4. With the aid of suitable diagrams describe the appearance and process of formation of any two of the following landforms (a) Rock pedestal (b) Depression...

As shown in the diagrams above, two desert landforms are described below.

(a) Rock pedestal

Appearance: A rock pedestal is an isolated pillar of rock with a broad upper part and a narrow, undercut base. It is sometimes called a mushroom rock because it may resemble a mushroom. Its sides are often smooth, grooved or polished by wind-blown sand. It is common in hot, dry desert areas where vegetation is sparse.

Formation:

• It develops where alternating layers of hard and soft rocks occur, or where the lower part of a rock mass is less resistant than the upper part.
• Strong winds carry sand particles close to the ground surface.
• The sand particles strike and scrape the lower part of the rock by abrasion or sand-blasting.
• Since most sand is carried within about one metre of the ground, erosion is greatest at the base of the rock.
• The softer lower layers are worn away faster than the harder upper layers.
• Continued undercutting leaves a narrow base supporting a wider, more resistant top, thereby producing a rock pedestal.

(d) Inselberg

Appearance: An inselberg is an isolated, steep-sided mass or hill of resistant bare rock rising abruptly from a flat or gently sloping plain. It may have a rounded, dome-shaped, conical or flat-topped summit. Inselbergs are usually made of hard rocks such as granite, gneiss or quartzite, and are common in arid and semi-arid regions.

Formation:

• An inselberg is formed from a former extensive upland, plateau or rock surface containing both resistant and less resistant rocks.
• Weathering attacks the weaker rocks more rapidly. In granite regions, water may enter joints and cause deep chemical weathering below the surface.
• The more resistant rock remains relatively unweathered, while surrounding rocks are broken down into loose weathered material.
• Running water, sheet wash and wind remove the weathered debris and softer materials from the surrounding area.
• As denudation continues, the resistant rock is left standing above the lowered plain as an isolated rocky hill, called an inselberg.

Thus, the rock pedestal is mainly produced by wind abrasion concentrated near ground level, whereas the inselberg results mainly from differential weathering and the removal of softer surrounding materials.