PHYSIOGRAPHY OF THE SEA-SHORE 27 



The Tides. — ^A factor which is of immense and funda- 

 mental importance in shaping the area we are studying and 

 in influencing the life upon it is that of the tides. These 

 may vary greatly in relative force and extent, ranging from 

 practically nil to a rise and fall of many feet, with correspond- 

 ing effects on the fauna. In Britain the biggest tides occur 

 in the Severn estuary, the mean range of spring tides at 

 Chepstow being 42 feet, and of neaps 21 feet. In France, 

 at St. Malo, spring tides range 42*5 feet and neap tides 

 18 feet, this being the maximum for that country. The tidal 

 range in the Dee is 26 feet at springs and 12 feet at neaps, 

 the mean range of spring tides round the coasts of Great 

 Britain being 16*4 feet and of neap tides 8*6 feet. (These 

 data are taken from an article in Nature, vol. 105, June 3, 

 1920.) The causation of the tides is too well known to need 

 lengthy description. Briefly, since the moon attracts the side 

 of the earth nearest to it more strongly than the centre, and 

 since, moreover, the water surrounding the earth is attracted 

 most strongly of all, the waters on this side yield and are 

 heaped up beneath the moon. On the other hand, since 

 the moon attracts the side of the earth remote from it less 

 strongly than the centre, and since, moreover, the water on 

 this side is attracted least strongly of all, the waters, which 

 are less attracted than the solid earth, bulge outwards on 

 the side away from the moon also. (For a more adequate 

 yet not too technical explanation reference should be made 

 to Lake, Physical Geography, Cambridge, 1908.) 



High tides are thus produced at two opposite sides of 

 the earth at the same time. This means that water has been 

 drawn away from other parts of the earth, and consequently 

 at two opposite points half-way between the two high tides 

 the sea is below its normal level and there is low tide. As 

 the earth rotates on its axis every meridian comes opposite 

 the moon in turn, and a rotation occupying twenty-four 

 hours, there are normally two high tides and two low tides in 

 that period. The lagging of the tides, or the difference of 

 roughly fifty minutes in the time of high or low tide on suc- 

 cessive days, is due to the fact that while the earth is rotating 



