With the aid of diagrams, describe the mode of formation of the following features: (a) stack; (b) fold mountalin.

Answer Details

(a) Formation of a stack

A stack is a pillar or column of rock standing in the sea, detached from the coast. It is formed by marine (wave) erosion of a headland:

1. Waves attack lines of weakness (joints or faults) at the base of a headland by hydraulic action, abrasion and solution.
2. Continued erosion widens the weakness into a sea cave on the side of the headland.
3. When two caves on opposite sides of the headland are eroded until they meet, a natural arch is formed through the headland.
4. Further erosion and weathering enlarge the arch until its roof becomes too heavy and collapses.
5. The collapse leaves an isolated column of rock standing in the sea, separated from the headland; this pillar is the stack. (When a stack is later worn down to a low stump it becomes a stump.)

Diagram should show, in stages: headland with cave, arch, and finally the detached stack.

(b) Formation of a fold mountain

Fold mountains are formed by the folding of sedimentary rocks under compression:

1. Over a long period, thick layers of sediment accumulate in a large shallow depression in the earth's crust called a geosyncline, usually on the floor of a sea.
2. The sediments are compacted and cemented into layers of sedimentary rock.
3. Powerful horizontal compressional forces (caused by the movement of crustal/tectonic plates towards each other) squeeze the rock layers from opposite sides.
4. The layers are crumpled and buckled into a series of up-folds called anticlines and down-folds called synclines.
5. These folded and uplifted rock layers form the fold mountains, for example the Alps, Himalayas and Atlas Mountains.

Diagram should show folded strata with labelled anticlines (upfolds) and synclines (downfolds).