With the aid of diagrams, describe the formation and appearance of any two of the following features: (i) Stack (ii) Blow Hole (iii) Sand Bar

Answer Details

Describe the formation and appearance of any two of the following (two are treated here).

(i) Stack

Formation: a stack develops on a headland of fairly resistant rock. Waves attack lines of weakness at the base by hydraulic action and corrasion, first cutting a cave. When two caves on opposite sides of a narrow headland join, or a cave is cut right through, an arch is formed. Continued erosion and weathering widen the arch until its roof becomes too heavy and collapses, leaving an isolated pillar of rock standing in the sea. This is a stack.

Appearance: a tall, steep-sided, isolated column or pillar of rock rising out of the sea just off the coast, separated from the mainland cliff.

(ii) Blow hole

Formation: a blow hole forms where a sea cave is cut into a cliff along a line of weakness (a joint). Waves force water and compressed air up through the joint towards the top of the cliff. Continued hydraulic action enlarges the joint until it breaks through to the cliff top, forming a vertical shaft. During high tides or storms, water is forced up the shaft and spouts out at the surface.

Appearance: a roughly circular opening or hole on top of a cliff, connected below to a sea cave, through which spray and water spout upward with a blowing sound.