The effectiveness of waves, whether they work by impact of water alone, or by impact of water and detritus, is dependent on their strength and on the concentration of their blows.[156] The strength of waves is dependent on the strength of the winds (or other generating cause) and the depth and expanse of the water, and the concentration of their blows is conditioned by the slope against which they break. On exposed ocean-coasts the fetch of the waves is always great. The winds are variable. For a given coast they have an average strength, but the effectiveness of wave-erosion is determined less by the average strength of waves than by the strength of the storm-waves. This is often very great. On the Atlantic and North Sea coasts of Britain, winter breakers which exert a pressure of three tons per square foot are not infrequent.[157] So great is the force of exceptional storm-waves that blocks of rock exceeding 100 tons in weight are known to have been moved by them. Ground-swells, “even when no wind is blowing, often cover the cliffs of north Scotland with sheets of water and foam up to heights of 100 or even nearly 200 feet. During northeasterly gales the windows of the Dunnet Head lighthouse, at a height of upwards of 300 feet above high-water mark, are said to be sometimes broken by stones swept up the cliffs by sheets of sea-water.”[158] The average force of waves on the Atlantic coast of Britain has been found to be 611 lbs. per square foot in summer, and 2086 lbs. in winter.[159] Where deep water extends up to the shore, the force of the wave is almost wholly expended near the water line; where shallow water borders the land, the force of the waves is expended over a greater area. Waves are, therefore, most efficient on bold coasts bordered by broad expanses of deep water. The less familiar phases of wave-work are accomplished by hydraulic pressure, compressed air, the use of ice, etc. When the water of a wave is driven into an open joint or a cave, the hydraulic pressure is great, and if the structure be weak, the rock may be broken. When water is driven with force into a cave, the compression of the air may be great if the wave be high enough to close the entrance. When the water runs out of a cave, the air within may be greatly rarefied, while that above exerts its normal pressure. In either case the roof of the cave, if it be weak, may be broken. At certain seasons of the year, especially during the spring, waves make destructive use of the ice which is then breaking up, but it is only in high latitudes that sea-ice is of consequence in this way. In general, the effect of its presence in keeping down waves overbalances its effect as an agent of erosion. [Illustration: +Fig.+ 302.—Showing blocks similar to those of Fig. 306, reduced and rounded by wave-action. Shore of Lake Champlain. The rock is Utica shale. (Perry.)]