In quarries and railway cuttings, and on cliff faces, the strata or layers of rock can often be seen to be tilted and bent, and sometimes broken, although we know that they were originally formed in flat, horizontal sheets. To explain how this came about would occupy a large and complicated book – and in any case, scientists are not sure that they know the whole explanation yet. So here we must just accept the fact that the Earth's crust – the rocks forming its outer layers about 30 miles thick – is and always has been under stress and strain, squeezing some parts together and pulling others apart raising some parts higher and lowering other parts. These 'earth movements', as they are called, take place so slowly that they are seldom noticed in a man's life time, but over periods of millions of years they can move the strata a long way. In certain regions, however, the movements are sufficiently rapid for their effects to be seen or felt. Earthquakes are a sign that the rocks have been displaced, and sometimes the amount of movement can be seen and measured. Volcanoes are another result of earth movements, which have caused the molten rocks from deep down to be forced out violently at the surface.

The effects of the earth-movements are very important to the miner, and we must therefore study some of them. The simplest effect is tilting. In some parts of Britain, especially in Scotland, Lancashire and North Staffordshire, the coal seams and other rocks lie at very steep angles – sometimes, indeed, they are now vertical though we know they were horizontal when they were first formed. The illustration shows a seam of moderate inclination, as would be seen if a block of country could be cut out of a coalfield. The angle of the downward slope or tilting of the rocks is called the 'dip', though the miner often prefers to think of the upward slope, which he calls 'the rise'. The direction at right angles to the dip is called the 'strike'. A line in this direction is always horizontal, because it is not affected by the dip.

Often the strata are not simply tilted but are also bent or folded. As shown in the illustrations, an upward or arch-fold is called an 'anticline' and a downward or trough fold is called a 'syncline'.

Instead of bending (or perhaps because they have been bent too far) the strata may break. These breaks are called 'faults'. The miner finds that the seam he is working in is suddenly cut off at the fault, and on the other side it lies at a higher or a lower level as shown in the illustration below. The displacement or difference in level is called the 'throw', and it may be only a few inches or as much as hundreds of feet. If the seam is found at a higher level the miner calls it an upthrow fault, and if a lower level, a downthrow fault.

The rocks close to the break or fault plane are generally crushed and broken, and the surfaces are often highly polished because one side has slipped under enormous pressure over the other. Such polished surfaces are called 'slickensides'.

The angle between the fault plane and the vertical is called the 'hade'. In the commonest type of fault, known as a 'normal fault', the hade or slope of the fault plane is generally less than 450 – that is, it is nearer the vertical than the horizontal. The slope is also always towards the lower or downthrow side. The 'normal' fault therefore always causes a 'want' or space between the two faulted edges of the seams and other layers.

A large break in the strata is often not a single fault but a number of parallel faults all throwing in the same direction. This gives rise to step-faulting, as shown in the illustration.

Another kind, known as a 'reversed fault', is caused by the strata on one side being pushed over those on the other side. As a result, the hade or slope of the fault plane is nearer to horizontal than in a normal fault, and there is repetition of the beds – for example, if a hole was bored downwards from the surface it would pass through some of the beds twice.

A trough fault is formed when two breaks in the strata allow the rocks between them to fall below their original position; a typical example is shown at 4 opposite.

Other disturbances of the strata affect the coal seams and complicate the work of the miner. As shown previously, seams vary in thickness from place to place, but occasionally the change is rapid. The seam thins out suddenly, its place being taken by sand and gravel, but if the miner tunnels through this he reaches the seam again. This is a 'washout', caused by a stream or river cutting through the seam shortly after it was formed.

Occasionally the seam is missing over a short distance because the underlying rock formed a ridge in the coal swamp. This is known as a 'roll' or 'horseback'.

In some coalfields, molten rocks from very deep down have been pushed up into the Coal Measures, and on cooling and solidifying they have formed very hard walls or 'dykes'. Sometimes, instead of cutting across the strata, the molten rocks have been squeezed between the beds, occasionally stepping suddenly from one level to another. In this form they are called 'sills'.