OUTLIER AND INLIER 319 



longer as ridges, but the wearing away of their summits gives 

 rise to subordinate ridges and valleys within the limits of each 

 anticlinal arch. Here also the ridges are the outcropping 

 harder beds, and the valleys are cut in the softer ones. Even 

 in mountain ranges denudation may reverse the original struct- 

 ural topography and give rise to anticlinal valleys and synclinal 

 mountains. 



If a region of folded rocks has once been planed down to 

 base-level or to a peneplain, and then reelevated and subjected 

 to denudation, the resulting topography will be determined by 

 the same laws. Indeed, this is the method in which regions 

 of tilted or inclined strata are produced, for, as we saw in 

 Chapter XII (p. 232), inclined beds are generally parts of trun- 

 cated folds. In such regions drainage is first in accordance 

 with the slopes of the planed and tilted surface, but as denuda- 

 tion proceeds, the structure and arrangement of the rocks make 

 themselves felt, and bring about changes and adjustments of the 

 drainage to the structure, as will be more fully explained in the 

 following chapter. 



The combined action of the displacement and dislocation of 

 the strata, on the one hand, and of denudation, on the other, often 

 results in the formation of disconnected patches of rock which, 

 according to their geological relations, are called outliers and 

 inliers. An outlier is an isolated mass of rock, like an island, 

 which has been cut off by denudation from its former connections. 

 Outliers are sometimes scores of miles from the nearest mass 

 of the same strata, and they stand as monuments which show, 

 partially at least, the former extension of the eroded beds. 

 An outlier rests upon the underlying strata, and when viewed 

 on a map, which brings all projections down to one plane, is 

 surrounded by beds which are geologically older than itself. 

 Outliers are almost always composed either of horizontal strata, 

 or of isolated synclines, for an isolated anticline would soon be 

 swept away by denudation, the dip coinciding with the slope of 

 the surface, an arrangement very favourable to landslips and rapid 

 erosion. 



