WAVE EROSION 221 



longshore currents. The actual impact of the waves upon a sandy 

 or rocky shore produces comparatively little effect. By compress- 

 ing the air in the caves or joint cracks, completely closed by the 

 water, or by hydraulic pressure, the waves may succeed in ulti- 

 mately shattering the rocks or loosening large masses. The most 

 effective wave erosion work is. however, accomplished by hurling 

 pebbles and even boulders or ice blocks against the cliffs, and so 

 undermining them and by removing the pieces broken off by this 

 process, by frost action or otlierwise. 



The force of the waves in great storms is often surprising. 

 Measurements with a dynamometer made by Stevenson on the 

 north coast of Scotland gave a force of impact equal to about 

 3,000 kgm. per square meter in summer (611 lb. per square foot), 

 and more than 10,000 kgm. per square meter in winter (2,086 lb. 

 per square foot), while in exceptional storms the force may exceed 

 30,000 kgm. per square meter. "The greatest result yet obtained 

 at Skerryvore was during the heavy westerly gale of 29th of March, 

 1845, when a pressure of 6,083 lbs. per square foot was registered. 

 The next highest is 5,323 lbs." (Stevenson-67 :i'5.) At Cher- 

 bourg, on the coast of France, and at Algiers, on the north coast of 

 Africa, similar measurements gave a force 3,000 to 3,500 kgm. to 

 the square meter, while at Civitavecchia, on the west coast of Italy, 

 a force of 16,000 kgm. per square meter has been obtained. 



Observations on the transporting powers of waves are also avail- 

 able. Rock masses weighing 100 tons have been known to be 

 moved by the great waves of winter storms on the north British 

 coast. Many times blocks weighing five tons or over have been 

 torn from the ledges or from foundations and dragged many yards 

 by the waves. At Plymouth, during a severe storm in November, 

 1824, granite blocks up to 14,000 pounds were broken from the 

 harbor embankment, and pushed uphill in some cases for 60 yards. 



"On more than one occasion at Plymouth during the construc- 

 tion of the breakwater large blocks of stone, some of them weigh- 

 ing 7 to 9 tons, were removed from the sea slope of the break- 

 water at the level of the low water, carried over the top a distance 

 of 138 feet, and piled up on the inside. In one night 200,000 tons 

 [British] of stones were thus removed; and on another occasion 

 9,000 tons." (Wheeler-73 :/('?.) At the Peterhead breakwater 

 waves 30 feet in height and 500 to 600 feet long have on three 

 occasions displaced blocks weighing over 40 tons each at levels 

 from 17 to 36 feet below tide, and at Cherbourg over 200 blocks 

 weighing each about 4 tons were lifted over the top of the break- 

 water, while blocks weighing over 12 tons were turned upside down. 



