Comprehensive Study Notes on Geomorphology and Landscape Evolution of Landscapes
Topography Associated with Horizontal Strata
Topography associated with horizontal strata represents a significant geomorphological study area, particularly within the South African context. These landscapes are characterized by rock layers that remain in their original horizontal orientation. The resulting landforms differ significantly depending on the climatic conditions of the region. In moist or wet areas, the landscape typically presents as a hilly terrain where slopes are rounded due to the prevalence of chemical weathering. In these regions, deep soil horizons develop on the slopes because the high rainfall facilitates the breakdown of materials. The underlying rocks are horizontal and show uniform resistance to erosion, leading to the formation of steep but generally smooth hilly landscapes.
In contrast, dry or arid regions characterized by horizontal strata produce landforms that are much rougher and more angular in appearance. The lack of significant rainfall limits chemical processes, making mechanical weathering the dominant force of landscape shapement. This results in the formation of steep and uneven slopes as weathering and erosion target jointing and specific rock layers. The soil in these regions is typically thin and coarse, reflecting the slow pace of rock disintegration. Landscape evolution here is marked by the exposure of resistant layers that create sharp, stepped profiles.
Utilization and Characteristics of Hilly Landscapes
The utilization of these landscapes by humans is largely dictated by their soil quality and water availability. In humid or moist hilly areas, the fine texture of the developed soils makes agricultural activities highly possible. However, the steepness of the terrain often necessitates restrictions on the use of heavy machinery to prevent excessive soil erosion and to accommodate the topography. Conversely, in dry hilly areas, land use is severely restricted. The combination of limited water resources and thin, nutrient-poor soil means these regions are generally unsuitable for intensive cropping and are instead used primarily for livestock grazing.
Basalt Plateaus and Canyon Landscapes
Basalt plateaus represent a specific category of horizontal strata landforms. These are characterized by high-altitude, flat-topped landscapes that are significantly elevated above sea level. They often terminate in steep cliffs. A distinct feature of these plateaus is the presence of cracks within the rock, which are caused by the repeated expansion and contraction of basalt as it cools and weathers. Despite the rugged exterior, weathered basalt produces highly fertile soil that is rich in iron. When these plateaus are situated in areas of high rainfall, they become very productive agricultural zones.
Canyon landscapes represent another dramatic manifestation of horizontal strata, particularly where vertical erosion is dominant. Canyons are characterized by deep valleys with extremely steep sides and narrow valley bottoms. These are primarily shaped by mechanical weathering and the vertical erosive force of a river. The river carves a course into the Earth's surface, and as downward erosion proceeds faster than the weathering of the valley walls, the valley becomes progressively deeper. From a human perspective, canyons present challenges; their steep slopes and narrow floors make farming nearly impossible, and the water at the bottom of the valley is often difficult to access. However, they serve as major tourism attractions due to their grand scale and aesthetic appeal.
Karoo Landscapes and Associated Landforms
The Karoo landscape is a classic example of horizontal strata evolution over millions of years. This topography consists of tablelands separated by wide, flat plains. The formation of these features is tied to the presence of dolerite sills, which are horizontal intrusions of magma that penetrated the layers of sandstone and shale covering most of South Africa. As the magma cooled, it formed hard, resistant horizontal sills. Over time, weathering and erosion stripped away the softer sedimentary layers, exposing the dolerite. These resistant dolerite caps protect the underlying softer rock, resulting in the preservation of flat-topped hills.
A specific sequence of landforms is associated with the erosion of these landscapes. A ridge is defined as a long, extended hill. A mesa is a large, flat-topped isolated hill that is extensive and usually stands more than high. As erosion continues to wear away the edges of a mesa, it shrinks into a butte, which is a smaller flat-topped isolated hill. Eventually, a butte may erode further into a spire or pinnacle. The surrounding plains are extensive level lands usually found between and above sea level. An escarpment refers to the extended steep slope at the edge of this higher land. While the flat nature of Karoo landscapes facilitates the construction of settlements and transport routes, the thin soil and limited rainfall pose continuous challenges for agricultural activities.
Scarp Retreat and Pediplanation
Scarp retreat, or back-wearing, is the process by which a cliff-like slope between a high area and a lower plain changes its position due to erosion. In this process, the scarp or cliff erodes backward while maintaining its original angle. Over time, as the scarp retreats, the lower flat areas join together to form an extensive, low-relief surface known as a pediment or pediplanes. This process is often driven by a river cutting into the base of the cliff, causing it to lose stability and collapse, as well as by general mass movement. The direction of retreat is away from the original position, and the debris from the eroded rock typically moves down the cliff to be transported away, allowing the scarp to retreat back on itself.
Topography of Inclined Strata
Topography associated with inclined or tilted strata occurs when rock layers experience tension from compression, volcanic intrusions, or tectonic movements, causing them to tilt relative to their original horizontal position. Faulting and folding are the primary drivers that cause these strata to be tilted in various directions. The resulting landforms feature two distinct slopes: a dip slope and an escarpment slope (or scarp slope). The dip slope is a gradual slope that follows the angle of the resistant rock layer, with a tilt commonly measured between and . The escarpment slope is the steep face where the rock layers have been cut across, typically exposing softer rock layers underneath.
Cuestas, Homoclinal Ridges, and Hogbacks
When inclined strata weather, they form asymmetrical ridges known as homoclinal ridges. These are formed where underlying strata are tilted in the same direction and have a uniform dip. These ridges are classified based on the angle of their dip slope. A Cuesta is a ridge with a very gradual dip slope, usually between and , and a steep escarpment slope. The fertile soil on cuesta dip slopes is often utilized for forestry and agriculture; an example is the Magaliesberg in Gauteng. If the dip angle increases to between and , the landform is called a homoclinal ridge. In these areas, rivers often cut through the ridges to form poorts, as seen in the Hex River Mountains. When the dip angle exceeds , it is called a hogback (skerprugbult). In a hogback, there is very little difference in the gradient between the dip slope and the escarpment slope, making the ridge look nearly symmetrical.
Inclined strata also form specific circular or oval structures. Cuesta basins (komvormige cuesta) form due to volcanic intrusions or crustal sagging, where the escarpment slope faces outward and the dip slope is curved inward. Conversely, Cuesta domes (koepelvormige cuesta) form when volcanic intrusions like batholiths or laccoliths push the strata upward, resulting in an inward-facing escarpment and an outward-facing dip slope. These landforms are significant for human use; cuestas are ideal for agriculture and forestry, while hogbacks are often used for recreation such as rock climbing and serve as tourist attractions.
Massive Igneous Rocks and Intrusions
Massive igneous rocks are characterized by their crystalline structure, extreme hardness, and light color. They often form as part of large batholiths and develop joints or cracks as the magma cools. These rocks can be categorized as extrusive, forming on the surface, or intrusive, cooling beneath the Earth's surface. Major intrusive features include the batholith, which is the largest feature, often made of granite, and forms deep within the crust between and below the surface. A laccolith is a mushroom-shaped intrusion with a convex top that arches the overlying sedimentary strata. A lopolith is saucer-shaped; when the overlying layers are eroded, it can form cuesta basins because the weight of the strata caused the magma chamber to collapse inward.
Other smaller intrusions include dykes and sills. A dyke is a vertical intrusion that cuts across rock layers through existing cracks. Their width and length vary from centimeters to hundreds of meters. When exposed at the surface, dykes can form hogback ridges. A sill is a horizontal intrusion between sedimentary layers, very common in the Karoo landscape, where dolerite is the most frequent rock type found in these sheets.
Development of Tors or Klipkoppies
The development of klipkoppies, also known as tors or stapelrots, is a multi-stage process beginning deep underground. As a granite batholith cools, it develops a network of vertical and horizontal joints. Groundwater seeps into these cracks, initiating chemical weathering that dissolves the minerals within the granite. Over time, the weathering deepens and widens the cracks. Eventually, the overlying layers of soil and rock are removed by erosion, exposing the core stones at the surface. There are two primary perspectives on this: one focuses on the underground chemical weathering creating the blocks, and the other emphasizes the subsequent removal of the weathered material (overburden) to reveal the stacked, loose boulders known as tors.
Slope Elements and Development Factors
Slopes serve as the fundamental units of the landscape and are typically composed of four distinct elements. The crest (kruin) is the top of the hill, featuring a gradual convex slope and thin soil; material here moves downward through soil creep. Below the crest is the cliff, or free face (krans/skarp), which is a vertical wall of hard, resistant rock where rockfalls occur. At the base of the cliff sits the talus slope (puinhelling), a uniform slope with an angle between and where debris accumulates. Finally, the pediment is the lowest part, featuring a concave slope with a very gentle angle between and .
Slope development is influenced by several factors. Climate plays a role as higher rainfall increases the volume of material washed away, and temperature fluctuations cause rock expansion and contraction. Soil stability is affected by thickness; thinner or saturated soils are less stable. Vegetation is crucial as plant roots hold soil in place; a lack of cover leads to faster erosion. Human activities such as deforestation, infrastructure construction, excavations, and overgrazing frequently destabilize slopes. Finally, rock type is a determinant, as resistant rocks erode more slowly and provide greater stability than softer rocks.
Theories of Slope Evolution
There are two primary theories regarding how slopes change over time. The theory of scarp retreat, developed by L.C. King, is based on the semi-arid Karoo landscape of South Africa. It suggests that the slope angle and the individual slope elements remain constant as the slope erodes and retreats parallel to itself. In this model, the pediment widens over time. The second theory, slope decline, is based on humid or temperate climates. It posits that slopes are initially steep but flatten out over time as the angle decreases. In this model, the top of the slope becomes more convex while the bottom becomes more concave, eventually leading to a peneplain.
Mass Movement Processes and Types
Mass movement is the downward movement of weathered material like soil and rock due to gravity. It is triggered by factors such as steep gradients, lack of vegetation, high rainfall, and human activities like road construction or blasting. Different types of mass movement occur at various speeds. Soil creep is a very slow process, moving approximately per year, often detected by curved tree trunks or tilted telephone poles. Solifluction occurs in icy regions when the top layer of soil thaws and flows over a frozen sub-layer. Earthflow involves the rapid movement of water-saturated clay soil, while mudflow is even faster, occurring in dry areas after heavy rain in a stream-like fashion.
More dramatic movements include landslides, where large masses of land break off along a crack and slide down, causing significant infrastructure damage. Slumping involves a block of land moving along a curved plane, often tilting backward. Rockfalls are the fastest type, occurring when rocks of various sizes break loose from a cliff and plummet to the talus slope below. These movements have severe impacts, leading to loss of life, destruction of homes, and ecological damage such as river blockages and habitat destruction.
Strategies for Mitigating Mass Movement
To minimize the effects of mass movement, various management strategies are employed. Planting vegetation on slopes is a primary method for stabilizing soil. Restricting the construction of large buildings and infrastructure on steep gradients is also essential. In mining, the gradient of mine heaps must be controlled. Engineering solutions include building drainage channels to divert excess water, installing artificial structures like nets and gabions (wire mesh cages filled with stones), and using rock bolts to secure unstable blocks to the bedrock. Thorough geological studies of rock structures are mandatory before any construction begins on a slope.