Parr Il. Seer. ii. §6] TIDES. _ 419 
raised up and Jet down again, there can hardly be any appreciable. 
effect upon the deep ocean bottom. When, however, the tidal wave. 
enters a narrow and shallow sea, it has to accommodate itself to a 
smaller channel, and encounters more and more the friction of the 
bottom. Hence, while its rate of motion is diminished, its height: 
and force are increased. It is in shallow water and along the shores 
of the land that the tides acquire their main geological importance. 
They there show themselves in an alternate advance upon and 
retreat from the coast. Their upper limit has received the name of 
high-water mark, their lower that of low-water mark, the littoral 
space between being termed the beach (Fig. 155). If the coast is 


BIGH WATER: 
—_—-_— 







“LOW WATER. 
| a = 
Yi) _ 
Fig. 155.—SerctTion oF A BEACH DEFINED BY HiIGH- AND LOwW-WATER Mark. 


precipitous, a beach can only occur in shelving bays and creeks, 
Since elsewhere the tides will rise and fall against a face of rock, as. 
they do on the piers of a port. On such rocky coasts the line of 
high water is sometimes admirably defined by the grey crust of 
barnacles adhering to the rocks. Where the beach is flat, and the 
rise and fall of the tide great, several hundred square miles of sand 
or mud may be laid bare in one bay at low-water. 
The height of the tide varies from zero up to 60 or 70 feet. It 
is greatest where, from the form of the land, the tidal wave is cooped 
vf \G 
, Loe oe 
eS one es f 
Fic. 156.—Errect or ConvERGING SHORES UPON THE TIDAL Wave. 
_ The tide wave running up in the direction of the arrows rises successively higher at a, b, 
and c to d, after which it slackens and dies away at the upper limit of tides, f. 
up within a narrow inlet or estuary. Under such circumstances the 
advancing tide sometimes gathers itself into one or more large waves, 
and rushes furiously up between the converging shores. ‘This is the 
- origin of the “bore.” of the Severn, which rises to a height 9 feet, 
2 
