With the aid of diagrams, describe the characteristic and mode of formation of any two of the following: (a) Rock pedestal (b) Yardang (c) Mesa

Answer Details

These are landforms produced mainly by wind action (deflation and abrasion) in hot desert regions. Two are described below with the aid of labelled diagrams.

(a) Rock pedestal (mushroom rock / gour)

Diagram: a rock with a broad, rounded top standing on a narrow, undercut stem, resembling a mushroom, with wind arrows striking the base.

• Characteristics: it has a wide upper part resting on a thin, narrow base (stem), giving a mushroom or table shape. The neck is deeply undercut and the whole feature stands isolated on the desert floor.
• Mode of formation: it develops where a mass of rock has harder rock on top and softer rock below. Wind carries sand grains that are heaviest and most concentrated near the ground, so abrasion (sand-blasting) is strongest at the base. The lower, softer part is worn away faster than the top, undercutting the rock until it stands on a narrow pedestal shaped like a mushroom.

(b) Yardang

Diagram: parallel ridges and grooves running in the direction of the prevailing wind, with wind arrows aligned along the furrows.

• Characteristics: it consists of long, steep-sided ridges of hard rock separated by grooves or furrows of softer rock. The ridges are aligned parallel to the prevailing wind and may be several metres high and long.
• Mode of formation: it forms where bands of hard and soft rock lie alternately and are aligned in the direction of the wind. Wind-driven sand abrades and scours out the softer bands, cutting them into deep grooves, while the more resistant hard bands are left standing up as ridges. Continued deflation and abrasion deepen the furrows and sharpen the ridges into yardangs.