240 ECONOMIC GEOLOGY 
which struck with a force of 7 cwt. per square foot in its 
advance, on its recoil gave a blow of 1 ton per square foot. 
The backwash by its undercurrent carries beach material 
outward. 
Swiftly moving water is a most powerful disruptive agent, 
for its blow is heavy and it attacks the weakest point like a 
pickaxe. The force of waves was measured in 1843-5 by 
Alan Stevenson when designing the lighthouse at Skerryvore, 
a low rock 12 miles from the coast of Tiree (Alan Stevenson, 
Skerryvore Lighthouse,” 1848; Thos. Stevenson, Lighthouse 
Construction, 1881). The average wave during the five 
summer months struck a blow of 611 Ib. per square foot ; the 
average for the six winter months was 2086 Ib. per square foot ; 
the maximum in a gale, 29 March, 1845, was 6086 1b., or 
nearly 3 tons per square foot. In narrow seas the wave force 
may be as great. Thus on the eastern coast of Scotland 
blows of 3 tons per square foot have been measured at Buckie, 
and of 3% tons per square foot at Dunbar. 
The height reached by waves varies with the slope of the 
sea-floor and the shape of the land. Thus at the Fastnet 
Lighthouse, off South-western Ireland, a chasm acts as a 
nozzle and water is flung against the tower at the height of 
120 feet, and a 3-ton block of stone was thrown off the cliff 
at the height of 82 ft. 
The direct blow of the waves tears away and shatters 
masses of hard rock. During the construction of the break- 
water at Wick a mass of concrete, 1350 tons in weight, 
was moved from its place. A storm at Genoa in 1898 swept 
the base of the breakwater bare of shingle, undercut it, and 
carried for 155 feet blocks of concrete 600 cubic feet in volume, 
and weighing 40 tons. A storm at Bilbao in 1804 swept 
away the huge blocks of stone placed to shield the break- 
water which was breached, and one mass of 1045 cubic yards 
and weighing 1700 tons was carried 175 feet into the harbour. 
Recession oF THE Lanp—With such effects on carefully 
built structures, it is not surprising that the sea rapidly 
wears away soft rocks. Waves hurl beach material against a 
cliff, and its fragments serve as ammunition for its further 
destruction. The cutting back of the land is aided by air 
being compressed in cracks and crevices in a cliff by an 
advancing wave, and on its fall expanding with explosive