SAND AND PEBBLES OF THR SEASHORE 53 



A beach is built up of water-worn pebbles, consisting 

 usually of bits of the rock of the immediate vicinity, 

 which have become rounded and shaped by continually 

 rolling and knocking against one another as the waves 

 of the sea throw them up or drag them down the sloping 

 heap of like pebbles which is accumulated near high- 

 water line. At Dover and such places, under chalk 

 cliffs, the beach consists of chalk pebbles oval in shape, 

 often of 8 or 9 inches in length, with a large number 

 of well-rounded flint pebbles as big as your fist inter- 

 spersed, or outnumbering the chalk pebbles. At Tenby, 

 in South Wales, the beach consists of assorted sizes of 

 limestone pebbles, well-worn bits of the limestone cliffs 

 of the neighbourhood. Large numbers of them are 

 literally " worm-eaten," being bored into, hard and dense 

 as they are, by a little marine worm (known as Polydora), 

 which may be sometimes found alive and at work in these 

 limestone pebbles lying between tide limits, or more 

 easily at other places in similarly placed chalk blocks 

 or pebbles. On a coast bounded by granite cliffs you 

 get a beach of granite pebbles ; where there are cliffs 

 of slate or of sandstone, pebbles of slate or of sandstone. 



But there are some beaches which, as remarked 

 above, are continually travelling along the coast. That 

 on the English shores of the North Sea, for instance, 

 is always moving southwards, except where it is held 

 by piles and breakwaters, locally called " shies." More- 

 over, the land of the East Coast, especially the Suffolk 

 and Norfolk coast, in the course of its erosion, has given 

 back to the sea old deposits of the glacial and post- 

 glacial period, consisting of gravels and " drift," made 

 up of flint pebbles and fragments of rocks from the 

 more northern regions over which the great European 

 ice-cap of the glacial epoch extended, and from which 



