It is evident that the stratified rocks which form the land must have been changed, at least relatively, from the position which they originally occupied, since the great bulk of them were laid down under the sea. Originally they must have been nearly horizontal, for this is a necessary result of the operation of gravity. Just as a deep fall of snow, when not drifted by the wind, gradually covers up the minor inequalities of the ground and leaves a level surface, so on the sea-bottom the sediments are spread out in nearly level layers, disregarding ordinary inequalities. We must remember, however, that this original horizontality is not exact, and departures from it are not infrequent. On a large scale, these departures from the horizontal position are very slight, while those that are conspicuous are local.

Examples of such original deviations from horizontality are the following: (1) When a sediment-laden stream or current empties abruptly into a deep basin with steeply sloping sides, the sediment is rapidly deposited in oblique layers, which follow the slope of the sides (i.e. foreset beds). (2) Alluvial cones, or fans (p. 202), have steeply inclined layers, for a similar reason. Both of these cases resemble the artificial embankments which are built out by dumping earth or gravel over the end, until each successive section is raised to the necessary level. In such embankments the obliquity of the layers is often plainly visible. (3) Sand beaches often have a considerable inclination, as much as 8 %, and newly added layers follow this slope. (4) On a large scale, the great sheets of sediment that cover the sea-bottom generally have a slight inclination away from the land, with a somewhat increased slope along lines of depression. These slight original inclinations of sedimentary masses, either as a whole, or along certain lines, are called initial dips, and have an important bearing upon the results of subsequent movements of displacement.

The displacements to which strata have been subjected after their formation are of two principal kinds: (1) In the first kind, the strata have been lifted vertically upward, often to great heights, without losing their horizontality. Over great areas of our Western States and those of the Mississippi valley, the beds are almost as truly horizontal as when they were first laid down. In some of the lofty plateaus through which the Grand Canon of the Colorado has been cut, almost horizontal strata are found 10,000 feet above the sea-level. (2) More frequent and typical are the displacements of the second class, by which the beds are tilted and inclined at various angles, sometimes bringing the strata into a vertical position, and occasionally even overturning and inverting them. In the comparatively small exposures of strata which may be seen in ordinary sections in cliffs and ravines, the rocks appear to be simply inclined, and the strata themselves to be nearly straight. But when the structure is determined on a large scale, it is often found that this appearance is due to the limited area visible in one view, and that the apparently straight beds are really portions of great curves.

Such curves are called folds.

Dip

The angle of inclination which a tilted stratum makes with the plane of the horizon is called the dip, and is measured in degrees. The line or direction of the dip is the line of steepest inclination of the dipping bed, and is expressed in terms of compass bearing. For example, a stratum is said to have a dip of 150 to the northwest. The angle of dip is measured by means of an instrument called a clinometer, of which many kinds are in use.