A sea cliff is a vertical or steep slope on the coastline. They can be seen on OS maps where contour lines along the coast are very close together.
High-water mark and low-water mark:
the levels reached by the sea at high and low tides.
Formation: • Waves erode a notch at a line of weakness on the coastline between the high-water mark and the low-water mark. Over time, this notch grows larger due to the hydraulic action of the waves.
• Undercutting (the cutting of the waves into the rock base) then occurs.
• The rock above the notch is called the overhang. It is left without support and eventually collapses.
• As the sea erodes further and the rock it erodes gets higher, a cliff is formed.
• The processes of compression and abrasion constantly erode the cliff so that it retreats into the coastline.
• The rock that is left at the former base of the cliff is called a wave-cut platform. Material eroded from the new cliff face is then deposited on top of this platform to form a wave-built terrace.
Example of cliffs: Cliffs of Moher in Co. Clare.
Hard rock cliffs
Cliff face Notch Wave-cut platform The cliff retreating
Overhang collapses into the sea
: Figure 10.7 Formation of sea cliffs NUMERACY ACT VITY
(a) Research the highest point of (i) the Cliffs of Moher in Co. Clare and (ii) the Cliffs of Dover in England. (b) What is the difference in height between the Cliffs of Moher and the Cliffs of Dover?
; Figure 10.8 The Cliffs of Moher
DID YOU KNOW? The cliffs of Croaghaun on Achill Island in Co. Mayo are the highest in Ireland. They are 688 metres high.
