In sand dunes, sand bars, alluvial fans, deltas, spits and beaches where there are cur rents, layers of sand or mud are often laid down on sloping instead of horizontal surfaces. This may produce for short distances inclined layering known as false bedding or cross bed ding which in small exposures may be mistaken for true bedding, and lead to the incorrect idea that the beds have been tilted, when as a matter of fact they have been formed in this inclined position.
Ripple marks may be defined as rhythmic undulations made on the surface of unconsoli dated sediments. They may be formed by run ning water on the surface of sand bars. Al most everyone has noticed the little closely spaced ridges occurring after a downpour along the bottoms of channels that were re cently full of running water. Wind also makes tiny ripples on sand dunes. The constant back and forth movement of waves makes similar small parallel ridges of even more regular pattern on the sandy bottom of shallow bodies of water. If these ridges become buried be fore they are destroyed and the material hardens, the real ripple may be preserved with its duplicate in reversed form as a cast on the lower surface of the overlying bed. Practiced observers can frequently tell the ripple from its reversed cast and are thus enabled to tell whether the beds are right side up, even in regions where there has been intense folding. Ripples are rarely found in deep sea sediments as the water is too deep for waves to disturb the bottom. See RIPPLE MARKS.
Mud flats that are long exposed to the sun's rays crack to considerable depth, the crack being widest at the top and pinching out below. If sand drifts over this surface and fills these little fissures, the beds may subsequently solidify and the filled crack be preserved in the rock. As the crack is widest at the top, this structure too can be used to tell whether or not the bed is right side up. Sun cracks are found• in ter restrial sediments, but obviously not in marine beds.
Slight showers sometimes leave rain drop imprints on mud surfaces. These may like wise be buried, harden, and be preserved. Little rill marks, channels made by running water, may be preserved in a similar manner, as may also foot prints left by animals in the mud. These are all characteristic of terrestrial rather than marine sediments and, as in the case of ripples and sun cracks may be used to tell top from bottom of beds.
A layer of sediment may be formed beneath the sea, the region may then emerge and be carved into hills and valleys. Another mergence may cause continued sedimentation, later beds being laid down on top of the old erosion surface. Such an erosional break in the continuity of deposition is said to be an unconformity, and the upper beds rest uncoil formably on the lower. At the time of emer gence there may be folding, the folds being planed off by the subsequent erosion. The
later series of beds may then be laid horizont ally across the truncated upturned edges of the older beds. This is an angular unconformity, and the beds are said to be discordant. A knowledge of unconformity is highly important in studying the geology of a given region. See UNCONFORMITY.
Secondary or Induced Structures.—Joints are fractures which traverse rock masses.
Sometimes they occur in definitely spaced sets, in which case they comprise a joint system. Two or more systems may occur in the same region dividing the rock into regular blocks. Columnar jointing in lavas is supposed to be due to shrinkage from cooling. (See Giasres CAUSEWAY). Other joints are the result of internal deformation that bends and cracks the rock. Still other joints may be due to shrinln age of sediments from loss of water on drying. Jointing modifies ground water circulation, the deposition of mineral matter as veins, and a'ds in quarrying. See Jonas.
Faults are fractures along which there has been slipping of one of the rock walls past the other. In extreme cases one wall has dropped several thousand feet with respect to the adja cent side, resulting in a cliff known as a fault scarp. Erosion, however, soon removes such a cliff and the majority of faults, unless very re cent, have no scarps. Sometimes the plane of breaking is nearly horizontal, and lower and older beds have been thrust far out, over younger formations. Such thrust faults are most common in closely folded mountain re gions, and are sometimes known with mover ments as great as fifteen miles or more. Min eral veins or seams of coal may be faulted off and portions of them never discovered. The entire subject of faulting is one of great inter est in mining. See FAULT.
In many regions, particularly in mountain ous ones, compression has caused the buckling of the rocks into great folds. Upward arches are known as anticlines, trough-like folds as synclines. Folds vary from a few inches across to many and from the gentlest swells to structures in which the beds are vertical or even overturned. An anticline when first formed constitutcs an elevation or mountain which however, may soon be completely re moved by erosion, though the structure is still called a fold. After peneplanation the beds on either side will be exposed in long parallel lines, and if there are many parallel folds the same formation may outcrop repeatedly. See FOLDS.
In intensely folded regions where rocks have undergone great pressure, new minerals usually grow with their longer dimension at right angles to the pressure, as that is the eas iest way for them to develop. As a result the rock manifests a tendency to split more easily parallel to the long dimensions of the minerals, Just as wood splits parallel to the grab. Such a tendency is called cleavage and is best shown in schists and slates. See 'Root CLEAVAGE. '