Classification of Slopes: Understanding the Variability in Hillslope Profiles

While an ideal hillslope consists of all four elements—summital convexity, free face, rectilinear slope, and concave base—these elements don’t always appear together. Their presence and arrangement depend on local conditions, geological structure, and the stage of erosion.

For example:
✔ In areas with low relief, a free face may not develop at all.
✔ If hard and soft rocks alternate, there may be multiple free faces and rectilinear elements rather than a smooth transition.
✔ The dominance of slope elements changes over time—in the youthful stage, summital convexity is more prominent; in the mature stage, rectilinear slopes become dominant; and in the old stage, concave slopes take over.

Now, let’s explore how slopes are classified.

1️⃣ Genetic Classification of Slopes

This method classifies slopes based on how they were formed—whether due to tectonic movements, erosion, or deposition.

A. Tectonic Slopes: The Product of Earth’s Movements

These slopes arise due to tensional and compressional forces in the Earth’s crust, leading to the formation of fault scarps (steep slopes formed by faulting) and tilted rock beds.
✔ Example: Scarp slopes formed by faulting, like the Western Ghats escarpments.

B. Erosional Slopes: Sculpted by Natural Forces

These slopes are shaped by rivers, glaciers, and sea waves, which gradually wear down the landscape.
✔ River valleys evolve through different stages—youth, maturity, and old age—modifying slope profiles over time.
✔ Example: River-cut slopes in the Chambal ravines.

C. Slopes of Accumulation: Built by Deposits

These slopes form due to the deposition of sediments carried by rivers, wind, and glaciers.
✔ Such slopes are also called aggradational slopes.
✔ Example: Alluvial fans and cones created by rivers, sand dunes formed by wind action in deserts.

2️⃣ Quantitative Classification of Slopes

This method, proposed by A. Young, categorizes slopes based on their angle of inclination.

Conclusion: Slopes are Dynamic and Diverse

Understanding slope classification helps in geological studies, construction planning, and environmental management. Whether formed by tectonic activity, erosion, or deposition, slopes evolve over time, continuously reshaping the landscape.

Sample Question

Discuss the geomorphological differences between convex and concave slopes with suitable examples. Explain how these slope profiles evolve over time according to the models of slope development. (15 Marks)

Answer:

A slope is the inclined surface of the land which varies in shape, gradient, and material composition. Based on their profile, slopes are classified into convex and concave types. Understanding slope evolution is crucial in geomorphology as it explains the transformation of landscapes over time.

Convex Slopes:

• Convex slopes have a bulging outward shape with a steep gradient at the top and a gentle slope downward. These slopes are formed due to rapid weathering and removal of material from the upper region.
• Example: The upper slopes of the Shivalik Hills. According to Davis’ Slope Decline Theory, convex slopes dominate in the youthful stage of landform development.

Concave Slopes:

• Concave slopes have a curved inward profile with a gentle gradient at the base. These slopes result from material deposition at the base due to continuous erosion from the upper slope.
• Example: Piedmont Plains of the Himalayas. Davis’ model suggests that concave slopes develop in the old stage of landscape evolution due to slope decline.

Slope Evolution Models:

• Davis’ Slope Decline Model: Slope angle progressively declines from convex to concave due to weathering and denudation.
• Penck’s Slope Replacement Model: Steep slopes are replaced by gentler ones through the accumulation of scree at the base.
• King’s Parallel Retreat Model: Slopes retreat parallel without significant change in angle, resulting in the formation of pediments.
• Wood’s Model: Emphasized the backwasting of free face, resulting in rectilinear slopes followed by concave slopes.

Understanding slope development helps in land use planning, agriculture, and infrastructure development, especially in hilly regions vulnerable to landslides and erosion.