=Restoration of mutilated folds.=—Since flexuring of the rocks takes place within the zone of flow at a distance of several miles below the earth’s surface, it is quite obvious that the results of the process can be studied only after some thousands of feet of superincumbent strata have been removed. We are a little later to see by what processes this lowering of the surface is accomplished, but for the present it may be sufficient to accept the fact, realizing that before foldings in the strata can reach the surface, they must have passed through the upper zone of fracture. It might perhaps be supposed that the anticlines would appear as the mountains upon the surface, and occasionally this is true; as, for example, in the folded Jura Mountains of western Europe. More generally, the mountains have a synclinal structure and the valleys an anticlinal one; but as no general rule can be applied, it is necessary to make a restoration of the truncated folds in each district before their character can be known. =The geological map and section.=—The earth’s surface is in most regions in large part covered with soil or with other incoherent rock material, so that over considerable areas the hard rocks are hidden from view. Each locality at which the rock is found at the earth’s surface “in place” is described as an _outcropping_ or _exposure_. In a study of the region each such exposure must be examined to determine the nature of the rock, especially for the purpose of correlation with neighboring exposures, and, in addition, both the probable direction in which it is continued along the surface—the _strike_—and the inclination of its beds—the _dip_. If the outcroppings are sufficiently numerous, and rock type, strike and dip, may all be determined, the folds of the district may be restored with almost as much accuracy as though their curves were everywhere exposed to view. A cross section through the surface which represents the observed outcrops with their inclinations and the assumed intermediate strata in their probable attitudes is described as a _geological section_ (Fig. 27). A map upon which the data have been entered in their correct locations, either with or without assumptions concerning the covered areas, is known as a _geological map_. [Illustration: FIG. 27.—A geological section based upon observations at outcrops, but with the truncated arches restored.]