Geological map reflects the geographical patterns of rocks and minerals components on the earth surface by means of lithology, structure, succession of beds formation. To properly understand it, drawing of cross-section & interpretation of geological maps is of utmost necessity. Before drawing cross-sections & interpretation geological maps, we firstly proceed to read the map.
If we want to read a geological map we must have knowledge about the basic terminology. I had already discussed it in my previous article. The link to the article is in the box below.
Drawing of cross-section:
On the basis of the reading knowledge of a geological map, we select the section line of the map and steep forward to drawing of the cross-section. The cross-section of a geological map exposed all inner formations of rock beds. It also helps us to interpret the total geology of the map. I clearly discussed the process of the cross-section of different types of geological maps in my YouTube videos. There are simple series, double series, triple series, and specially annotated map with broadly elaborated their cross-section in my video classes. All the links are given below.
|Introduction of Geological Map with Basic Terminology Part -1||See the video|
|Drawing of Cross profile of Geological Map. Part -3 (Triple & specially annotated map)||See the video|
YouTube videos on Geological Map
Steps for drawing of geological cross-section:
1. Selection of a Section Line
2. Identification of Structure and series of the map
(Demarcation of Line of Unconformity in multi-series)
3. Drawing of Strike line
4. Observation of Dip, Dip direction & thickness of the beds
5. Rough Sketches & estimation for drawing of Cross Section
6. Making of the paper strip
7. Drawing of Cross Section
- Relief Profile drawing
- Beds and unconformity drawing
- The sequence of beds and annotation
- Succession of beds Heading & Caging
- Scale of drawing
Ten important geological maps with geological cross-section:
1. Single Series Maps
Geological Map No:1
Geological Map No:2
Geological Map No:3
2. Double series map
Geological Map No:4
Geological Map No:5
Geological Map No:6
Here vertical igneous body below the upper series is a dike.
Geological Map No:7
Geological Map No:8
Here vertical igneous body in both series is a dike.
Triple series and spacially annotated map
Geological Map No:9 with sample complete interpretation
Interpretation of Geological Map
In the topic drawing of cross-section and interpretation of geological maps we will now proceed for interpretation of the map. The interpretation of the geological map is the outcome of its different attributes in detail. It is the final part of a geological map understanding. After drawing of cross-section we generally proceed for interpretation of the geological map. A sample format of geological map interpretation for map number 9, we can discuss in the following subheadings.
- Map No: 9
- The scale of the map: 1inch to 1000ft.
- Area of the map: 8inch x 10 inch =80 sq. inch in the map is equivalent of 0.0199 sq miles in the ground.
- Maximum & minimum altitude of the map: 1100ft in western part & 400ft in the southern part.
- Cross-section line and its length and direction: AB. A is in the western end and B is in the eastern end. The length of AB is 10.5 inch which is about 0.166 miles in the ground.
- The number of series of the given map: It is a triple series map with two distinct unconformities.
The geological map is a part of the dissected plateau region or dissected high land region. The highest elevation of the entire region is 1100ft at the western part of the map. We see the next highest elevated almost half circled flat-topped region in the northern part which is encircled by the contour 1000 ft. The lowest altitude of the region is in the southernmost part of the region where 400ft contour passes.
Therefore the relative relief of the region is 700ft. There are five river valleys in the mapped region. The main river flows from north-northeast to south. The river dissects the upper series rock beds in the northern part and upper with lower series in middle to south part. Two almost parallel tributaries join to the left bank of the main river from the eastern region. There is a probability of river capture found in the north-eastern part of the map. Here two opposite-direction-flowing rivers will meet soon with each other by their headword erosion. From the western region, other tributaries join the river on its right bank. In the middle to the south portion of the region lower series rock beds exposed by the downward river erosion
3. Succession of rock beds:
I clearly represent the succession of eight rock beds and three series in the following table in detail. It will be helpful to a proper understanding of the map.
D. Geological Structure :
There are three series of rock beds in the map. The upper series is horizontal in structure. It is composed of two rock beds S & T. The thickness of the rock cannot be measurable from the map due to its horizontal manner but it can be measure from the cross-section. Both the rock beds are the same thickness. It is 100 ft each.
The middle series is Uniclinal in structure. In section line AB, it shows an apparent dip of about 2 degrees. The middle series is composed of two rock beds O & P. The thickness of the O bed is 100 ft and the thickness of the P bed is not measurable due to the absence of its upper bedding plane.
The lower series is folded in structure. There is a prominent anticline in the middle of the region and a syncline on the eastern side of the region. Therefore the entire region is consisting of three folded limbs with their different dip and direction at lower series rock beds. The lower series is composed of D, E, F & G beds where D is the oldest lower bed, and G is the top bed in this series. The thicknesses of the beds are 200 ft and 300 ft for E & F respectably, whereas the thicknesses of D & G beds cannot be measured due to the absence of lower and upper bedding planes for D and G beds respectably.
E. The relation between Structure and topography:
There is a close interrelationship between the structure and topography of the given geological map. There are five major valleys in the mapped area that creates a positive and negative relation between structure and topography. In the middle part of the region, the altitude is low due to the high level of erosion which makes positive relation and in the north-eastern and south-western regions, the relation is negative due to the low level of erosion. In the said both sides of the region, a steep escarpment and cap rock are formed in the horizontal structure of ‘P’ bed.
In the middle part of the region, a folded anticline structure is prominent in the lower series of rock beds, where the main river creates a wide and steep valley by gradual valley deepening and valley widening process. Hence the topographic expression here is an Anticlinal valley. The oldest rock bed ‘D’ is exposed here.
In the north- west part of the region the folded lower series rock beds create a syncline on which cap rock ‘O’ of the middle series is located. Hence the topographic expression here is a synclinal ridge. So in compare with these two parts we can easily interpret the region is a ideal example of inversion of relief. The total region is a dissected highland with Questa, Escarpment, Mesa, and Butte topographic expression.
F. Geological History:
In the early unknown geological time the entire region was under the sea beds where the rock beds D, E, F, and G were formed in horizontal manner. At the time of uplift due to the tectonic movement, the rock beds were folded. After a long time the region were submerged again and middle series rock beds ‘O’ and ‘P’ formed in horizontal fashion. The region were uplifted with tilting and that is why ‘O’ & ‘P’ rock beds slightly tilt towards the east disrupting their previous horizontal structure. The region was exposed to nature and after a long period the entire region submerged under the sea. The rock beds ‘S’ and ‘T’ were formed in horizontal style. The total region is uplifted and exposed to nature for erosion and deposition. The present relief is the outcome of intensive fluvial erosion for a long time.
Geological Map No:10
The succession of rock beds in Geological Map No:10 for drawing and interpretation clue
So this is the Drawing of Cross Section And Interpretation of Geological Maps.