45. _Inclination of Strata._--Beds of aqueous strata must have been deposited in horizontal or approximately horizontal planes; but we now find them most frequently inclined at various angles to the horizon, and often even standing on end. They sometimes, however, retain a horizontal position over a large tract of country. The angle which the inclined strata make with the horizon is called the _dip_, the degree of inclination being the _amount_ of the dip; and a line drawn at right angles to the dip is called the _strike_ of the beds. Thus, a bed dipping south-west will have a north-west and south-east strike. The _crop_ or _outcrop_ (sometimes also, but rarely, called the _basset edge_) of a bed is the place where the edge of the stratum comes to view at the surface. We may look upon inclined beds as being merely parts of more or less extensive undulations of strata, the tops of the undulations having been removed so as to expose the truncated edges of the beds. In the following diagram, for example, the outcrops of limestone seen at _l_, _l_, are evidently portions of one and the same stratum, the dotted lines indicating its former extent. The trough-shaped arrangement of the beds at _s_ is called a _synclinal curve_, or simply a syncline; the arched strata at _a_ forming, on the contrary, an _anticlinal curve_ or _anticline_. [Illustration: Fig. 9.--Anticlines and Synclines.] [Illustration: Fig. 10.--Contorted Strata.] When strata shew many and rapid curves, they are said to be contorted. The diagram section (fig. 10) will best explain what is meant by this kind of structure. 46. In certain regions, the strata often dip in one and the same direction for many miles, at an angle approaching verticality, as in the following section. It might be inferred, therefore, that from A to B we had a gradually ascending series--that as we paced over the outcrop we were stepping constantly from a lower to a higher geological horizon. But, in such cases, the dip is deceptive, the same beds being repeated again and again in a series of great foldings of the strata. Such is the case over wide areas in the upland districts of the south of Scotland. The section (fig. 11) shews that the beds are actually inverted, the strata at × × being bent back upon strata which really overlie them. [Illustration: Fig. 11.--Inversion of Strata.] [Illustration: Fig. 12.--Contemporaneous Erosion.]