The movements which affect beds may occur at different times. One set of beds may be laid down flat, then thrown into folds by movement, the tops of the beds worn off, and another set of beds laid down upon the worn surface of the older beds, the edges of which will abut against the oldest of the new set of flatly deposited beds, which latter may in turn undergo disturbance and removal of their upper portions. Again, after the formation of the beds many changes may occur in them. They may become hardened, pebble-beds being changed into conglomerates, sands into sandstones, muds and clays into mudstones and shales, soft deposits of lime into limestone, and loose volcanic ashes into exceedingly hard rocks. They may also become cracked, and the cracks are often very regular, running in two directions at right angles one to the other. Such cracks are known as joints, and the joints are very important in affecting the physical geography of a district. As the result of great pressure applied sideways, the rocks may be so changed that they can be split into thin slabs, which usually, though not necessarily, split along planes standing at high angles to the horizontal. Rocks affected in this way are known as slates. If we could flatten out all the beds of England, and arrange them one over the other and bore a shaft through them, we should see them on the sides of the shaft, the newest appearing at the top and the oldest at the bottom. Such a shaft would have a depth of between 50,000 and 100,000 feet. The beds are divided into three great groups called Primary or Palaeozoic, Secondary or Mesozoic, and Tertiary or Cainozoic, and at the base of the Primary rocks are the oldest rocks of Britain, which form as it were the foundation stones on which the other rocks rest, and are termed Precambrian rocks. The three great groups are divided into minor divisions known as systems. NAMES OF SUBDIVISIONS CHARACTERS OF ROCKS SYSTEMS { { Metal Age Deposits } { Recent { Neolithic ,, } Superficial Deposits { Pleistocene { Palaeolithic ,, } { { Glacial ,, } T { E { { Cromer Series } R { { Weybourne Crag } T { Pliocene { Chillesford and Norwich Crags } Sands chiefly I { { Red and Walton Crags } A { { Coralline Crag } R { Y { Miocene Absent from Britain { { { Fluviomarine Beds of Hampshire} { { Bagshot Beds } { Eocene { London Clay } Clays and Sands { { Oldhaven Beds, Woolwich and } chiefly { { Reading Groups } { { Thanet Sands }