Page:South African Geology - Schwarz - 1912.djvu/102

 the river or marine deposits once universally spread over it, but sometimes it appears that the land has tilted since the cutting of the level The main impression, however, is that South Africa has risen as an iceberg would when the ice melts.

SECTION III

TECTONIC GEOLOGY

Tectonic Geology is concerned with structures exhibited in the earth's crust.

Stratification.— When a river comes down in flood it pours out over the sea floor an immense amount of sand and mud, which eventually subsides and forms a more or less horizontal stratum or bed of sediment. The surface of this deposit will be smoothed by the currents in the water, so that, when the next flood carries fresh sand and mud over the previous deposit, the two will be separated by a plane of division marked by the smoothed surface of the underlying one. This plane of division is called a bedding plane. When the sand and mud are consolidated into sandstone and shale, and elevated above the sea, the bedding plane will be clearly shown as a division. All sedimentary rocks are arranged in strata which were deposited intermittently, each stratum being marked above and below by a bedding plane; hence sedimentary beds are stratified rocks. It is evident that the supply of sediment is limited, therefore the stratum of deposit will thin out on the edges, and consequently the strata of rocks are always more or less lenticular, that is, they thin out at the edges.