The basic concept of geological map

The geological map is an important part of Geography as well as Geology. It helps to understand the internal structure of rock beds of a particular area. The basic concept of the Geological map is very important to understand. The basic concept of the Geological map consists of some basic terminology. It helps to read it, to draw its cross-section, and to interpret it. I elaborately discuss the basic concept of the Geological map in the following subdivided headings.

Basic Terminology-1 :

1. Beds: 

Beds are the layers of sedimentary rocks that are distinctly different from overlying and underlying subsequent beds of other sedimentary rocks.

2. Bedding plain:  

It is the surface that separates each successive layer of a stratified sedimentary rock from its preceding layer.

3. Section line:

The line based on it we draw a cross-section of a geological map. Generally, it passes through all the geological features and contours.

4. Strike line:

The strike line of a bed is a line representing the intersection of that feature with the same horizontal plane or contour.

5. Dip or Angle of dip:

It is the acute angle (> 0-degree c&<90degree) that a rock surface or bedding plane makes with a horizontal plane.

     Dip direction is the azimuth of the direction of an inclined plane or rock bed. In another word, it is the clockwise angular direction of an inclined plane from the magnetic north. We use a Clinometer compass to measure dip amount and dip direction. It can be a true dip direction and apparent dip direction.  True dip direction is the shortest line in between two successive strike lines and its amount always is the highest in value with respect to all other dip directions. Apparent dip direction is the all others dip direction in between two same successive strike lines

The thickness of a bed: It can be determined if any strike line passes through the point of contour crossing on the upper and lower bedding plane, the difference of contour values is the thickness of that particular bed.

You can see the Article on Clinometer Compass for measuring dip of rock beds.

Basic Terminology-2 :

1. Geological Structure:

In the geological map, we see the different geological structures of rock beds which disclose their past geological history. The main geological structures are as follows.

2. Horizontal Structure:

The dip of rock beds is 0 degrees in a horizontal structure. In this structure contours never cross-bedding plane, they are always run parallel to each other.

3. Uniclinal Structure:

The dip of rock beds varies from >0 to <90 degrees in a Uniclinal structure. Above 45 degrees it can be defined as a high angle and below 45 degrees can be defined as a low angle Uniclinal Structure.

4. Folded Structure:

In a folded structure the repetition of rock beds with different dips is usually found. This structure contours cross-bedding plane. Here different sets of strike lines use to draw for different limbs of fold.

5. Faulted Structure:  

In a faulted structure displacement of rock, beds occur in a particular line and we consider this line as a fault line. So for the drawing of a cross-section of a geological map, we measure the displacement of rock beds. We also measure its direction either upward or downward.

Basic Terminology-3 :

1. Line of Unconformity: 

Line of Unconformity reveals the long geological time gap of sedimentation and erosional features in between two successive series of beds. In a geological map, one Line of Unconformity represents two series of rock beds and two Line of Unconformityrepresent three series of rock beds present in the map.

2. Buried beds:

In a geological map, buried beds are occasionally found just below the line of unconformity.  So we do not see the buried beds on the top layer of the map in a particular point of a map, but we easily understand its continuation under the line of unconformity.  In spite of its absence in the top layer we use it to draw the buried beds in geological cross-sections at specific order in rock beds by using projected or eye estimation methods.

3. Igneous intrusions:

Sometimes we see the igneous body in sedimentary rocks. It occurs when magma cools and solidifies before it reaches the surface. Three common types of intrusion sills, dikes, and batholiths are found in the Geological map.

Basic Terminology-4 :

1.Series of rock beds:

 In geological map series of rock, beds are the combination of some rock beds in the same structural formation. As per the depositional sequence, lower, middle, and upper series of rock beds were deposited from bottom to top in order of alignment. Each of the series may compose of different types of geological structures. The following chart represents the different series of rock beds and their structural types.

Important types :

Different types of Geological maps:  We generally see the different types of geological maps for drawing of the cross-section. So we use to identify firstly the types of the map it is. That is why in the following diagram, I represent the different important types of geological maps. The important types of maps are a single series map, double series map, triple series map, and special annotation map.  In my video, I will discuss its drawing of the cross-section.  

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Answer the questions in the comment section:

Q1.Write True or False

a) The value of true dip is always higher than the value of the apparent dip of a rock bed.

b) Dip of rock beds is a vector quantity.

c) We can measure the dip direction by prismatic Compass.

Q2. Go through the topic and answer the questions in a word.

a) In a geological map if there are three series of rock beds, how many unconformity lines are there?

b) We measure the displacement of rock beds in a particular line in which structure of the geological map? What is the name of the line?

c) In which type of structure do we see the repetition of rock beds with different dips?

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