200 The Realm of Nature CHAP. 



266. Wave Action. The measurements of Mr. Thomas 

 Stevenson on the coast of Scotland show that during severe 

 storms the waves may exert a force equal to 3 tons on 

 eVery square foot of the cliffs they beat against. A force 

 of i ton per square foot is commonly exerted by the waves 

 of the Atlantic in winter, and 600 Ibs. on the square foot in 

 summer. This ponderous surge of the waves tears off 

 loose pieces of rock, and the deluge of spray and pebbles 

 which the breakers toss into the air has been known to 

 break the windows of a lighthouse 300 feet above the sea. 

 When a wave swells up against a cliff it powerfully com- 

 presses the air in all the cracks of the rock, thus striking a 

 sudden blow throughout the whole mass. An explosive 

 expansion of the air follows when the wave subsides, and 

 the loosened fragments are sucked out along the lines of 

 bedding or jointing ( 290). This action and the bombard- 

 ment by pebbles are the chief agents in forming sea-caves, 

 of which one of the finest examples is Fingal's Cave in Staffa, 

 carved out of columnar basalt As the cave extends into the 

 cliff it grows narrower, and finally a long diagonal tunnel may 

 be drilled out, opening on to the upland far from the shore. 

 Such openings or blow-holes are common along all cliff- 

 girdled coasts, and throw up columns of spray during 

 storms often with a noise resembling the outburst of a 

 geyser. Blow -holes naturally widen as the sides are 

 weathered (31 o), and form deep isolated pools where the 

 tidal water rises and falls at the bottom of a nearly vertical 

 rocky shaft. When softer and harder rocks alternate along 

 a coast, the former are in time cut back by the waves and 

 form bays, while the latter project as headlands. Currents, 

 or tidal eddies, attacking a narrow headland on both sides, 

 and driving the pebbles against one part of the cliff, often 

 break a cave right through, which when wide forms a 

 tunnel, when high and narrow a natural bridge. Atmo- 

 spheric erosion may cut as rapidly above as the waves do 

 below, and the headland become separated from the main- 

 land as an isolated rock or stack, round the base of which 

 the water sweeps. Some of the finest examples of such 

 cliff scenery occur on the north coast of Scotland and in 



