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 interspersed, 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." Moreover, 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 it ground and tore the surface rock and carried large and small masses--boulders and incredible millions of tons of broken up fragments--and spread them over East Anglia (where they form the so-called "glacial drift"), and over regions still submerged in the North Sea. Consequently the beach on the Suffolk seashore has a specially variegated assortment of pebbles from all sorts of more northerly situated rocks--though small flint pebbles, derived directly from glacial drift and by the drift from the chalk land-surface (the chalk itself not now reaching the shore-line of East Anglia), are greatly predominant. It is in the chalk that flint takes its origin, being found there as large irregular nodules and sheets. CHAPTER VII THE CONSTITUENTS OF A SEABEACH