method, one can locate his position in the field with reference to map of the area.

Geological mapping of an area includes the methods by which exposures of various rock types as recognised in the field and their attitude are represented in the topographical map. Accordingly the structure of the area from the geological point of view and the occurrences of various rock types of a particular can be identified from its geological map,

In case of Geological surveying, the representation of the important features of an area, is being made and it indicates the location of various important objects, along with their direction, in a particular area according to a given scale.