Concept of geomorphic cycles and landscape development
CONCEPT OF UNIFORMITARIANISM
The concept of uniformitarianism, first proposed by James Hutton, states that the processes that shape the Earth’s surface today are the same processes that have been acting throughout Earth’s history. In other words, the present is the key to the past. This concept emphasizes the cyclical nature of Earth’s history and suggests that the consequences of processes observed today are similar to those that occurred in the past.
DAVISIAN CYCLE OF EROSION
The Davisian cycle of erosion was influenced by various ideas and concepts from scientists like Darwin, Hutton, and Powell. Darwin’s concept of evolution, which emphasized orderly and predictable changes occurring in a sequential manner, resonated with Davis. Hutton’s idea that processes operate continuously without a beginning or end also shaped Davis’s thinking. Additionally, Powell’s concept of base level, which refers to the position of stability in a landscape, provided the foundation for Davis’s understanding of the cycle of erosion.
Davis assumed a constant and unchanging base level, which serves as the driving force behind the fluvial process of erosion. He proposed the concept of a geomorphic cycle, where landforms are considered as a function of three aspects: process, structure, and stage. However, Davis focused primarily on riverine erosional processes, considering other agents of denudation as the products of climatic accidents. This approach led to some limitations and biases in his theory, as he ignored the structural control on landforms and the influence of non-fluvial processes.
PROCESS AND SEQUENCES
According to Davis, the cycle of erosion begins with a flat surface that undergoes rapid upliftment for a short period. Afterward, it enters a stage of prolonged relative stability with an unchanging base level. Once upliftment ceases, the landform is subjected to fluvial erosion, leading to predictable changes in a manner analogous to the life cycle. This process involves the birth, youth, maturity, old age, and death of the landform.
During this cycle, there is a competition between exogenetic forces (such as erosion) and endogenetic forces (such as upliftment). However, it is important to note that erosion and upliftment can coexist, challenging the assumption of an isolated cycle of erosion.
STAGES OF RIVER EROSION
The cycle of erosion proposed by Davis involves three stages of river erosion:
- Zero Stage: In this stage, a flat surface experiences upliftment without any erosion cycle operating.
- Stage I – Youthful Stage: This stage is characterized by small and swift rivers flowing at high elevations. The river valley sides and floors are convex in cross-section. The primary erosional processes during this stage are valley deepening and stream lengthening. Valley widening does not occur significantly, resulting in no summit erosion.
- Stage II – Mature Stage: As the river grows larger, it starts performing both valley widening and deepening processes. The river develops meanders, and the summit of the landform widens. The river is more mature at this stage.
- Stage III – Senile Stage: In this stage, the river is large and almost reaches the base level. It becomes slow and sluggish, with valley deepening almost stopped. The crest or summit of the landform becomes flattened, and an extensive erosional plane known as a pene plane is formed. The slope profile can be straight or concave.
EVALUATION
The Davisian cycle of erosion has both positive aspects and drawbacks. It provides a historical and evolutionary approach to landform development, emphasizing the importance of time and the concept of a cycle. Davis’s work was the first comprehensive attempt to understand landform development.
However, the theory has limitations. It overlooks the significance of processes other than fluvial erosion, ignores the control of structure on landforms, and neglects depositional processes and landforms. It also fails to consider all agents of denudation and focuses excessively on the stage aspect of the cycle. The Davisian cycle of erosion may not accurately represent all environmental conditions and is biased towards fluvial erosion cycles.
CYCLE OF EROSION
Davis’s work primarily focused on temperate humid conditions in North America, which he considered as representative of the world. He referred to this as the “normal cycle of erosion,” where rivers act as agents of denudation, and the base level remains unchanged. Davis acknowledged the possibility of other climatic conditions and agents of denudation but considered them as “abnormal cycles of erosion” or climatic accidents.
Davis proposed the existence of partial or interrupted cycles of erosion with fluctuations in the base level, either positive or negative. Positive fluctuations lead to a quick termination of the cycle, while negative fluctuations prolong the cycle, resulting in rejuvenation. Rejuvenation occurs when a mature or senile river is re-energized, leading to the development of youthful features superimposed on the existing landscape.
SLOPE DEVELOPMENT
The concept of slope development in Davisian theory is based on the idea of slope decline, which is a function of the age of the river. Slope decline operates from top to bottom. This idea was countered by Penck’s theory of slope replacement, which suggests that gentle slopes grow from below. Richard Chorley and Berry introduced the concept of a system’s approach in geography, stating that Davis’s model operates as a closed system with orderly and predictable sequences of events.
They proposed that the system works from high energy and low entropy towards low energy and high entropy, representing the river’s journey towards the base level. However, it is important to note that the Davisian cycle of erosion may not hold true in all environmental conditions and cannot be generalized as a universal model for slope development.
PENCK’S MODEL
The Penck’s model of landform development was proposed by Penck, a process theorist and geologist. Unlike Davis, Penck was more concerned about the factors and processes that influence and control landforms. His model focused on the shape and intensity of landforms, which are determined by the ratio of the velocity of endogenous movement to the intensity of erosion.
According to Penck, landform development occurs through three possible phases: growth/waxing, constant growth, and decline/waning. These phases are not time-bound and progressive like Davis’s stages but are rather flexible and represent different stages of landform evolution. Penck explained the interaction between endogenetic forces and exogenetic denudation processes in shaping the landscape.
Growth/Waxing Phase:
In this phase, the landform experiences rapid upliftment and acceleration of upliftment. Erosion operates alongside upliftment, but initially, the rate of summit lowering is less than the rate of valley deepening. The absolute relief, relative relief, summit height, absolute height of the valley floor, and valley depth all increase. Upliftment is more significant than erosion during this phase.
Constant Growth Phase:
In this phase, the rate of summit lowering equals the rate of valley deepening. There are three possibilities depending on the net upliftment rate in relation to erosion. If the net upliftment rate is more than erosion, the absolute height and relative height increase. If the net upliftment rate is equal to erosion, both the absolute relief and relative relief remain constant. If the net upliftment rate is less than erosion, the absolute relief remains constant.
Decline/Waning Phase:
In this phase, upliftment declines, and the rate of valley deepening is less. Summit erosion and net erosion become more significant than upliftment. As a result, the absolute relief and relative relief decrease. The landscape gradually flattens into a gentle sloping convexo-concave profile, known as the endrumpf or pediplane.
The endrumpf or pediplane is characterized by hard resistant rocks, called bornhardts or inselbergs, which have steeper scarp faces. This concept was further developed by L.C. King in his pediplanation or parallel retreat theory.
L.C. King Theory: Pediplanation or Parallel Retreat Theory
L.C. King’s theory of pediplanation focuses on the parallel retreat of scarps. The process involves the retreat of steep slopes through weathering and rainwash, resulting in the formation of rock pediments flanking river valleys. The landscape passes through three stages: youth, mature, and old.
In the youth stage, the landscape undergoes upliftment, rapid downcutting by rivers, and the appearance of small pediments at the valley bottom. In the mature stage, lateral erosion narrows down the interfluve area, leading to the development of steep-sided rounded top features known as inselbergs. Vertical erosion stops during this stage. In the old stage, residual hills disappear, and the entire landscape becomes a flat, extensive, low-angled pediment.
L.C. King’s theory shares similarities with Davis’s concept of upliftment and crustal stability. It is mainly applicable to arid, semi-arid, and savanna regions. The features of King’s theory include rock pediments flanking river valleys with concave slopes, scarps bounding the uplands undergoing parallel retreat, and steep-sided residual hills known as inselbergs or bornhardts.
Comparing the slope profile in King’s theory with Davis and Penck’s models, the waxing slope (crest) is convex, the free face (scarp) is a bedrock outcrop, the debris slope has a constant slope, and the waning slope (pediment) is concave.
Evaluation:
Penck’s model is considered more comprehensive and forward-looking compared to Davis’s theory. It accounts for both endogenetic and exogenetic processes and includes detailed considerations of structure in landform development. Penck’s approach as a process theorist rejects time dependency and emphasizes the control of structure.
However, Penck’s model has limitations. It lacks field observations and is limited to arid and subtropical regions. It fails to explain the climatic influence on fluvial geomorphology, which Davis’s work successfully addressed.
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