Large Scale Geomorphological Maps: Complex Thematic Mapping

by Syfujjaman

Geomorphic maps, also known as geological maps, are a special type of complex thematic map that presents multiple features of landforms and geological characteristics simultaneously through various colors and symbols.

Geomorphic maps differ from other maps related to landforms (such as topographic maps) in that they do not comprehensively depict the overall nature of the landforms. We have discussed various methods for representing different characteristics of landforms on maps for cartographic purposes. These methods collectively represent partial characteristics of landforms.

In engineering, urban planning, land management, and other fields where a comprehensive understanding of the terrain is needed, a comprehensive landform map is necessary. In such maps, various aspects of the terrain, including the nature of the landform, different characteristics of landforms, the relationship between the surface and subsurface, the processes responsible for landform creation, and more, are expressed using different colors and symbols. Such maps are referred to as geomorphological maps.

Difference with Topographic Map:

The differences between a geomorphological map and a topographic map are as follows:

• A topographic map provides information to the map reader about specific characteristics of the Earth's surface in a particular area, but it does not provide information about the processes responsible for the formation of landforms.
• Although some may argue that rivers are visible on a topographic map and the presence of various features such as stream bars, gullies, or ravines, snowmelt, snowlines, boulder presence, rocky surfaces, etc., can be inferred through various symbols, it cannot be said that a topographic map accurately depicts the entire terrain of an area. For instance, natural features, geological structures, and characteristics of rocks such as erosion resistance cannot be accurately represented.
• Additionally, since topographic maps often rely on contour lines to represent the relief, it is not always possible to accurately delineate the boundaries of different landforms, and the changes in small-scale terrain slopes are not always captured.

Use of Geomorphological Map:

From an engineering, urban planning, or land management perspective, a geomorphological map serves as an essential tool because it presents a comprehensive view of the physical features, their controllers, and the geomorphic layout of the entire terrain.

Since the post-World War II era, geomorphological maps have been utilized in various countries around the world for planning in different development projects.

Thus, geomorphological maps, with their ability to incorporate diverse physical and cultural data, play a crucial role as a geomorphic tool for planning in various development projects of human society.

As mentioned earlier, geomorphological maps are being widely used as a significant tool in the planning of regional development, urban planning, multi-purpose river planning, river basin management, disaster management, mineral extraction, and related environmental management.

These maps are contributing to the success of various engineering projects such as railway construction, road construction, bridge construction, tunneling, watercourse management, and dam construction.

Types of Information Incorporated in Geomorphological Maps:

Geomorphological maps primarily present three types of information simultaneously. These are:

1. Mapping of Surface Form:

Since geomorphological maps use various symbols, this type of map easily represents the nature of the terrain and the characteristics of each landform in a simple and detailed manner. For example, angular disintegration on a mountain slope is represented by the English letter 'V.' However, based on the nature of this 'V' symbol, it can be understood whether the slope is steep, gentle, or linear. Although such subtle details may not be visible on any other map where landforms are represented by contour lines.
To express the steepness of a slope, symbols are used below the slope arrow, with the quantity of slope written beside it. Different symbols are used for the northern or southern slope portions.
The presence of ravines is indicated by a black arrow.
To measure the steepness of the slope, sometimes shading is used along with contour lines. The International Geographical Union (I.G.U.) has classified the slope steepness into different categories:

• 0° to 2 degrees
• 2 degrees to 5 degrees
• 5 degrees to 15 degrees
• 15 degrees to 35 degrees
• 35 degrees to 55 degrees
• More than 55 degrees

• Notably, slope steepness can be measured in three ways: in degrees, percentage, and gradient ratio.

Mapping of Geological Nature:

Depending on the nature of the terrain, i.e., if you use it extensively, a 1:10,000 scale is employed. However, if the terrain is more uniform, a scale below 1:70,000 can be used. Different contour lines can be used for the same nature but with different characteristics.

Mapping of Different Surface Forms:

Different symbols for different surface forms are necessary in the case of unique identification. For example, a shale area may be indicated by a symbol, and an eroded or incised portion of a channel may be represented by another symbol.
For showing very small features on the map, small symbols can be used, even smaller than the minimum area. Different symbols are used for identifying different surface forms, for instance, an area prone to erosion or an expanded part due to deposition."

Note: Translating technical terms may require context-specific knowledge, and some terms may have multiple possible interpretations.

2. Mapping of Surface Material in Geomorphological Maps:

Various surface materials on the Earth's surface are formed by different substances, each of which is distinct in its natural and structural characteristics. Therefore, it is crucial to represent all these materials separately on the map with the help of symbols, facilitating easy identification of elements related to the earth's surface processes. This helps in understanding the relationship established between elements prone to erosion or easily erodible, based on the nature of geomorphic processes.

The purpose of mapping geological elements can be divided into two parts:

• Solid Rocks: These are rocks, such as granite, which are exposed on the Earth's surface as part of the lithosphere.

Superficial Rocks: These rocks are positioned separately on the Earth's surface in the form of debris from rocks on each side.
This can further be classified into:

i. Rocks that can be eroded, or

• ii. Elements that can be transported and transferred elsewhere by means such as river, glacial, wind, or sea waves.

In this context, it is essential to map distinct features of both soil and rocks separately. It is not only about how the rocks are formed but also about explicitly mentioning the characteristics of these rock layers since the formation of these rock layers depends on the nature of the earth's surface.

Various deposits accumulate on the Earth's surface, and differences can be observed among them in different areas. These differences include:

• Compaction or compactness,
• Composition or structural elements,
• Porosity or permeability,
• Permeability or the ability to allow fluid to pass through,
• Depth of the weather mantle, or the depth of rock fragments.

Similar to rock debris, soil exhibits behavioral characteristics. Therefore, the characteristics of soil, such as compaction strength, porosity, and permeability, need to be represented on the map. Measurement of characteristics like compressive strength, similar to rock debris, should be portrayed on the map.

Furthermore, soil moisture retention and its relationship with rock debris should also be delineated. Hence, in the creation of geomorphological maps, information about soil genesis, composition, quantity of biotic elements in the soil, soil depth, and more is crucial for agricultural planning.

Mapping of Processes in Geomorphological Maps:

In every corner of the Earth's surface, some geomorphic processes have always been at work, shaping various landforms. Therefore, it is essential to accurately represent the geomorphic processes in the preparation of geomorphological maps. This is because each process operates in different directions, distinguishing one from the other. For example, all the landform processes in an upland region are entirely distinct from those in a riverine process.

These processes create separate landform groups on the Earth's surface, and they are intricately connected with different actions and reactions associated with various characteristics of the Earth's surface and lithosphere.

Hence, any ancient landform present on the Earth's surface needs to be indicated on the map along with the processes responsible for its formation. Although, in such cases, it is crucial to mention the current landform-generating processes as well. For instance, consider a glacial landform created by glacial activity, but over time, the area has undergone a shift due to river processes. In this case, the map should acknowledge both the ancient and current landform processes.

As human activities have significantly impacted the Earth's surface, influencing both the geological and geomorphic structures on both the lithosphere and the surface debris, it is imperative to appropriately represent human activities on geomorphological maps. This includes various construction activities on the Earth's surface, such as mining, road construction, water diversion, dam construction, etc. These activities should be represented with distinct symbols on the map to illustrate the diversity of processes.

Types of geomorphological maps:

Verstappen (1970) has mentioned four types of geomorphological maps. These are:

• Morphographic maps: These maps present various landforms according to their specific names.
• Morphogenetic maps: Generally, these maps describe and present the origin and changes of landforms.
• Morphometric maps: These maps establish information related to different measurements and landform characteristics.
• Morpho-chronological maps: This type of map describes the history of landforms from their origin to the present, mentioning the causes of their origin. Usually, age is indicated by symbolic signs.

Colors used in geomorphological maps:

The International Cartographic Association (I.T.C.) has suggested the use of 12 different colors to represent various features, including landforms, in geomorphological maps.

How to prepare a geomorphological map (Steps for the preparation of a geomorphological map):

Since geomorphological maps involve a comprehensive review of a field, utilizing field techniques where topographical maps are filled with actual field observations and various measurement methods are applied to incorporate information related to different landforms. The following steps are generally followed:

• First step: Gather knowledge about the area where the geomorphological map will be prepared. This includes information about the geological history, landform processes, past topography, and landform creation processes, obtained from published maps and various documents.
• Second step: Identify the reasons for preparing the map and clarify the purpose of creating it by defining the actual problem.
• Third step: Collect topographical maps at a scale of 1:50,000 or larger for the region. Simultaneously, prepare a base map for the specific area from the topographical map.
• Fourth step: This step is crucial, involving multiple tasks such as:

• Reading the topographical map thoroughly to understand the landform characteristics of the area.
• Highlighting various landforms and presenting them on the base map with specific symbols.
• Determining how the land division will be done and marking the main river axes.
• Identifying different landforms based on the processes of landform creation and highlighting them.
• Representing all landforms separately on the map using distinct symbols.

• Fifth step: This step also involves multiple tasks, such as:

• Measuring the slope of the land using Wentworth's method or measuring the gradient and preparing maps accordingly.
• Indicating the direction and declination of the slope using correct symbols.

• Sixth step: In this phase, specific techniques are followed based on the type of geomorphological map to be prepared. For example:

• Using landform techniques when preparing a morphometric map to identify various significant individuals.
• When preparing morphogenetic or morpho-chronological maps, first understanding the geological history of the landform is necessary, then determining the age of the landform using age determination methods. All this information is presented in the specific location on the map.

• Seventh step: In this phase, field verification is done to cross-verify the correct information. Additionally, specific elements such as the sub-surface elements of the land and various aspects of the slope, including the foothills, rocky areas, plains, and mountain peaks, are marked using distinct symbols.
• Eighth step: In this phase, all the above-mentioned pieces of information are presented on the main topographic map with the help of specific legal symbols.
• Ninth step: In this phase, various human-created subjects are presented based on their importance to human-nature.
• Tenth step: Finally, the specific and unique characteristics of the land surface are presented on the map's surface using specific symbols or signs.

This comprehensive and systematic process ensures that all relevant geomorphological information is accurately and clearly presented on the main topographic map, following the principles of specific laws.

Geological mapping:

There is a close relationship between geomorphology and geology. As the formation of all landforms on Earth is influenced by some geological structure, and since the various characteristics of landforms such as elevation, slope, etc., are controlled by geological structures to a considerable extent, geological mapping plays a vital role in expressing various geological structures and their cartographic representation. Often, geological structures and landforms are represented on the same map.

Various types of geological structures are represented differently.

For example:

Geological section: It is essentially a geological cross-section that indicates the relationship between the surface topography and directs attention to the geological structure within.

To understand the relationship between geomorphology and geology, the easiest method is to use a geological section. For the representation of geological sections, a line is drawn on the geological map (section line), and the values along this line, such as rock types, their dip, strike, age, etc., are displayed according to various symbols. Different symbols are used for different types of features.

Outcrop map: Different colors or shading are used to represent various geological formations on the map.