An outlier is an isolated mass of strata, which is surrounded on all sides by beds older than itself. This definition does not imply that the older beds must actually rise to the level of the outlier and enclose it, but as viewed on a map, which brings all irregularities of surface down to one plane, the older beds appear to surround the outlier. An outlier has been cut off by denudation from its former connections, from which it is separated, in some cases by a few feet, in others by scores or even hundreds of miles. Outliers thus stand as monuments which show, partially at least, the former extension of strata long subject to denudation, though we never can be sure that the farthest outlier was at the actual original margin of the beds, and generally may be confident that it was not. Outliers are almost always composed of horizontal strata, or of isolated synclines.

If the outlier be brought to the surface by faulting, it is called a faulted outlier, in distinction from one which is entirely due to erosion. A faulted outlier may be found on the downthrow side of a fault-block, especially in a trough-fault, which is downcast with reference to the blocks on each side of it. In such a case the older beds actually surround and enclose the isolated mass of newer beds.

Inliers differ from outliers in not necessarily being isolated masses of rock, but merely isolated outcrops of older beds which are surrounded by newer strata, though underground they may be continuous with very extensive areas of beds. An inlier is thus a larger or smaller mass of rock surrounded by beds which are geologically younger than itself. The summit of an anticline or dome which has been truncated by denudation exposes older strata in the middle, newer ones on the sides. Inliers may also be due to faulting and occur on the upthrow side, as in a fault-block which is on the upthrow side with reference to the blocks on each side of it, or Horst.

Outliers may be converted into inliers by the deposition of newer beds around them. The isolated "stacks" and pillars on the sea-coast, as shown in Figs. 76 and 77, are outliers, but a movement of depression submerging them in the sea would eventually result in their being buried in newer deposits, thus changing them into inliers. There is abundant evidence that such changes have actually occurred in past times.