Geomorphic Model of Penck

Imagine you are on a trek across a vast landscape, watching mountains rise, rivers carve valleys, and eventually, the entire region flatten into a plain. This is not just a sightseeing experience—it is a deep dive into the geomorphic model of Walther Penck, a German geologist who saw landforms as the result of continuous interactions between internal (endogenetic) and external (exogenetic) forces.

Unlike William Morris Davis, who believed in a time-dependent “cycle of erosion” (where landforms evolve in distinct stages over time), Penck focused on processes rather than time. He argued that landscapes do not follow a strict historical sequence but evolve based on the balance between upliftment and erosion.

Penck vs. Davis: A Different Perspective

To understand Penck’s approach, let’s compare it with Davis’s model.

Penck asserted that a landform is a consequence of interaction between endogenetic and exogenetic processes.–this is the logic for time independency

Davis envisioned landforms as following a strict evolutionary path—youth, maturity, and old age. In contrast, Penck saw landforms as evolving dynamically based on how fast or slow upliftment and erosion occur together.

The Three Phases of Landform Evolution According to Penck

1. Phase of Growth (Waxing Phase / Aufsteigende Entwicklung)

In this phase the land is slowly rising due to tectonic forces. Over time, this uplift accelerates, leading to:

• Steeper slopes and an increase in river gradient.
• More vigorous river downcutting, forming deep V-shaped valleys.
• Both absolute relief (height of landmass above sea level) and relative relief (difference in height between highest and lowest points in an area) increase.
• Summits begin eroding, but their rate of erosion is still less than valley deepening (R_U > R_VD > R_SE).

It’s like a young mountain range being pushed upwards by tectonic forces while rivers aggressively carve through it.

2. Phase of Constant Growth (Gleichförmige Entwicklung)

At this stage, the landscape is in a state of equilibrium, where uplift and erosion occur at comparable rates. It is divided into three sub-phases:

• First Sub-phase (R_U > R_VD = R_SE)
  • Upliftment still exceeds erosion, so absolute relief keeps increasing.
  • Valleys continue deepening, and summits lower at the same rate, keeping relative relief constant.

• Second Sub-phase (R_U = R_VD = R_SE)
  • Upliftment and erosion rates are exactly balanced, so absolute relief remains constant.
  • The landscape maintains its relative ruggedness.

• Third Sub-phase (R_U = 0; R_VD = R_SE)
  • Uplift stops entirely, and absolute relief starts decreasing due to continued erosion.
  • Relative relief remains unchanged as valleys and summits lower at the same rate.

This phase can be visualized as a mountain range reaching its peak form and then gradually stabilizing before its decline begins.

3. Waning Phase (Absteigende Entwicklung)

Now, the landmass has stopped rising, and erosion dominates. This phase leads to:

• Extensive lateral erosion, making valleys wider and reducing their depth.
• A rapid decrease in absolute relief and gradual lowering of summits because of total absence of upliftment.
• Declining relative relief as peaks erode faster than valleys deepen (R_U = 0; R_VD < R_SE).

This stage results in a nearly flat, featureless landscape that Penck called Endrumpf—comparable to Davis’s Peneplain but reached through a different process

Final Thoughts: Why Penck’s Model Stands Out

Penck’s model offers a dynamic perspective on landform evolution, focusing on the continuous interaction of uplift and erosion rather than a rigid time-based cycle. His ideas were not widely accepted initially due to:

• His incomplete work (he passed away before refining his concepts).
• Complex German terminology that made it hard for English-speaking scholars to interpret.
• Misrepresentation by Davis, who downplayed Penck’s ideas in reviews.

However, today, Penck’s model is valued for its realistic representation of landscape evolution in tectonically active regions.

Slope Replacement by Penck

Imagine standing at the edge of a towering cliff, gazing down at the slopes below. Over time, this steep cliff will not just erode but will undergo a systematic transformation—a process Walther Penck described as slope replacement.

Unlike parallel retreat, which was later better explained by L.C. King in his model of pedeplanation, slope replacement focuses on how different segments of a slope evolve as erosion progresses.

Key Components of Slope Replacement

1. Gravity Slope (Boschungen)
   • This is the uppermost segment of the slope, which maintains its steep angle for a long time.
   • It represents the original landform before significant erosion takes place.
2. Wash Slope (Haldenhang)
   • Found at the lower segment of the valley sides.
   • Formed by the accumulation of talus materials (eroded debris from the upper slope).
   • It has a gentler inclination compared to the gravity slope.

How Slope Replacement Happens?

• Initially, the gravity slope dominates the landscape.
• Due to erosion, material from the upper segment moves downward and accumulates at the base, forming the wash slope (haldenhang).
• Over time, the haldenhang expands, while the gravity slope shrinks.
• Eventually, the gravity slope is reduced to isolated steep-sided conical hills, known as Inselbergs (meaning “island mountains”).
• As erosion continues, even the Inselbergs disappear, leaving behind an almost flat surface, known as Endrumpf—the final stage of landscape evolution in Penck’s model.

Final Thought

Penck’s concept of slope replacement provides a more dynamic perspective on how landforms change. Unlike Davis’s time-dependent model, Penck’s process-based approach explains how landscapes continuously adjust based on erosion and uplift. If you travel through regions like the Deccan Plateau, you might witness remnants of this process—isolated hills standing in a vast plain, silently narrating their geological history.