Rocks are of two kinds, (1) those laid down mostly under water; (2) those due to the action of heat.
The first kind may be compared to sheets of paper one over the other. These sheets are called beds, and such beds are usually formed of sand (often containing pebbles), mud or clay, and limestone, or mixtures of these materials. They are laid down as flat or nearly flat sheets, but may afterwards be tilted as the result of movement of the earth’s crust, just as you may tilt sheets of paper, folding them into arches and troughs, by pressing them at either end. Again, we may find the tops of the folds so produced worn away as the result of the constant action of rivers, glaciers, and sea-waves upon them, as one might cut off the tops of the folds of the paper with a pair of shears. This has happened with the ancient beds forming parts of the earth’s crust, and we therefore often find them tilted, with the upper parts removed. Tilted beds are said to dip, the direction of dip being that in which the beds plunge downwards, thus the beds of an arch dip away from its crest, those of a trough towards its middle. The dip is at a low angle when the beds are nearly horizontal, and at a high angle when they approach the vertical position. The horizontal line at right angles to the direction of the dip is called the line of strike. Beds form strips at the surface, and the portion where they appear at the surface is called the outcrop. On a large scale the direction of outcrop generally corresponds with that of the strike. Beds may also be displaced along great cracks, so that one set of beds abuts against a different set at the sides of the crack, when the beds are said to be faulted.
The other kinds of rocks are known as igneous rocks, which have been melted under the action of heat and become solid on cooling. When in the molten state they have been poured out at the surface as the lava of volcanoes, or have been forced into other rocks and cooled in the cracks and other places of weakness. Much material is also thrown out of volcanoes as volcanic ash and dust, and is piled up on the sides of the volcano. Such ashy material may be arranged in beds, so that it partakes to some extent of the qualities of the two great rock groups.
The production of beds is of great importance to geologists, for by means of these beds we can classify the rocks according to age. If we take two sheets of paper, and lay one on the top of the other on a table, the upper one has been laid down after the other. Similarly with two beds, the upper is also the newer, and the newer will remain on the top after earth-movements, save in very exceptional cases which need not be regarded by us here, and for general purposes we may regard any bed or set of beds resting on any other in our own country as being the newer bed or set.
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 ofSystems | Subdivisions | Characters ofRocks | ||||||
| TERTIARY |  | Recent Pleistocene |  | Metal Age | Deposits |  | Superficial Deposits | |
| Neolithic |
| |||||||
| Palaeolithic |
| |||||||
| Glacial |
| |||||||
| Pliocene |  | Cromer Series |  | Sands chiefly | ||||
| Weybourne Crag | ||||||||
| Chillesford and Norwich Crags | ||||||||
| Red and Walton Crags | ||||||||
| Coralline Crag | ||||||||
| Miocene | Absent from Britain | |||||||
| Eocene | | Fluviomarine Beds of Hampshire | | Clays and Sands chiefly | ||||
| Bagshot Beds | ||||||||
| London Clay | ||||||||
| Oldhaven Beds, Woolwich and Reading Groups | ||||||||
| Thanet Sands | ||||||||
| SECONDARY |  | Cretaceous | | Chalk | | Chalk at top Sandstones, Mud andClays below | ||
| Upper Greensand and Gault | ||||||||
| Lower Greensand | ||||||||
| Weald Clay | ||||||||
| Hastings Sands | ||||||||
| Jurassic |  | Purbeck Beds |  | Shales, Sandstones and OoliticLimestones | ||||
| Portland Beds | ||||||||
| Kimmeridge Clay | ||||||||
| Corallian Beds | ||||||||
| Oxford Clay and Kellaways Rock | ||||||||
| Cornbrash | ||||||||
| Forest Marble | ||||||||
| Great Oolite with Stonesfield Slate | ||||||||
| Inferior Oolite | ||||||||
| Lias—Upper, Middle, and Lower | ||||||||
| Triassic | | Rhaetic | | Red Sandstones and Marls, Gypsum andSalt | ||||
| Keuper Marls | ||||||||
| Keuper Sandstone | ||||||||
| Upper Bunter Sandstone | ||||||||
| Bunter Pebble Beds | ||||||||
| Lower Bunter Sandstone | ||||||||
| PRIMARY | | Permian |  | Magnesian Limestone and Sandstone |  | Red Sandstones and MagnesianLimestone | ||
| Marl Slate | ||||||||
| Lower Permian Sandstone | ||||||||
| Carboniferous | | Coal Measures | | Sandstones, Shales and Coals at topSandstones in middle Limestone and Shales below | ||||
| Millstone Grit | ||||||||
| Mountain Limestone | ||||||||
| Basal Carboniferous Rocks | ||||||||
| Devonian | | Upper | | Devonian and Old Red Sandstone | | Red Sandstones, Shales, Slates andLimestones | ||
| Mid | ||||||||
| Lower | ||||||||
| Silurian | | Ludlow Beds | | Sandstones, Shales and ThinLimestones | ||||
| Wenlock Beds | ||||||||
| Llandovery Beds | ||||||||
| Ordovician | | Caradoc Beds | | Shales, Slates, Sandstones and ThinLimestones | ||||
| Llandeilo Beds | ||||||||
| Arenig Beds | ||||||||
| Cambrian | | Tremadoc Slates | | Slates and Sandstones | ||||
| Lingula Flags | ||||||||
| Menevian Beds | ||||||||
| Harlech Grits and Llanberis Slates | ||||||||
| Pre-Cambrian | No definite classification yetmade | Sandstones, Slates and VolcanicRocks | ||||||
In the preceding table (p. [29]) a representation of the various great subdivisions or ‘systems’ of the beds which are found in the British Islands is shown. The names of the great divisions are given on the left-hand side, in the centre the chief divisions of the rocks of each system are enumerated, and on the right-hand the general characters of the rocks of each system are given.
