Page:South African Geology - Schwarz - 1912.djvu/104

 rock the more the load the greater the compactness of the rock. When the sediments rise and form dry land, the overlying beds become washed away and cut away by weathering and erosion, so that successively older and older beds become laid bare on the surface of the earth. When the pressure under which they have lain for ages is thus relieved, the rocks expand, and two systems of vertical cracks develop at right angles to one another rarely at other angles due to the movement consequent on the relief of pressure. These cracks are called joints, and can be seen in any quarry, for they allow square blocks to be split off, while the bedding planes give horizontal planes of cleavage.

Folding.—In most cases where there is an elevation of the land, disturbances occur which occasion horizontal pressure; the simplest case is where a mass of igneous rock like the dolerite of the Karroo is intruded into, and pushes outwards, the sedimentary rocks it invades. If the horizontal thrust is brought up against a buffer, such as a core of granite anchored to the substructure of the crust, then the beds are slowly bent. The movement is brought about by the solution of the grains in position of pressure, and deposition of the material dissolved in positions of less pressure, so that a massive bed of hard sandstone may be twisted into bends and crinkles while it remains solid all the while. A fold which is upwards, like an A, is called an anticline; a fold which is downwards, like a V, is called a syncline. The centre of the