Page:South African Geology - Schwarz - 1912.djvu/103

 If the currents in the water are violent, the sand is heaped up in one place, and the layers or strata are steeply inclined. When the direction of the current changes, owing to the tide or wind, the tops of the mounds will be washed away, and new material will be laid down on the side of the mound opposite to that on which it was previously deposited. This action is called contemporaneous erosion and deposit, and the irregular stratification is called false or current bedding. Windblown sand is always markedly false-bedded.

Unconformity. — The old rock surface of the sea floor on which the sediment is deposited must have been cut when it was exposed as dry land, as below low tide no erosion takes place. The submerged land surface is then said to be covered unconformably by the newer sediment, and the plane of division between the two is called an unconformity. An unconformity always implies a land surface; so that, if in the side of a hill one set of beds is seen to lie unconformably upon another, we know for a certainty that the two are widely separated in time, for the lower sediments were laid down in the sea; compacted into rock; elevated into dry land; cut into peak, valley, and plain; were again submerged, before the overlying sediments were deposited upon them. The most conspicuous example of an unconformity is the sandstone forming the top of Table Mountain at Cape Town, resting on the eroded surface of the granite and clay-slate below.

Joints. — When the sediments have accumulated to any great depth, the sea floor sinking continuously, as explained under the heading "secular upheaval and subsidence", the lower portions become compacted by the weight of the superincumbent material and turned into