Models of Slope Evolution

Various theories have been proposed to describe the mechanisms of slope development, each offering a unique perspective on how slopes transform. From W.M. Davis’ concept of slope decline, which emphasizes a gradual reduction in slope steepness, to W. Penck’s slope replacement model, which focuses on the accumulation of debris at the base, these theories highlight different pathways of evolution. L.C. King’s parallel retreat theory explains how slopes maintain their shape while shifting backward, whereas A. Wood’s and R.A. Savigear’s models incorporate elements of both decline and retreat to present a more dynamic view of slope evolution. By understanding these models, we gain valuable insights into the forces shaping our landscapes, from towering mountain ridges to gently rolling plains.

Slope Decline Theory of W.M. Davis

Imagine you are on a time-lapse journey through a landscape, watching how a newly formed hill slowly transforms over millions of years. This is exactly what W.M. Davis proposed in his Slope Decline Theory, which explains how hillslopes gradually change shape over time through a cyclic process of erosion.

Davis believed that just like landscapes evolve in stages, slopes also undergo a systematic transformation—starting steep and youthful, becoming smoother in maturity, and finally turning into a gentle, concave form in old age.

The Three Stages of Slope Evolution

1️⃣ Youthful Stage: Steep & Convex Slopes

🔹 In the beginning, a slope is steep and convex due to active vertical erosion (downcutting) by rivers.
🔹 Weathering starts, but slope retreat is minimal, meaning the steepness either remains the same or increases slightly.
🔹 The landscape is dominated by sharp ridges, deep valleys, and steep hilltops.

💡 Example: Imagine a newly formed valley in a mountainous region, where rivers rapidly carve deep valleys, leaving behind steep slopes.

2️⃣ Mature Stage: Rectilinear Slopes & Slope Decline Begins

🔹 Over time, lateral erosion (side-cutting by rivers) becomes dominant over vertical erosion.
🔹 The sharp ridges of the youthful stage start to wear down, and the slopes begin to decline in steepness.
🔹 The hilltops, or water divides, erode, forming a smoother curve.
🔹 Slopes become more uniform and rectilinear (straight-line appearance).

💡 Example: Look at a hilly region with rolling slopes and valleys that are wider and less steep compared to mountainous areas.

3️⃣ Old Stage: Concave Slopes & Gentle Terrain

🔹 As erosion continues, the slopes decline further and become concave.
🔹 Slope angles become very gentle, often not exceeding 5 degrees.
🔹 The landscape turns into low-lying hills with broad valleys, almost merging with the surrounding plains.
🔹 This stage marks the final flattening of the landscape, often leading to peneplains (almost flat surfaces with few remaining uplands).

💡 Example: The Deccan Plateau in India, where millions of years of erosion have created a gently undulating landscape with few high ridges.

Key Takeaways from Davis’ Theory

✅ Slopes decline progressively over time in a cyclic manner.
✅ The process is controlled by erosion, weathering, and time.
✅ The slope angle reduces from steep (youthful) → moderate (mature) → gentle (old).
✅ The theory explains why old landscapes appear more subdued and less rugged compared to youthful terrains.

However, modern geomorphologists argue that slope evolution is not always so systematic, as climate, geology, and external forces (like tectonic uplift) can disrupt this smooth cycle. But Davis’ theory still remains a foundational concept in geomorphology.

🔹 Think of it like aging—just as humans go through youth, adulthood, and old age, landscapes also evolve through distinct stages over time!

Slope Replacement Theory by W. Penck

Imagine a steep rocky cliff that slowly transforms into a gentler slope over time. Unlike Davis, who proposed that slopes decline in angle progressively, W. Penck suggested a different process called Slope Replacement.

According to Penck, steep cliffs do not simply wear down—instead, they are replaced by a lower-angle slope that gradually extends upward from the base.

How Does Slope Replacement Work?

🔹 The initial slope is steep, often featuring a free face (bare rock wall).
🔹 Due to weathering and erosion, small rock fragments, called scree, start accumulating at the base of the cliff.
🔹 Over time, the scree pile grows and buries the free face, covering it entirely.
🔹 If basal sapping (erosion at the base) does not remove the scree, it eventually replaces the entire cliff, forming a gentler slope at a constant angle.

💡 Example: Imagine a mountain cliff where falling rocks pile up at the bottom. If these loose rocks are not washed away, they gradually build up, replacing the cliff with a sloping surface.

Key Features of Slope Replacement Theory

✅ Free face disappears as scree accumulates and buries it.
✅ New slope forms at a constant angle, extending upward.
✅ Weathering is uniform, affecting the entire cliff face.
✅ If the scree is removed by erosion, the free face remains.

Penck vs. Davis: How Are They Different?

Why Is This Important?

Penck’s theory helps explain landscapes where scree accumulation dominates, such as:

✔️ Mountain cliffs gradually getting buried by rock debris.
✔️ Regions with slow weathering and weak erosion, where scree is not carried away.
✔️ Steep slopes in dry or arid regions, where mechanical weathering (like frost action) produces a lot of loose rocks.

While both Davis’ and Penck’s theories offer valuable insights, modern geomorphologists believe that slope evolution is influenced by multiple factors like climate, rock type, and tectonic activity, rather than following a single universal pattern.

🔹 In simple terms, Penck’s theory tells us that cliffs don’t just wear down—they are buried from the bottom up!

Parallel Retreat Theory by L.C. King (1948)

L.C. King proposed that in semi-arid regions, hillslopes do not just wear down—they retreat backward while maintaining their original shape. This is called parallel retreat of slopes.

Key Features of Parallel Retreat

🔹 Slopes in semi-arid areas have a complex profile with four distinct elements:
1️⃣ Summital Convexity – the curved crest at the top
2️⃣ Free Face – a steep rock wall
3️⃣ Rectilinear Debris Slope – a straight slope below the free face
4️⃣ Concave Pediment – a gently sloping or nearly flat surface at the base

🔹 Each upper part of the slope retreats backward at the same rate, keeping the original angles constant.
🔹 The pediment (lower concave slope) expands outward as the upper slope retreats, creating a wider and flatter landform.

What Is a Pediment?

✔️ A gently sloping surface at the base of the slope.
✔️ Covered by a thin layer of weathered material (regolith).
✔️ Expands as slopes retreat, making the landscape flatter over time.

💡 Example: Imagine a mountain with a steep face. As time passes, instead of the slope becoming less steep, the entire mountain shifts backward, keeping the same steepness. Meanwhile, a flat plain (pediment) forms at the base and expands outward.

Why Is Parallel Retreat Important?

✅ Explains why steep cliffs in deserts seem to remain unchanged for long periods.
✅ Shows how landscapes in semi-arid regions evolve, forming pediments.
✅ Helps understand why some mountain slopes appear to “move” backward instead of eroding downward.

King vs. Davis vs. Penck: Comparing Theories

A. Wood’s Model of Slope Evolution

A. Wood proposed a progressive slope evolution model, combining elements of parallel retreat and adjustment between weathering and debris transport. His concept revolves around the transformation of a cliff slope with a free face over time.

Key Processes in Wood’s Model

1️⃣ Free Face Retreat Through Backwasting

• The free face undergoes weathering, causing retreat through backwasting (erosion from behind).
• Weathered debris moves downslope and accumulates at the base.

2️⃣ Formation of a Constant Slope

• As debris builds up, it covers the lower part of the free face, burying it under a constant slope of debris.
• The length of the free face decreases over time.

3️⃣ Development of a Convexo-Rectilinear Slope

• A convex rock slope forms under the debris-covered segment.
• As backwasting continues, the rectilinear segment extends upslope, eventually reaching the summit.
• Free face disappears, leading to the rounding of the divide summit and formation of summital convexity.

4️⃣ Formation of a Convexo-Concave Slope

• Over time, the lower constant slope becomes concave.
• The entire slope profile transforms into three elements:
  ✔️ Summital convexity (rounded summit)
  ✔️ Rectilinear segment (straight slope)
  ✔️ Basal concavity (concave lower slope)
• Eventually, the rectilinear segment disappears, leaving behind a convexo-concave slope.

Evaluation of Wood’s Model: Strengths & Criticism

✅ Strengths:
✔️ Incorporates parallel retreat and weathering-transport balance.
✔️ Explains progressive slope transformation over time.
✔️ Justifies the disappearance of the free face and formation of summital convexity.

❌ Criticism:
❌ Assumes convexity and concavity appear only in the final stage, which is debatable.
❌ The idea that a convex rock slope transforms into a concave form is difficult to justify.
❌ Does not fully account for external influences like climate and lithology on slope evolution.

Comparison of Wood’s Model with Other Theories

Conclusion: Why Does Wood’s Model Matter?

Wood’s model offers a dynamic perspective on slope evolution by integrating weathering, backwasting, and sediment transport. It highlights the progressive burial of free faces and the formation of a convexo-concave slope over time. However, some of its assumptions—like the transformation of buried convex slopes into concave forms—remain questionable.

💡 Key Takeaway: Wood’s theory provides valuable insight into how debris accumulation affects slope development, but it may not be universally applicable to all landscapes.

Concept of R. A. Savigear

R. A. Savigear’s theory is empirically based, developed from his observations of slope profiles in Carmarthen Bay, South Wales. His model integrates both parallel retreat and slope decline, suggesting that these processes can act simultaneously in a region with uniform environmental conditions.

Key Observations and Slope Profiles Identified

1️⃣ Eastern Carmarthen Bay: Parallel Retreat Dominates

• Identified a three-part slope profile:

✔ Basal free face
✔ Middle rectilinear slope (32° angle)
✔ Limited summital convexity

• The rectilinear slope maintains a constant angle because gravity removes weathered material, transporting it downslope to the free face.
• At the free face base, waves instantly remove debris, ensuring the scarp slope retreats parallelly.

2️⃣ Western Carmarthen Bay: Slope Decline Due to Debris Accumulation

• Convex-rectilinear-concave slopes are found here.
• The sea has receded, preventing waves from reaching the slope base.
• Since debris cannot be removed, it accumulates at the base, leading to the formation of a concave slope.

3️⃣ Mechanism of Slope Decline

• If debris is effectively removed, the rectilinear slope maintains a 32° angle.
• If debris accumulates at the base, it protects the lower slope from further erosion, causing only the upper slope to retreat, leading to slope decline over time.

Shoreline Division in Savigear’s Model

Savigear also classified the shoreline into three key zones:

🔹 Backshore:

• Extends from the cliff base to the limit of frequent storm waves.
• Represents the upper beach zone, mostly affected by storm waves rather than daily tidal action.

🔹 Foreshore:

• Extends from low tide water to high tide water.
• The most dynamic zone, where waves and tides shape the beach.

🔹 Offshore:

• Represents the shallow bottom of the continental shelf.
• Lies beyond the wave-breaking zone, where sediment transport is relatively slower.

Evaluation of Savigear’s Concept

✅ Strengths:
✔ Recognizes that parallel retreat and slope decline can coexist.
✔ Based on empirical observations, making it more practical than purely theoretical models.
✔ Explains how coastal erosion and slope processes interact.

❌ Criticism:
❌ Limited to specific coastal settings; may not apply to inland slopes.
❌ Assumes constant slope angles, which may vary due to external factors like climate, lithology, and tectonics.
❌ The role of vegetation and biological weathering is not considered.

Conclusion: Why Is Savigear’s Model Important?

Savigear’s approach bridges the gap between different slope evolution theories by showing how both parallel retreat and slope decline operate under varying coastal conditions. His real-world observations make his model valuable for coastal geomorphology and slope stability studies. However, it is less applicable to non-coastal landscapes, where different forces might dominate slope evolution.

💡 Key Takeaway: Slope evolution is not a one-size-fits-all process; different mechanisms can work together depending on environmental conditions.