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. Then, 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_. NAMES OF SYSTEMS CHARACTERS OF ROCKS { Recent & Pleistocene+------+ { Pliocene +------+ sands, superficial deposits TERTIARY { Eocene +------+ { | | clays and sands chiefly +------+ { Cretaceous | | chalk at top { | | sandstones, mud and clays below { +------+ SECONDARY{ Jurassic | | shales, sandstones and { | | oolitic limestones { +------+ { Triassic | | red sandstones and marls, { | | gypsum and salt +------+ { Permian | | red sandstones & magnesian { +------+ limestone { | | { | | sandstones, shales and coals { Carboniferous | | at top { | | sandstones in middle { | | limestone and shales below { +------+ { | | { Devonian | | red sandstones, { | | shales, slates and limestones { +------+ { | | { Silurian | | sandstones and shales { | | thin limestones PRIMARY { +------+ { | | shales, slates, { Ordovician | | sandstones and { | | thin limestones { | | { +------+ { | | { Cambrian | | slates and { | | sandstones { | | { | |