How to Identify the type and nature of the underlying bedrock on a topographic map

 Geological information is not directly indicated on topographic maps. It can, however, be identified through a thorough analysis of the relationship between various geographical aspects.
In topographic map, factors that can help to determine the type and nature of the underlying bedrock of an area include, but are not limited to, the following;

(A) Water bodies and water features

1. The presence of water bodies such as oceans, seas, lakes, swamps, rivers suggest the presence of an impermeable rock.
2. Disappearing rivers i.e. Rivers shown on map by a blue line that ends abruptly in the ground may suggest the presence of permeable/porous rocks.
3. The presence of hot springs may suggest igneous rocks.
4. The presence of the presence of salt /soda ash lakes may suggest sedimentary rocks.
5. The presence of crater lakes may suggest igneous rocks.
6. Lack of surface water may suggest permeable/porous rocks.
7. Drainage patterns

• Radial drainage pattern may suggest igneous rocks.
• Dendritic drainage patterns may suggest the rock types must be impermeable and non-porous.
• Trellis drainage pattern may suggest an alternate of hard and soft rocks.
• The centripetal drainage pattern may suggest the rock type must be an impermeable sedimentary rock.
• Rectangular drainage pattern may suggest the presence of rocks that have uniform resistance to erosion.

(B) Relief features or landforms

1. The presence of cliffs and plateaus may suggest the presence of hard rocks.
2. The presence of headlands and bays along the coast may suggest an alternate of hard and soft rocks.
3. The presence of volcanic landforms e.g. craters and conical hills may suggest igneous rocks
4. The presence of depositional landforms such as delta, flood plain and coral reef may suggest sedimentary rocks.
5. The presence of a range of Fold Mountains may suggest metamorphic rocks.

(C) Vegetation

1. The presence of thick forests in the map may suggest that the underlying bedrock is likely to be igneous.
2. Poor vegetation cover such as shrubs, thickets and grassland may suggest that the underlying bedrock is likely to be sedimentary.
3. Plantation crops such as paddy/rice may suggest that that the underlying bedrock is impermeable.
4. Crops like tea and coffee may suggest that that the underlying bedrock is likely to be igneous.
5. Human activities.
6. The presence of quarries, cement working, brick works, and coalmines may suggest the presence of sedimentary rocks.

Tips for students: when required to describe the underlying geology on topographic maps, you should follow the following hints;

1. Identify as many landforms as possible and then interpret the likely bedrock type underlying each of them.
2. Identify all the vegetation types indicated on the map and then interpret the likely type and nature of the bedrock underlying each of them.
3. Identify all the water bodies and water features indicated on the map, and then interpret the likely type and nature of the underlying bedrock for each them.
4. Examine for any human activity in the map that may indicate anything about the type of the underlying bedrock.
5. 
   

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