Slopes, and Drainage | Geology Optional Notes for UPSC PDF Download

Slope Elements and Slope Evolution

• The physical landscape consists of slopes, which are integral to landform study.
• Geomorphologists have long been intrigued by the origin and form of slopes.
• Various theories and models have been formulated to explain slope origin and form.
• Slope study faces challenges in determining nature, process rate, and trajectory of development.
• Two key aspects of landform study are form and process.

Form and Process

• Form: Refers to the morphology of a region at a given time.
• Process: Denotes the operation of agents causing changes in the physical environment.
• Agents include soil creep, surface wash, weathering, etc.

Approaches to Understanding Slope Development

• Historical Approach:
  • This approach focuses on the historical evolution of slopes from origin to present form.
  • Challenges include correct reconstruction of past forms and determining slope age.
  • Various theories attempt to explain slope evolution but often rely on speculation.
• Process-Form Approach:
  • This approach assumes a causal relationship between weathering, erosion, transportation, and deposition in shaping slope form.
  • Challenges include observing slow processes and determining direct relationships between processes and form.
  • Climate, rock types, and vegetation influence the types of slope forms produced.

Challenges and Considerations

• Determining slope age and reconstructing past forms pose difficulties.
• Slow processes like weathering and creep are not readily observable.
• Tools and methods for recording processes accurately are necessary.
• Relationships between slope form and climate are complex and not always straightforward.

Conclusion

• Both historical and process-form approaches aid in understanding slope evolution.
• Each approach has its challenges and considerations in studying slope development.
• A comprehensive understanding requires considering both historical sequences and present-day processes.

Genetic Classification of Slopes

• Slopes result from both endogenetic and exogenetic processes.
• They are broadly categorized into Endogenetic Slopes and Exogenetic Slopes.

Endogenetic Slopes

• Originate from processes within the Earth, such as earth movements leading to folds, faults, and rift valleys.
• Also known as tectonic slopes.
• Volcanic eruptions contribute to the formation of new features like volcanic hills, plateaus, and cones.
• Features formed undergo modification by subaerial processes.

Exogenetic Slopes

• Result from external processes near or at the Earth's surface, including weathering, mass wasting, erosion, and deposition.
• Can be erosional (degradational) or depositional (aggradational).
• Erosional slopes formed by wind, running water, waves, and glaciers, producing various landforms.
• Depositional slopes formed by similar agents, resulting in features like alluvial fans, natural levees, and sand dunes.

Elements of Slope

Convex Slope

• Often found at the top of a slope, assumed to be characteristic of humid temperate regions.
• Formed due to denudational processes, associated with certain rock types like chalk and limestone.
• Commonly referred to as "crest" or "summit slope."
• Angle of the slope increases downslope from the crest.
• Weathering and soil creep contribute to summital convexity.

Cliff or Free Face

• Steep wall-like slope, bare due to steepness, often called a scarp or free face.
• Develops along coastlines, in river valleys, glacial regions, and faulted landscapes.
• Material falls or slides down and accumulates at the base, forming talus slopes.
• Talus slope angle determined by size of weathered materials.
• Continuous supply of weathered material gradually covers the free face, eventually replaced by a depositional slope.

Rectilinear Slope

• Straight slope section, mostly below the cliff or free face, also known as constant slope.
• Angle largely remains constant, varies in dimension, may extend from summit to valley bottom.
• Can be denudational forms underlain by solid rock with a veneer of detritus.
• Referred to as "debris-controlled slope" or "repose slope."

Concave Slope

• Located at the lowest part of the slope profile, extends to the river valley.
• Usually covered with a layer of debris, rainwash spreads finer particles farther, leading to concavity.
• Sharp break of gradient in arid regions, smooth transition in humid conditions.
• Referred to as "waning slope."

Combination of Elements

• Not all elements may be present in a slope profile, combinations vary.
• Various factors influence the presence and combination of slope elements.
• Multiple combinations are theoretically possible, but some occur more frequently than others.

Common Composite Slope Profiles

• Composite slope profiles exhibit various combinations of slope elements.
• Three common combinations have been identified:

Convexo-Rectilinear-Concave Slope Profile

• Consists of upper convex, middle rectilinear, and lower concave forms.
• Smoothly graded elements give a curving slope profile.
• Most common in regions with weak rock types, such as lowland England.
• Variations in length of slope elements observed in landscapes.

Composite Slope Profile in Regions with Alternating Massive and Thinly Bedded Weak Strata

• Characterized by several free faces and rectilinear slopes.
• Summital convexity and basal concavity limited or absent.
• Free face formed where massive strata occur, rectilinear slopes where weak and thinly bedded rock exists.
• Occurs in regions with high relief, deep valleys, and active weathering.

Composite Slope Profile in Arid Regions with Hard Crystalline Rocks

• Consists of a free face, boulder-controlled mid-section, and concave (pediment) slope.
• Free face has a slope of 40° or more, mid-section has a slope angle of 25° or more.
• Concave slope at the bottom is very gentle, with angles below 7 degrees.
• Develops due to the occurrence of hard crystalline rocks in arid regions.

Factors Influencing Specific Slope Elements

Historical Explanations

• Early explanations attributed specific slope forms to processes like rainwash and soil creep.
• N. M. Fenneman associated convexo-concave profiles with the action of running water.
• Fenneman's hypothesis suggested that convexity develops due to greater erosion away from the summit and concavity forms where water gets concentrated into small channels.

Opposing Views and Support

• Opponents argued that Fenneman's hypothesis neglects the role of soil creep in shaping slopes.
• Horton supported Fenneman's idea, stating that erosion is absent in certain sections of the slope due to lack of energy in runoff.
• Gilbert attributed soil creep to rounding hilltop summits and developing convexity.
• Lawson disagreed with Fenneman, suggesting that wash is most effective at the slope summit.

Complex Interplay of Factors

• Various scientists and geomorphologists proposed theories based on soil creep and rainwash as key processes in shaping slopes.
• Interplay of factors influencing slope form is highly complex.
• Dominant factors may play a key role, but other factors also contribute to specific slope forms.