THE SEA SHORE 27 



The waves are the greatest influence at work moulding the 

 shore, breaking with fury against the land, washing away 

 loose material or, with the aid of pebbles and stones which 

 they dash against it, gradually eating into a hard, rocky 

 coast, forming a flat " abrasion platform " at the base often 

 of towering cliffs. Here we find a typical rocky shore. 

 In other regions, on the other hand, the action of powerful 

 cross currents deposits great banks of sand, formed by the 

 breaking down of rocks. At the mouths of rivers or in 

 sheltered creeks and gulleys there are mud flats where the 

 sediment brought from land is deposited. The action of 

 ice and of weathering in general also assists in the wearing 

 away of the land and the formation of the shore, while the 

 influence of plants and animals is not to be neglected. 

 The former often help to bind together sand and mud and 

 convert them into firm, dry land ; encrusting animals, such 

 as barnacles and mussels, may help to protect rocks, but 

 usually the action of animals is destructive, notably that 

 of the various rock borers. 



A feature of the greatest importance is the variability of 

 conditions on the shore. Not only does the sea cover and 

 uncover it twice daily, but the ranges of temperature are 

 far greater, both yearly and often daily, than in any other 

 part of the sea. An animal in moderately cool water at 

 high tide may be left stranded at low tide in a small rock 

 pool where the temperature rises to great heights under the 

 influence of a hot summer sun. The shallow water near the 

 shore is both warmer in summer and cooler in winter (when 

 ice may form on the shore) than the deeper water further 

 out. The constitution of the sea is also apt to be variable, 

 especially near the mouths of rivers where much fresh- 

 water is mixed with it ; also in the height of summer when, 

 in enclosed areas, evaporation may make the sea more than 

 usually salty. The influence of light is very great, greater 



