With the aid of suitable diagrams, describe the characteristics and mode of formation of any two of the following (a) Ox-bow lake (b) Zeugen (c) Block mount...

Answer Details

As shown in the diagrams above, the following are examples of landforms formed by river and wind action, and by earth movements.

(a) Ox-bow lake

An ox-bow lake is a crescent-shaped or horse-shoe-shaped lake formed on a river flood plain when a meander is cut off from the main river channel.

Characteristics

• It is a curved, crescent-shaped body of standing water.
• It occurs on the wide flood plain of a mature or old river, where the gradient is gentle.
• It is usually found beside the present river channel.
• It has no continuous connection with the main river, except possibly during floods.
• With time, it may become shallower through deposition of silt and may eventually dry up or become a swamp.

Mode of formation

1. A river flowing over a gentle gradient develops bends called meanders.
2. On the outer sides of the bends, the river flows faster and erodes laterally by hydraulic action and abrasion. This produces river cliffs.
3. On the inner sides of the bends, the water is slower and deposits sand and silt, forming slip-off slopes.
4. Continued lateral erosion narrows the neck of the meander.
5. During a period of flooding, the river cuts through the narrow neck and follows the shorter, straighter channel.
6. Deposition of alluvium seals both ends of the abandoned meander loop. The isolated loop remains as an ox-bow lake, as illustrated in the diagram.

(b) Zeugen

A zeugen is a table-like or ridge-like desert landform produced mainly by wind abrasion and differential erosion of alternating hard and soft rock layers.

Characteristics

• It occurs mainly in hot, dry desert regions where vegetation is sparse.
• It consists of a resistant layer of hard rock resting on a softer rock layer.
• It has a broad, relatively flat hard-rock cap and steep or undercut sides.
• Its lower soft-rock portion is more deeply eroded than the upper hard cap, giving it a pedestal or mushroom-like appearance in cross-section.
• Zeugen commonly occur as elongated ridges separated by grooves or furrows, often aligned with the direction of the prevailing wind.

Mode of formation

1. In a desert, horizontal or nearly horizontal beds of hard rock may overlie less resistant soft rock.
2. Intense heating and cooling cause the surface rocks to crack. These cracks and joints provide openings through which wind-blown sand can attack the rock.
3. Wind abrasion is strongest near the ground surface, where sand particles are carried by the wind.
4. The softer rock is worn away more rapidly along the cracks and weaker parts, forming furrows or trenches.
5. The resistant hard rock is eroded slowly and remains as a protective cap over the softer rock beneath it.
6. The remaining blocks stand out as ridges with hard tops and undercut soft bases. These are called zeugen, as shown in the diagram.

(c) Block mountain

A block mountain, also called a horst, is a large uplifted or relatively upstanding block of the earth's crust bounded by faults.

Characteristics

• It is a high, steep-sided mass of land.
• It is bounded by one or two major fault scarps.
• It is often long and narrow, with a fairly level or gently sloping summit.
• It may stand above lower down-faulted areas called grabens or rift valleys.
• Its sides are usually steep because they are formed by faulting rather than by ordinary erosion.

Mode of formation

1. Tensional forces within the earth's crust cause it to crack along two approximately parallel faults.
2. Movements occur along the fault planes.
3. The central block may be uplifted, or it may remain relatively stationary while the land on both sides subsides.
4. The central block then stands above the surrounding land.
5. The upstanding block is called a horst or block mountain, while the lowered blocks beside it may form grabens or rift valleys, as indicated in the diagram above.