232 GLIMPSES OF NATURE. 



of rock, weighing thirteen tons, have been excavated 

 from the cliffs by the force of the waves acting at a 

 height of 70 feet above the level of the sea. When 

 a large wave strikes upon a rock, the pressure of the 

 blow may equal three tons to each square foot. Thus- 

 wise, the air which all rocks contain is forcibly com- 

 pressed, but as quickly, of course, expands when the 

 wave has retreated. 



It is chiefly this compression and expansion of the 

 air in rocks which aids and effects their breaking-up 

 by the waves ; but the sea has other methods of pro- 

 cedure in its attack on the land. The boulders and 

 blocks which have become detached from the rocks 

 accumulate at the foot of the cliffs. The waves seize 

 these blocks in their grasp, and dash them with fury 

 against the cliffs ; using thus the material it has stolen 

 from the rocks as a kind of natural artillery wherewith 

 fresh assault and battery are made upon the land. 

 There is often a definite plan of sea-action to be noted 

 in the case of many coasts. An outjutting portion of 

 a cliff is first of all tunnelled by the waves, so that a 

 natural archway is formed. Then the tunnel collapses, 

 and its outermost part is left standing as a sea-stack 

 or " needle." This stack, bit by bit, is worn down to 

 the sea-level. It grows small by degrees, until it 

 becomes a mere tangle-covered rock, which will be 

 further planed down and worn to nothingness by the 

 irresistible action of the waves. 



The Yorkshire coast, as well as the east coasts 

 farther south, present us with many characteristic 

 historical examples of the action of the sea in robbing 

 us of the land. There is one section of the Yorkshire 

 coast, from the mouth of the Tees to the Humber 

 estuary, which has suffered severely through sea- 



