5. Sedimentary rocks are often found with the planes of their strata more or less _inclined_. It is evident that they were not thus formed. The depositions of sediment from water will always be horizontal, or, at most, only slightly inclined. But there is often evidence in the rock itself that its strata were once horizontal. It is frequently observed that vertical strata contain pebbles with their longer axes in the plane of the strata. (Fig. 51.) When these pebbles were deposited, the longer axes would take, on an obvious mechanical principle, a horizontal position. Their present vertical position must have resulted from a change in the position of the strata in which they are enclosed. The same thing is shown by the position of a petrified forest in the south of England, known as the Portland dirt-bed. Some parts of it are inclined at an angle of forty-five degrees. The position of the vegetable remains (Fig. 52) shows that when they were growing the surface was horizontal. The line _b d_ (Fig. 53), on inclined strata which makes with the horizon the greatest angle, is called the _direction of the dip_. The angle thus formed (_a b d_) is the _angle of inclination_. When inclined strata come to the surface, the exposed edge, _b c_, is the _outcrop_, and the line of outcrop on a horizontal surface is called the _strike_ of the strata. [Illustration: Fig. 53.] When the inclined position is produced by an uplift of the strata, along a given line, so that they dip in opposite directions, this line is called an _anticlinal axis_, as at Fig. 54. If, however, the strata are fractured along this line, as at _b_, the fracture becomes a _valley of elevation_. [Illustration: Fig. 54.] If depression take place along a given line, as at _c_, the strata will dip towards this line, and it will be a _synclinal axis_. The depression will be a _valley of subsidence_. A synclinal axis would also be produced by an elevation of the strata, as at _d_ and _e_, on each side of it, and the valley thus produced is one of elevation. When successive sets of strata, as _f_ and _d_, Fig. 53, are not parallel, they are said to be _unconformable_. [Illustration: Fig. 55.] [Illustration: Fig. 56.] 6. When the strata are subjected to displacement, they do not always take a merely inclined position, but are often _contorted_ (Fig. 55), or folded together (Fig. 56). These _folded axes_ frequently succeed each other for many miles. (See Figs. 7 and 82.) In the case represented by Fig. 56, if the highest portion has been removed, so that the line _a b_ represents the actual surface, we shall have apparently a succession of deposits, of which those at _b_ would be the newest, and the oldest would be found at _a_, when in fact the strata at the extremities are parts of the same layer.