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Gr 11: Geomorphology- Horizontally layered rocks
Geography with Dave
Overview
This video explains landforms created by horizontally layered rocks, focusing on their characteristics and formation processes. It details how different geological conditions, such as humid versus dry climates, influence the resulting landscapes. The summary covers hilly terrains, basaltic plateaus, canyons, and the erosional processes that lead to features like mesas, buttes, and conical hills within a 'kero' landscape. It also touches upon the concept of scarp retreat and its role in shaping these landforms, concluding with the practical significance and human uses of these geological formations and how to interpret them on maps.
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Chapters
- Horizontally layered rocks are sedimentary rocks uplifted without changing their horizontal position, or igneous layers formed by cooled lava flows.
- These rocks can be of sedimentary origin or igneous origin, such as cooled lava flows.
- Intruded magma that cools horizontally underground can also form a 'sill', which becomes a horizontal layer when exposed at the surface.
Understanding the origin of horizontally layered rocks is fundamental to recognizing the geological structures that will later be sculpted into various landforms.
Sedimentary layers stacked on top of each other, or a layer of cooled lava on top of older rock.
- In humid areas, abundant rainfall leads to erosion, primarily through sheet wash, resulting in smooth, rounded hills.
- Dry areas experience less rainfall, leading to more rugged, uneven, and steep hilly landscapes where resistant rocks stand out.
- Dry terrains with extensive erosion of softer rocks are often called 'badlands', characterized by steep slopes and minimal vegetation.
Climate significantly influences the erosional processes, determining whether horizontally layered rocks form gentle, rounded hills or sharp, rugged badlands.
Smooth, rounded hills in a humid region versus steep, uneven slopes with exposed resistant rock in a dry region.
- Basaltic plateaus are formed by widespread, flat lava flows from fissures that cool and accumulate, creating large, flat-topped elevated areas.
- Canyons are formed when rivers erode through plateaus over long periods, cutting deep into the rock.
- The resistant rock layers in a plateau remain exposed on the canyon walls, creating rugged cliffs, while softer rocks are eroded away.
These features demonstrate how volcanic activity and fluvial erosion can dramatically shape landscapes, creating distinct elevated plains and deep gorges.
The Drakensberg mountains showing a basaltic plateau, and the Grand Canyon illustrating a river cutting through a plateau to form a canyon.
- Kero landscapes represent older, more eroded terrains that develop from plateaus and canyons over time.
- Key features include mesas (wider than tall, flat-topped isolated hills), buttes (taller than wide, steep-sided rock towers), and conical hills (smaller, often without a cap rock).
- These features retain the height of the original plateau as long as they are capped by a resistant rock layer (cap rock).
This chapter explains the progressive stages of erosion on horizontally layered rocks, leading to a variety of distinctive landforms that are identifiable by their shape and dimensions.
A mesa with a width greater than its height, and a butte with a height greater than its width, both capped by resistant rock.
- Scarp retreat (or back wasting) is the process where steep slopes are eroded and recede backwards, widening valleys.
- This erosion, often parallel, reduces the size of elevated features like buttes and mesas.
- Pediments are gently sloping erosion surfaces that form at the base of retreating scarps, and when joined, form a pediplain (a large, flat rock surface).
Understanding scarp retreat is crucial as it explains the mechanism behind the widening of valleys and the formation of extensive flat erosion surfaces like pediplain.
Water flowing down a steep slope (scarp) eroding it backwards, gradually reducing the size of an adjacent mesa or butte.
- Features like mesas and buttes can be identified on contour maps by their steep slopes near the summit and gentler slopes further away, with mesas having a wider flat top than buttes.
- Conical hills show more uniform, continuous slopes on contour maps.
- These landscapes offer various human uses, including farming (especially in humid areas), tourism (hiking, sightseeing), and livestock farming (on flat areas in kero landscapes).
This section bridges geological concepts with practical application, showing how to interpret landforms on maps and highlighting their value to human activities.
Distinguishing a mesa from a butte on a contour map by observing the width of the contour lines representing the flat summit area.
Key takeaways
- Horizontally layered rocks, whether sedimentary or igneous, form the foundation for a variety of distinct landforms.
- Climate is a primary driver in shaping these landscapes, influencing whether erosion results in smooth or rugged terrains.
- Plateaus are formed by extensive lava flows, and canyons are carved into them by rivers over geological time.
- Mesa, buttes, and conical hills are erosional remnants of plateaus, distinguished by their shape and the presence of a resistant cap rock.
- Scarp retreat is a fundamental process that explains the recession of steep slopes and the formation of pediplain.
- Interpreting contour maps is essential for identifying and differentiating these landforms based on their topographic characteristics.
- Geomorphological features associated with horizontally layered rocks have significant economic and recreational value for humans.
Key terms
Horizontally layered rocksSheet washBadlandsBasaltic plateauFissuresCanyonMesaButteConical hillCap rockScarp retreatBack wastingPedimentPediplainContour map
Test your understanding
- How does the presence of a cap rock influence the formation and preservation of mesas and buttes?
- What are the key differences in the formation and appearance of hilly landscapes in humid versus dry areas?
- Explain the process of scarp retreat and how it contributes to the formation of pediplain.
- How can one differentiate between a mesa and a butte when analyzing a contour map?
- What are the primary human uses for landscapes formed by horizontally layered rocks, and how do these uses vary by region?